KVM: LAPIC: Inject timer interrupt via posted interrupt

[linux.git] / mm / vmscan.c

// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/mm/vmscan.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *
 *  Swap reorganised 29.12.95, Stephen Tweedie.
 *  kswapd added: 7.1.96  sct
 *  Removed kswapd_ctl limits, and swap out as many pages as needed
 *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
 *  Zone aware kswapd started 02/00, Kanoj Sarcar ([email protected]).
 *  Multiqueue VM started 5.8.00, Rik van Riel.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/mm.h>
#include <linux/sched/mm.h>
#include <linux/module.h>
#include <linux/gfp.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/pagemap.h>
#include <linux/init.h>
#include <linux/highmem.h>
#include <linux/vmpressure.h>
#include <linux/vmstat.h>
#include <linux/file.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>  /* for try_to_release_page(),
                                        buffer_heads_over_limit */
#include <linux/mm_inline.h>
#include <linux/backing-dev.h>
#include <linux/rmap.h>
#include <linux/topology.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/compaction.h>
#include <linux/notifier.h>
#include <linux/rwsem.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/memcontrol.h>
#include <linux/delayacct.h>
#include <linux/sysctl.h>
#include <linux/oom.h>
#include <linux/pagevec.h>
#include <linux/prefetch.h>
#include <linux/printk.h>
#include <linux/dax.h>
#include <linux/psi.h>

#include <asm/tlbflush.h>
#include <asm/div64.h>

#include <linux/swapops.h>
#include <linux/balloon_compaction.h>

#include "internal.h"

#define CREATE_TRACE_POINTS
#include <trace/events/vmscan.h>

struct scan_control {
        /* How many pages shrink_list() should reclaim */
        unsigned long nr_to_reclaim;

        /*
         * Nodemask of nodes allowed by the caller. If NULL, all nodes
         * are scanned.
         */
        nodemask_t      *nodemask;

        /*
         * The memory cgroup that hit its limit and as a result is the
         * primary target of this reclaim invocation.
         */
        struct mem_cgroup *target_mem_cgroup;

        /* Writepage batching in laptop mode; RECLAIM_WRITE */
        unsigned int may_writepage:1;

        /* Can mapped pages be reclaimed? */
        unsigned int may_unmap:1;

        /* Can pages be swapped as part of reclaim? */
        unsigned int may_swap:1;

        /* e.g. boosted watermark reclaim leaves slabs alone */
        unsigned int may_shrinkslab:1;

        /*
         * Cgroups are not reclaimed below their configured memory.low,
         * unless we threaten to OOM. If any cgroups are skipped due to
         * memory.low and nothing was reclaimed, go back for memory.low.
         */
        unsigned int memcg_low_reclaim:1;
        unsigned int memcg_low_skipped:1;

        unsigned int hibernation_mode:1;

        /* One of the zones is ready for compaction */
        unsigned int compaction_ready:1;

        /* Allocation order */
        s8 order;

        /* Scan (total_size >> priority) pages at once */
        s8 priority;

        /* The highest zone to isolate pages for reclaim from */
        s8 reclaim_idx;

        /* This context's GFP mask */
        gfp_t gfp_mask;

        /* Incremented by the number of inactive pages that were scanned */
        unsigned long nr_scanned;

        /* Number of pages freed so far during a call to shrink_zones() */
        unsigned long nr_reclaimed;

        struct {
                unsigned int dirty;
                unsigned int unqueued_dirty;
                unsigned int congested;
                unsigned int writeback;
                unsigned int immediate;
                unsigned int file_taken;
                unsigned int taken;
        } nr;
};

#ifdef ARCH_HAS_PREFETCH
#define prefetch_prev_lru_page(_page, _base, _field)                    \
        do {                                                            \
                if ((_page)->lru.prev != _base) {                       \
                        struct page *prev;                              \
                                                                        \
                        prev = lru_to_page(&(_page->lru));              \
                        prefetch(&prev->_field);                        \
                }                                                       \
        } while (0)
#else
#define prefetch_prev_lru_page(_page, _base, _field) do { } while (0)
#endif

#ifdef ARCH_HAS_PREFETCHW
#define prefetchw_prev_lru_page(_page, _base, _field)                   \
        do {                                                            \
                if ((_page)->lru.prev != _base) {                       \
                        struct page *prev;                              \
                                                                        \
                        prev = lru_to_page(&(_page->lru));              \
                        prefetchw(&prev->_field);                       \
                }                                                       \
        } while (0)
#else
#define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
#endif

/*
 * From 0 .. 100.  Higher means more swappy.
 */
int vm_swappiness = 60;
/*
 * The total number of pages which are beyond the high watermark within all
 * zones.
 */
unsigned long vm_total_pages;

static LIST_HEAD(shrinker_list);
static DECLARE_RWSEM(shrinker_rwsem);

#ifdef CONFIG_MEMCG_KMEM

/*
 * We allow subsystems to populate their shrinker-related
 * LRU lists before register_shrinker_prepared() is called
 * for the shrinker, since we don't want to impose
 * restrictions on their internal registration order.
 * In this case shrink_slab_memcg() may find corresponding
 * bit is set in the shrinkers map.
 *
 * This value is used by the function to detect registering
 * shrinkers and to skip do_shrink_slab() calls for them.
 */
#define SHRINKER_REGISTERING ((struct shrinker *)~0UL)

static DEFINE_IDR(shrinker_idr);
static int shrinker_nr_max;

static int prealloc_memcg_shrinker(struct shrinker *shrinker)
{
        int id, ret = -ENOMEM;

        down_write(&shrinker_rwsem);
        /* This may call shrinker, so it must use down_read_trylock() */
        id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
        if (id < 0)
                goto unlock;

        if (id >= shrinker_nr_max) {
                if (memcg_expand_shrinker_maps(id)) {
                        idr_remove(&shrinker_idr, id);
                        goto unlock;
                }

                shrinker_nr_max = id + 1;
        }
        shrinker->id = id;
        ret = 0;
unlock:
        up_write(&shrinker_rwsem);
        return ret;
}

static void unregister_memcg_shrinker(struct shrinker *shrinker)
{
        int id = shrinker->id;

        BUG_ON(id < 0);

        down_write(&shrinker_rwsem);
        idr_remove(&shrinker_idr, id);
        up_write(&shrinker_rwsem);
}
#else /* CONFIG_MEMCG_KMEM */
static int prealloc_memcg_shrinker(struct shrinker *shrinker)
{
        return 0;
}

static void unregister_memcg_shrinker(struct shrinker *shrinker)
{
}
#endif /* CONFIG_MEMCG_KMEM */

#ifdef CONFIG_MEMCG
static bool global_reclaim(struct scan_control *sc)
{
        return !sc->target_mem_cgroup;
}

/**
 * sane_reclaim - is the usual dirty throttling mechanism operational?
 * @sc: scan_control in question
 *
 * The normal page dirty throttling mechanism in balance_dirty_pages() is
 * completely broken with the legacy memcg and direct stalling in
 * shrink_page_list() is used for throttling instead, which lacks all the
 * niceties such as fairness, adaptive pausing, bandwidth proportional
 * allocation and configurability.
 *
 * This function tests whether the vmscan currently in progress can assume
 * that the normal dirty throttling mechanism is operational.
 */
static bool sane_reclaim(struct scan_control *sc)
{
        struct mem_cgroup *memcg = sc->target_mem_cgroup;

        if (!memcg)
                return true;
#ifdef CONFIG_CGROUP_WRITEBACK
        if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
                return true;
#endif
        return false;
}

static void set_memcg_congestion(pg_data_t *pgdat,
                                struct mem_cgroup *memcg,
                                bool congested)
{
        struct mem_cgroup_per_node *mn;

        if (!memcg)
                return;

        mn = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
        WRITE_ONCE(mn->congested, congested);
}

static bool memcg_congested(pg_data_t *pgdat,
                        struct mem_cgroup *memcg)
{
        struct mem_cgroup_per_node *mn;

        mn = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
        return READ_ONCE(mn->congested);

}
#else
static bool global_reclaim(struct scan_control *sc)
{
        return true;
}

static bool sane_reclaim(struct scan_control *sc)
{
        return true;
}

static inline void set_memcg_congestion(struct pglist_data *pgdat,
                                struct mem_cgroup *memcg, bool congested)
{
}

static inline bool memcg_congested(struct pglist_data *pgdat,
                        struct mem_cgroup *memcg)
{
        return false;

}
#endif

/*
 * This misses isolated pages which are not accounted for to save counters.
 * As the data only determines if reclaim or compaction continues, it is
 * not expected that isolated pages will be a dominating factor.
 */
unsigned long zone_reclaimable_pages(struct zone *zone)
{
        unsigned long nr;

        nr = zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_FILE) +
                zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_FILE);
        if (get_nr_swap_pages() > 0)
                nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
                        zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);

        return nr;
}

/**
 * lruvec_lru_size -  Returns the number of pages on the given LRU list.
 * @lruvec: lru vector
 * @lru: lru to use
 * @zone_idx: zones to consider (use MAX_NR_ZONES for the whole LRU list)
 */
unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone_idx)
{
        unsigned long lru_size;
        int zid;

        if (!mem_cgroup_disabled())
                lru_size = lruvec_page_state_local(lruvec, NR_LRU_BASE + lru);
        else
                lru_size = node_page_state(lruvec_pgdat(lruvec), NR_LRU_BASE + lru);

        for (zid = zone_idx + 1; zid < MAX_NR_ZONES; zid++) {
                struct zone *zone = &lruvec_pgdat(lruvec)->node_zones[zid];
                unsigned long size;

                if (!managed_zone(zone))
                        continue;

                if (!mem_cgroup_disabled())
                        size = mem_cgroup_get_zone_lru_size(lruvec, lru, zid);
                else
                        size = zone_page_state(&lruvec_pgdat(lruvec)->node_zones[zid],
                                       NR_ZONE_LRU_BASE + lru);
                lru_size -= min(size, lru_size);
        }

        return lru_size;

}

/*
 * Add a shrinker callback to be called from the vm.
 */
int prealloc_shrinker(struct shrinker *shrinker)
{
        unsigned int size = sizeof(*shrinker->nr_deferred);

        if (shrinker->flags & SHRINKER_NUMA_AWARE)
                size *= nr_node_ids;

        shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
        if (!shrinker->nr_deferred)
                return -ENOMEM;

        if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
                if (prealloc_memcg_shrinker(shrinker))
                        goto free_deferred;
        }

        return 0;

free_deferred:
        kfree(shrinker->nr_deferred);
        shrinker->nr_deferred = NULL;
        return -ENOMEM;
}

void free_prealloced_shrinker(struct shrinker *shrinker)
{
        if (!shrinker->nr_deferred)
                return;

        if (shrinker->flags & SHRINKER_MEMCG_AWARE)
                unregister_memcg_shrinker(shrinker);

        kfree(shrinker->nr_deferred);
        shrinker->nr_deferred = NULL;
}

void register_shrinker_prepared(struct shrinker *shrinker)
{
        down_write(&shrinker_rwsem);
        list_add_tail(&shrinker->list, &shrinker_list);
#ifdef CONFIG_MEMCG_KMEM
        if (shrinker->flags & SHRINKER_MEMCG_AWARE)
                idr_replace(&shrinker_idr, shrinker, shrinker->id);
#endif
        up_write(&shrinker_rwsem);
}

int register_shrinker(struct shrinker *shrinker)
{
        int err = prealloc_shrinker(shrinker);

        if (err)
                return err;
        register_shrinker_prepared(shrinker);
        return 0;
}
EXPORT_SYMBOL(register_shrinker);

/*
 * Remove one
 */
void unregister_shrinker(struct shrinker *shrinker)
{
        if (!shrinker->nr_deferred)
                return;
        if (shrinker->flags & SHRINKER_MEMCG_AWARE)
                unregister_memcg_shrinker(shrinker);
        down_write(&shrinker_rwsem);
        list_del(&shrinker->list);
        up_write(&shrinker_rwsem);
        kfree(shrinker->nr_deferred);
        shrinker->nr_deferred = NULL;
}
EXPORT_SYMBOL(unregister_shrinker);

#define SHRINK_BATCH 128

static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
                                    struct shrinker *shrinker, int priority)
{
        unsigned long freed = 0;
        unsigned long long delta;
        long total_scan;
        long freeable;
        long nr;
        long new_nr;
        int nid = shrinkctl->nid;
        long batch_size = shrinker->batch ? shrinker->batch
                                          : SHRINK_BATCH;
        long scanned = 0, next_deferred;

        if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
                nid = 0;

        freeable = shrinker->count_objects(shrinker, shrinkctl);
        if (freeable == 0 || freeable == SHRINK_EMPTY)
                return freeable;

        /*
         * copy the current shrinker scan count into a local variable
         * and zero it so that other concurrent shrinker invocations
         * don't also do this scanning work.
         */
        nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);

        total_scan = nr;
        if (shrinker->seeks) {
                delta = freeable >> priority;
                delta *= 4;
                do_div(delta, shrinker->seeks);
        } else {
                /*
                 * These objects don't require any IO to create. Trim
                 * them aggressively under memory pressure to keep
                 * them from causing refetches in the IO caches.
                 */
                delta = freeable / 2;
        }

        total_scan += delta;
        if (total_scan < 0) {
                pr_err("shrink_slab: %pS negative objects to delete nr=%ld\n",
                       shrinker->scan_objects, total_scan);
                total_scan = freeable;
                next_deferred = nr;
        } else
                next_deferred = total_scan;

        /*
         * We need to avoid excessive windup on filesystem shrinkers
         * due to large numbers of GFP_NOFS allocations causing the
         * shrinkers to return -1 all the time. This results in a large
         * nr being built up so when a shrink that can do some work
         * comes along it empties the entire cache due to nr >>>
         * freeable. This is bad for sustaining a working set in
         * memory.
         *
         * Hence only allow the shrinker to scan the entire cache when
         * a large delta change is calculated directly.
         */
        if (delta < freeable / 4)
                total_scan = min(total_scan, freeable / 2);

        /*
         * Avoid risking looping forever due to too large nr value:
         * never try to free more than twice the estimate number of
         * freeable entries.
         */
        if (total_scan > freeable * 2)
                total_scan = freeable * 2;

        trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
                                   freeable, delta, total_scan, priority);

        /*
         * Normally, we should not scan less than batch_size objects in one
         * pass to avoid too frequent shrinker calls, but if the slab has less
         * than batch_size objects in total and we are really tight on memory,
         * we will try to reclaim all available objects, otherwise we can end
         * up failing allocations although there are plenty of reclaimable
         * objects spread over several slabs with usage less than the
         * batch_size.
         *
         * We detect the "tight on memory" situations by looking at the total
         * number of objects we want to scan (total_scan). If it is greater
         * than the total number of objects on slab (freeable), we must be
         * scanning at high prio and therefore should try to reclaim as much as
         * possible.
         */
        while (total_scan >= batch_size ||
               total_scan >= freeable) {
                unsigned long ret;
                unsigned long nr_to_scan = min(batch_size, total_scan);

                shrinkctl->nr_to_scan = nr_to_scan;
                shrinkctl->nr_scanned = nr_to_scan;
                ret = shrinker->scan_objects(shrinker, shrinkctl);
                if (ret == SHRINK_STOP)
                        break;
                freed += ret;

                count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
                total_scan -= shrinkctl->nr_scanned;
                scanned += shrinkctl->nr_scanned;

                cond_resched();
        }

        if (next_deferred >= scanned)
                next_deferred -= scanned;
        else
                next_deferred = 0;
        /*
         * move the unused scan count back into the shrinker in a
         * manner that handles concurrent updates. If we exhausted the
         * scan, there is no need to do an update.
         */
        if (next_deferred > 0)
                new_nr = atomic_long_add_return(next_deferred,
                                                &shrinker->nr_deferred[nid]);
        else
                new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);

        trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan);
        return freed;
}

#ifdef CONFIG_MEMCG_KMEM
static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
                        struct mem_cgroup *memcg, int priority)
{
        struct memcg_shrinker_map *map;
        unsigned long ret, freed = 0;
        int i;

        if (!memcg_kmem_enabled() || !mem_cgroup_online(memcg))
                return 0;

        if (!down_read_trylock(&shrinker_rwsem))
                return 0;

        map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,
                                        true);
        if (unlikely(!map))
                goto unlock;

        for_each_set_bit(i, map->map, shrinker_nr_max) {
                struct shrink_control sc = {
                        .gfp_mask = gfp_mask,
                        .nid = nid,
                        .memcg = memcg,
                };
                struct shrinker *shrinker;

                shrinker = idr_find(&shrinker_idr, i);
                if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) {
                        if (!shrinker)
                                clear_bit(i, map->map);
                        continue;
                }

                ret = do_shrink_slab(&sc, shrinker, priority);
                if (ret == SHRINK_EMPTY) {
                        clear_bit(i, map->map);
                        /*
                         * After the shrinker reported that it had no objects to
                         * free, but before we cleared the corresponding bit in
                         * the memcg shrinker map, a new object might have been
                         * added. To make sure, we have the bit set in this
                         * case, we invoke the shrinker one more time and reset
                         * the bit if it reports that it is not empty anymore.
                         * The memory barrier here pairs with the barrier in
                         * memcg_set_shrinker_bit():
                         *
                         * list_lru_add()     shrink_slab_memcg()
                         *   list_add_tail()    clear_bit()
                         *   <MB>               <MB>
                         *   set_bit()          do_shrink_slab()
                         */
                        smp_mb__after_atomic();
                        ret = do_shrink_slab(&sc, shrinker, priority);
                        if (ret == SHRINK_EMPTY)
                                ret = 0;
                        else
                                memcg_set_shrinker_bit(memcg, nid, i);
                }
                freed += ret;

                if (rwsem_is_contended(&shrinker_rwsem)) {
                        freed = freed ? : 1;
                        break;
                }
        }
unlock:
        up_read(&shrinker_rwsem);
        return freed;
}
#else /* CONFIG_MEMCG_KMEM */
static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
                        struct mem_cgroup *memcg, int priority)
{
        return 0;
}
#endif /* CONFIG_MEMCG_KMEM */

/**
 * shrink_slab - shrink slab caches
 * @gfp_mask: allocation context
 * @nid: node whose slab caches to target
 * @memcg: memory cgroup whose slab caches to target
 * @priority: the reclaim priority
 *
 * Call the shrink functions to age shrinkable caches.
 *
 * @nid is passed along to shrinkers with SHRINKER_NUMA_AWARE set,
 * unaware shrinkers will receive a node id of 0 instead.
 *
 * @memcg specifies the memory cgroup to target. Unaware shrinkers
 * are called only if it is the root cgroup.
 *
 * @priority is sc->priority, we take the number of objects and >> by priority
 * in order to get the scan target.
 *
 * Returns the number of reclaimed slab objects.
 */
static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
                                 struct mem_cgroup *memcg,
                                 int priority)
{
        unsigned long ret, freed = 0;
        struct shrinker *shrinker;

        if (!mem_cgroup_is_root(memcg))
                return shrink_slab_memcg(gfp_mask, nid, memcg, priority);

        if (!down_read_trylock(&shrinker_rwsem))
                goto out;

        list_for_each_entry(shrinker, &shrinker_list, list) {
                struct shrink_control sc = {
                        .gfp_mask = gfp_mask,
                        .nid = nid,
                        .memcg = memcg,
                };

                ret = do_shrink_slab(&sc, shrinker, priority);
                if (ret == SHRINK_EMPTY)
                        ret = 0;
                freed += ret;
                /*
                 * Bail out if someone want to register a new shrinker to
                 * prevent the regsitration from being stalled for long periods
                 * by parallel ongoing shrinking.
                 */
                if (rwsem_is_contended(&shrinker_rwsem)) {
                        freed = freed ? : 1;
                        break;
                }
        }

        up_read(&shrinker_rwsem);
out:
        cond_resched();
        return freed;
}

void drop_slab_node(int nid)
{
        unsigned long freed;

        do {
                struct mem_cgroup *memcg = NULL;

                freed = 0;
                memcg = mem_cgroup_iter(NULL, NULL, NULL);
                do {
                        freed += shrink_slab(GFP_KERNEL, nid, memcg, 0);
                } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
        } while (freed > 10);
}

void drop_slab(void)
{
        int nid;

        for_each_online_node(nid)
                drop_slab_node(nid);
}

static inline int is_page_cache_freeable(struct page *page)
{
        /*
         * A freeable page cache page is referenced only by the caller
         * that isolated the page, the page cache and optional buffer
         * heads at page->private.
         */
        int page_cache_pins = PageTransHuge(page) && PageSwapCache(page) ?
                HPAGE_PMD_NR : 1;
        return page_count(page) - page_has_private(page) == 1 + page_cache_pins;
}

static int may_write_to_inode(struct inode *inode, struct scan_control *sc)
{
        if (current->flags & PF_SWAPWRITE)
                return 1;
        if (!inode_write_congested(inode))
                return 1;
        if (inode_to_bdi(inode) == current->backing_dev_info)
                return 1;
        return 0;
}

/*
 * We detected a synchronous write error writing a page out.  Probably
 * -ENOSPC.  We need to propagate that into the address_space for a subsequent
 * fsync(), msync() or close().
 *
 * The tricky part is that after writepage we cannot touch the mapping: nothing
 * prevents it from being freed up.  But we have a ref on the page and once
 * that page is locked, the mapping is pinned.
 *
 * We're allowed to run sleeping lock_page() here because we know the caller has
 * __GFP_FS.
 */
static void handle_write_error(struct address_space *mapping,
                                struct page *page, int error)
{
        lock_page(page);
        if (page_mapping(page) == mapping)
                mapping_set_error(mapping, error);
        unlock_page(page);
}

/* possible outcome of pageout() */
typedef enum {
        /* failed to write page out, page is locked */
        PAGE_KEEP,
        /* move page to the active list, page is locked */
        PAGE_ACTIVATE,
        /* page has been sent to the disk successfully, page is unlocked */
        PAGE_SUCCESS,
        /* page is clean and locked */
        PAGE_CLEAN,
} pageout_t;

/*
 * pageout is called by shrink_page_list() for each dirty page.
 * Calls ->writepage().
 */
static pageout_t pageout(struct page *page, struct address_space *mapping,
                         struct scan_control *sc)
{
        /*
         * If the page is dirty, only perform writeback if that write
         * will be non-blocking.  To prevent this allocation from being
         * stalled by pagecache activity.  But note that there may be
         * stalls if we need to run get_block().  We could test
         * PagePrivate for that.
         *
         * If this process is currently in __generic_file_write_iter() against
         * this page's queue, we can perform writeback even if that
         * will block.
         *
         * If the page is swapcache, write it back even if that would
         * block, for some throttling. This happens by accident, because
         * swap_backing_dev_info is bust: it doesn't reflect the
         * congestion state of the swapdevs.  Easy to fix, if needed.
         */
        if (!is_page_cache_freeable(page))
                return PAGE_KEEP;
        if (!mapping) {
                /*
                 * Some data journaling orphaned pages can have
                 * page->mapping == NULL while being dirty with clean buffers.
                 */
                if (page_has_private(page)) {
                        if (try_to_free_buffers(page)) {
                                ClearPageDirty(page);
                                pr_info("%s: orphaned page\n", __func__);
                                return PAGE_CLEAN;
                        }
                }
                return PAGE_KEEP;
        }
        if (mapping->a_ops->writepage == NULL)
                return PAGE_ACTIVATE;
        if (!may_write_to_inode(mapping->host, sc))
                return PAGE_KEEP;

        if (clear_page_dirty_for_io(page)) {
                int res;
                struct writeback_control wbc = {
                        .sync_mode = WB_SYNC_NONE,
                        .nr_to_write = SWAP_CLUSTER_MAX,
                        .range_start = 0,
                        .range_end = LLONG_MAX,
                        .for_reclaim = 1,
                };

                SetPageReclaim(page);
                res = mapping->a_ops->writepage(page, &wbc);
                if (res < 0)
                        handle_write_error(mapping, page, res);
                if (res == AOP_WRITEPAGE_ACTIVATE) {
                        ClearPageReclaim(page);
                        return PAGE_ACTIVATE;
                }

                if (!PageWriteback(page)) {
                        /* synchronous write or broken a_ops? */
                        ClearPageReclaim(page);
                }
                trace_mm_vmscan_writepage(page);
                inc_node_page_state(page, NR_VMSCAN_WRITE);
                return PAGE_SUCCESS;
        }

        return PAGE_CLEAN;
}

/*
 * Same as remove_mapping, but if the page is removed from the mapping, it
 * gets returned with a refcount of 0.
 */
static int __remove_mapping(struct address_space *mapping, struct page *page,
                            bool reclaimed)
{
        unsigned long flags;
        int refcount;

        BUG_ON(!PageLocked(page));
        BUG_ON(mapping != page_mapping(page));

        xa_lock_irqsave(&mapping->i_pages, flags);
        /*
         * The non racy check for a busy page.
         *
         * Must be careful with the order of the tests. When someone has
         * a ref to the page, it may be possible that they dirty it then
         * drop the reference. So if PageDirty is tested before page_count
         * here, then the following race may occur:
         *
         * get_user_pages(&page);
         * [user mapping goes away]
         * write_to(page);
         *                              !PageDirty(page)    [good]
         * SetPageDirty(page);
         * put_page(page);
         *                              !page_count(page)   [good, discard it]
         *
         * [oops, our write_to data is lost]
         *
         * Reversing the order of the tests ensures such a situation cannot
         * escape unnoticed. The smp_rmb is needed to ensure the page->flags
         * load is not satisfied before that of page->_refcount.
         *
         * Note that if SetPageDirty is always performed via set_page_dirty,
         * and thus under the i_pages lock, then this ordering is not required.
         */
        if (unlikely(PageTransHuge(page)) && PageSwapCache(page))
                refcount = 1 + HPAGE_PMD_NR;
        else
                refcount = 2;
        if (!page_ref_freeze(page, refcount))
                goto cannot_free;
        /* note: atomic_cmpxchg in page_ref_freeze provides the smp_rmb */
        if (unlikely(PageDirty(page))) {
                page_ref_unfreeze(page, refcount);
                goto cannot_free;
        }

        if (PageSwapCache(page)) {
                swp_entry_t swap = { .val = page_private(page) };
                mem_cgroup_swapout(page, swap);
                __delete_from_swap_cache(page, swap);
                xa_unlock_irqrestore(&mapping->i_pages, flags);
                put_swap_page(page, swap);
        } else {
                void (*freepage)(struct page *);
                void *shadow = NULL;

                freepage = mapping->a_ops->freepage;
                /*
                 * Remember a shadow entry for reclaimed file cache in
                 * order to detect refaults, thus thrashing, later on.
                 *
                 * But don't store shadows in an address space that is
                 * already exiting.  This is not just an optizimation,
                 * inode reclaim needs to empty out the radix tree or
                 * the nodes are lost.  Don't plant shadows behind its
                 * back.
                 *
                 * We also don't store shadows for DAX mappings because the
                 * only page cache pages found in these are zero pages
                 * covering holes, and because we don't want to mix DAX
                 * exceptional entries and shadow exceptional entries in the
                 * same address_space.
                 */
                if (reclaimed && page_is_file_cache(page) &&
                    !mapping_exiting(mapping) && !dax_mapping(mapping))
                        shadow = workingset_eviction(page);
                __delete_from_page_cache(page, shadow);
                xa_unlock_irqrestore(&mapping->i_pages, flags);

                if (freepage != NULL)
                        freepage(page);
        }

        return 1;

cannot_free:
        xa_unlock_irqrestore(&mapping->i_pages, flags);
        return 0;
}

/*
 * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
 * someone else has a ref on the page, abort and return 0.  If it was
 * successfully detached, return 1.  Assumes the caller has a single ref on
 * this page.
 */
int remove_mapping(struct address_space *mapping, struct page *page)
{
        if (__remove_mapping(mapping, page, false)) {
                /*
                 * Unfreezing the refcount with 1 rather than 2 effectively
                 * drops the pagecache ref for us without requiring another
                 * atomic operation.
                 */
                page_ref_unfreeze(page, 1);
                return 1;
        }
        return 0;
}

/**
 * putback_lru_page - put previously isolated page onto appropriate LRU list
 * @page: page to be put back to appropriate lru list
 *
 * Add previously isolated @page to appropriate LRU list.
 * Page may still be unevictable for other reasons.
 *
 * lru_lock must not be held, interrupts must be enabled.
 */
void putback_lru_page(struct page *page)
{
        lru_cache_add(page);
        put_page(page);         /* drop ref from isolate */
}

enum page_references {
        PAGEREF_RECLAIM,
        PAGEREF_RECLAIM_CLEAN,
        PAGEREF_KEEP,
        PAGEREF_ACTIVATE,
};

static enum page_references page_check_references(struct page *page,
                                                  struct scan_control *sc)
{
        int referenced_ptes, referenced_page;
        unsigned long vm_flags;

        referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
                                          &vm_flags);
        referenced_page = TestClearPageReferenced(page);

        /*
         * Mlock lost the isolation race with us.  Let try_to_unmap()
         * move the page to the unevictable list.
         */
        if (vm_flags & VM_LOCKED)
                return PAGEREF_RECLAIM;

        if (referenced_ptes) {
                if (PageSwapBacked(page))
                        return PAGEREF_ACTIVATE;
                /*
                 * All mapped pages start out with page table
                 * references from the instantiating fault, so we need
                 * to look twice if a mapped file page is used more
                 * than once.
                 *
                 * Mark it and spare it for another trip around the
                 * inactive list.  Another page table reference will
                 * lead to its activation.
                 *
                 * Note: the mark is set for activated pages as well
                 * so that recently deactivated but used pages are
                 * quickly recovered.
                 */
                SetPageReferenced(page);

                if (referenced_page || referenced_ptes > 1)
                        return PAGEREF_ACTIVATE;

                /*
                 * Activate file-backed executable pages after first usage.
                 */
                if (vm_flags & VM_EXEC)
                        return PAGEREF_ACTIVATE;

                return PAGEREF_KEEP;
        }

        /* Reclaim if clean, defer dirty pages to writeback */
        if (referenced_page && !PageSwapBacked(page))
                return PAGEREF_RECLAIM_CLEAN;

        return PAGEREF_RECLAIM;
}

/* Check if a page is dirty or under writeback */
static void page_check_dirty_writeback(struct page *page,
                                       bool *dirty, bool *writeback)
{
        struct address_space *mapping;

        /*
         * Anonymous pages are not handled by flushers and must be written
         * from reclaim context. Do not stall reclaim based on them
         */
        if (!page_is_file_cache(page) ||
            (PageAnon(page) && !PageSwapBacked(page))) {
                *dirty = false;
                *writeback = false;
                return;
        }

        /* By default assume that the page flags are accurate */
        *dirty = PageDirty(page);
        *writeback = PageWriteback(page);

        /* Verify dirty/writeback state if the filesystem supports it */
        if (!page_has_private(page))
                return;

        mapping = page_mapping(page);
        if (mapping && mapping->a_ops->is_dirty_writeback)
                mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
}

/*
 * shrink_page_list() returns the number of reclaimed pages
 */
static unsigned long shrink_page_list(struct list_head *page_list,
                                      struct pglist_data *pgdat,
                                      struct scan_control *sc,
                                      enum ttu_flags ttu_flags,
                                      struct reclaim_stat *stat,
                                      bool force_reclaim)
{
        LIST_HEAD(ret_pages);
        LIST_HEAD(free_pages);
        unsigned nr_reclaimed = 0;
        unsigned pgactivate = 0;

        memset(stat, 0, sizeof(*stat));
        cond_resched();

        while (!list_empty(page_list)) {
                struct address_space *mapping;
                struct page *page;
                int may_enter_fs;
                enum page_references references = PAGEREF_RECLAIM_CLEAN;
                bool dirty, writeback;
                unsigned int nr_pages;

                cond_resched();

                page = lru_to_page(page_list);
                list_del(&page->lru);

                if (!trylock_page(page))
                        goto keep;

                VM_BUG_ON_PAGE(PageActive(page), page);

                nr_pages = 1 << compound_order(page);

                /* Account the number of base pages even though THP */
                sc->nr_scanned += nr_pages;

                if (unlikely(!page_evictable(page)))
                        goto activate_locked;

                if (!sc->may_unmap && page_mapped(page))
                        goto keep_locked;

                may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
                        (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));

                /*
                 * The number of dirty pages determines if a node is marked
                 * reclaim_congested which affects wait_iff_congested. kswapd
                 * will stall and start writing pages if the tail of the LRU
                 * is all dirty unqueued pages.
                 */
                page_check_dirty_writeback(page, &dirty, &writeback);
                if (dirty || writeback)
                        stat->nr_dirty++;

                if (dirty && !writeback)
                        stat->nr_unqueued_dirty++;

                /*
                 * Treat this page as congested if the underlying BDI is or if
                 * pages are cycling through the LRU so quickly that the
                 * pages marked for immediate reclaim are making it to the
                 * end of the LRU a second time.
                 */
                mapping = page_mapping(page);
                if (((dirty || writeback) && mapping &&
                     inode_write_congested(mapping->host)) ||
                    (writeback && PageReclaim(page)))
                        stat->nr_congested++;

                /*
                 * If a page at the tail of the LRU is under writeback, there
                 * are three cases to consider.
                 *
                 * 1) If reclaim is encountering an excessive number of pages
                 *    under writeback and this page is both under writeback and
                 *    PageReclaim then it indicates that pages are being queued
                 *    for IO but are being recycled through the LRU before the
                 *    IO can complete. Waiting on the page itself risks an
                 *    indefinite stall if it is impossible to writeback the
                 *    page due to IO error or disconnected storage so instead
                 *    note that the LRU is being scanned too quickly and the
                 *    caller can stall after page list has been processed.
                 *
                 * 2) Global or new memcg reclaim encounters a page that is
                 *    not marked for immediate reclaim, or the caller does not
                 *    have __GFP_FS (or __GFP_IO if it's simply going to swap,
                 *    not to fs). In this case mark the page for immediate
                 *    reclaim and continue scanning.
                 *
                 *    Require may_enter_fs because we would wait on fs, which
                 *    may not have submitted IO yet. And the loop driver might
                 *    enter reclaim, and deadlock if it waits on a page for
                 *    which it is needed to do the write (loop masks off
                 *    __GFP_IO|__GFP_FS for this reason); but more thought
                 *    would probably show more reasons.
                 *
                 * 3) Legacy memcg encounters a page that is already marked
                 *    PageReclaim. memcg does not have any dirty pages
                 *    throttling so we could easily OOM just because too many
                 *    pages are in writeback and there is nothing else to
                 *    reclaim. Wait for the writeback to complete.
                 *
                 * In cases 1) and 2) we activate the pages to get them out of
                 * the way while we continue scanning for clean pages on the
                 * inactive list and refilling from the active list. The
                 * observation here is that waiting for disk writes is more
                 * expensive than potentially causing reloads down the line.
                 * Since they're marked for immediate reclaim, they won't put
                 * memory pressure on the cache working set any longer than it
                 * takes to write them to disk.
                 */
                if (PageWriteback(page)) {
                        /* Case 1 above */
                        if (current_is_kswapd() &&
                            PageReclaim(page) &&
                            test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
                                stat->nr_immediate++;
                                goto activate_locked;

                        /* Case 2 above */
                        } else if (sane_reclaim(sc) ||
                            !PageReclaim(page) || !may_enter_fs) {
                                /*
                                 * This is slightly racy - end_page_writeback()
                                 * might have just cleared PageReclaim, then
                                 * setting PageReclaim here end up interpreted
                                 * as PageReadahead - but that does not matter
                                 * enough to care.  What we do want is for this
                                 * page to have PageReclaim set next time memcg
                                 * reclaim reaches the tests above, so it will
                                 * then wait_on_page_writeback() to avoid OOM;
                                 * and it's also appropriate in global reclaim.
                                 */
                                SetPageReclaim(page);
                                stat->nr_writeback++;
                                goto activate_locked;

                        /* Case 3 above */
                        } else {
                                unlock_page(page);
                                wait_on_page_writeback(page);
                                /* then go back and try same page again */
                                list_add_tail(&page->lru, page_list);
                                continue;
                        }
                }

                if (!force_reclaim)
                        references = page_check_references(page, sc);

                switch (references) {
                case PAGEREF_ACTIVATE:
                        goto activate_locked;
                case PAGEREF_KEEP:
                        stat->nr_ref_keep += nr_pages;
                        goto keep_locked;
                case PAGEREF_RECLAIM:
                case PAGEREF_RECLAIM_CLEAN:
                        ; /* try to reclaim the page below */
                }

                /*
                 * Anonymous process memory has backing store?
                 * Try to allocate it some swap space here.
                 * Lazyfree page could be freed directly
                 */
                if (PageAnon(page) && PageSwapBacked(page)) {
                        if (!PageSwapCache(page)) {
                                if (!(sc->gfp_mask & __GFP_IO))
                                        goto keep_locked;
                                if (PageTransHuge(page)) {
                                        /* cannot split THP, skip it */
                                        if (!can_split_huge_page(page, NULL))
                                                goto activate_locked;
                                        /*
                                         * Split pages without a PMD map right
                                         * away. Chances are some or all of the
                                         * tail pages can be freed without IO.
                                         */
                                        if (!compound_mapcount(page) &&
                                            split_huge_page_to_list(page,
                                                                    page_list))
                                                goto activate_locked;
                                }
                                if (!add_to_swap(page)) {
                                        if (!PageTransHuge(page))
                                                goto activate_locked_split;
                                        /* Fallback to swap normal pages */
                                        if (split_huge_page_to_list(page,
                                                                    page_list))
                                                goto activate_locked;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
                                        count_vm_event(THP_SWPOUT_FALLBACK);
#endif
                                        if (!add_to_swap(page))
                                                goto activate_locked_split;
                                }

                                may_enter_fs = 1;

                                /* Adding to swap updated mapping */
                                mapping = page_mapping(page);
                        }
                } else if (unlikely(PageTransHuge(page))) {
                        /* Split file THP */
                        if (split_huge_page_to_list(page, page_list))
                                goto keep_locked;
                }

                /*
                 * THP may get split above, need minus tail pages and update
                 * nr_pages to avoid accounting tail pages twice.
                 *
                 * The tail pages that are added into swap cache successfully
                 * reach here.
                 */
                if ((nr_pages > 1) && !PageTransHuge(page)) {
                        sc->nr_scanned -= (nr_pages - 1);
                        nr_pages = 1;
                }

                /*
                 * The page is mapped into the page tables of one or more
                 * processes. Try to unmap it here.
                 */
                if (page_mapped(page)) {
                        enum ttu_flags flags = ttu_flags | TTU_BATCH_FLUSH;

                        if (unlikely(PageTransHuge(page)))
                                flags |= TTU_SPLIT_HUGE_PMD;
                        if (!try_to_unmap(page, flags)) {
                                stat->nr_unmap_fail += nr_pages;
                                goto activate_locked;
                        }
                }

                if (PageDirty(page)) {
                        /*
                         * Only kswapd can writeback filesystem pages
                         * to avoid risk of stack overflow. But avoid
                         * injecting inefficient single-page IO into
                         * flusher writeback as much as possible: only
                         * write pages when we've encountered many
                         * dirty pages, and when we've already scanned
                         * the rest of the LRU for clean pages and see
                         * the same dirty pages again (PageReclaim).
                         */
                        if (page_is_file_cache(page) &&
                            (!current_is_kswapd() || !PageReclaim(page) ||
                             !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
                                /*
                                 * Immediately reclaim when written back.
                                 * Similar in principal to deactivate_page()
                                 * except we already have the page isolated
                                 * and know it's dirty
                                 */
                                inc_node_page_state(page, NR_VMSCAN_IMMEDIATE);
                                SetPageReclaim(page);

                                goto activate_locked;
                        }

                        if (references == PAGEREF_RECLAIM_CLEAN)
                                goto keep_locked;
                        if (!may_enter_fs)
                                goto keep_locked;
                        if (!sc->may_writepage)
                                goto keep_locked;

                        /*
                         * Page is dirty. Flush the TLB if a writable entry
                         * potentially exists to avoid CPU writes after IO
                         * starts and then write it out here.
                         */
                        try_to_unmap_flush_dirty();
                        switch (pageout(page, mapping, sc)) {
                        case PAGE_KEEP:
                                goto keep_locked;
                        case PAGE_ACTIVATE:
                                goto activate_locked;
                        case PAGE_SUCCESS:
                                if (PageWriteback(page))
                                        goto keep;
                                if (PageDirty(page))
                                        goto keep;

                                /*
                                 * A synchronous write - probably a ramdisk.  Go
                                 * ahead and try to reclaim the page.
                                 */
                                if (!trylock_page(page))
                                        goto keep;
                                if (PageDirty(page) || PageWriteback(page))
                                        goto keep_locked;
                                mapping = page_mapping(page);
                        case PAGE_CLEAN:
                                ; /* try to free the page below */
                        }
                }

                /*
                 * If the page has buffers, try to free the buffer mappings
                 * associated with this page. If we succeed we try to free
                 * the page as well.
                 *
                 * We do this even if the page is PageDirty().
                 * try_to_release_page() does not perform I/O, but it is
                 * possible for a page to have PageDirty set, but it is actually
                 * clean (all its buffers are clean).  This happens if the
                 * buffers were written out directly, with submit_bh(). ext3
                 * will do this, as well as the blockdev mapping.
                 * try_to_release_page() will discover that cleanness and will
                 * drop the buffers and mark the page clean - it can be freed.
                 *
                 * Rarely, pages can have buffers and no ->mapping.  These are
                 * the pages which were not successfully invalidated in
                 * truncate_complete_page().  We try to drop those buffers here
                 * and if that worked, and the page is no longer mapped into
                 * process address space (page_count == 1) it can be freed.
                 * Otherwise, leave the page on the LRU so it is swappable.
                 */
                if (page_has_private(page)) {
                        if (!try_to_release_page(page, sc->gfp_mask))
                                goto activate_locked;
                        if (!mapping && page_count(page) == 1) {
                                unlock_page(page);
                                if (put_page_testzero(page))
                                        goto free_it;
                                else {
                                        /*
                                         * rare race with speculative reference.
                                         * the speculative reference will free
                                         * this page shortly, so we may
                                         * increment nr_reclaimed here (and
                                         * leave it off the LRU).
                                         */
                                        nr_reclaimed++;
                                        continue;
                                }
                        }
                }

                if (PageAnon(page) && !PageSwapBacked(page)) {
                        /* follow __remove_mapping for reference */
                        if (!page_ref_freeze(page, 1))
                                goto keep_locked;
                        if (PageDirty(page)) {
                                page_ref_unfreeze(page, 1);
                                goto keep_locked;
                        }

                        count_vm_event(PGLAZYFREED);
                        count_memcg_page_event(page, PGLAZYFREED);
                } else if (!mapping || !__remove_mapping(mapping, page, true))
                        goto keep_locked;

                unlock_page(page);
free_it:
                /*
                 * THP may get swapped out in a whole, need account
                 * all base pages.
                 */
                nr_reclaimed += nr_pages;

                /*
                 * Is there need to periodically free_page_list? It would
                 * appear not as the counts should be low
                 */
                if (unlikely(PageTransHuge(page))) {
                        mem_cgroup_uncharge(page);
                        (*get_compound_page_dtor(page))(page);
                } else
                        list_add(&page->lru, &free_pages);
                continue;

activate_locked_split:
                /*
                 * The tail pages that are failed to add into swap cache
                 * reach here.  Fixup nr_scanned and nr_pages.
                 */
                if (nr_pages > 1) {
                        sc->nr_scanned -= (nr_pages - 1);
                        nr_pages = 1;
                }
activate_locked:
                /* Not a candidate for swapping, so reclaim swap space. */
                if (PageSwapCache(page) && (mem_cgroup_swap_full(page) ||
                                                PageMlocked(page)))
                        try_to_free_swap(page);
                VM_BUG_ON_PAGE(PageActive(page), page);
                if (!PageMlocked(page)) {
                        int type = page_is_file_cache(page);
                        SetPageActive(page);
                        stat->nr_activate[type] += nr_pages;
                        count_memcg_page_event(page, PGACTIVATE);
                }
keep_locked:
                unlock_page(page);
keep:
                list_add(&page->lru, &ret_pages);
                VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
        }

        pgactivate = stat->nr_activate[0] + stat->nr_activate[1];

        mem_cgroup_uncharge_list(&free_pages);
        try_to_unmap_flush();
        free_unref_page_list(&free_pages);

        list_splice(&ret_pages, page_list);
        count_vm_events(PGACTIVATE, pgactivate);

        return nr_reclaimed;
}

unsigned long reclaim_clean_pages_from_list(struct zone *zone,
                                            struct list_head *page_list)
{
        struct scan_control sc = {
                .gfp_mask = GFP_KERNEL,
                .priority = DEF_PRIORITY,
                .may_unmap = 1,
        };
        struct reclaim_stat dummy_stat;
        unsigned long ret;
        struct page *page, *next;
        LIST_HEAD(clean_pages);

        list_for_each_entry_safe(page, next, page_list, lru) {
                if (page_is_file_cache(page) && !PageDirty(page) &&
                    !__PageMovable(page) && !PageUnevictable(page)) {
                        ClearPageActive(page);
                        list_move(&page->lru, &clean_pages);
                }
        }

        ret = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
                        TTU_IGNORE_ACCESS, &dummy_stat, true);
        list_splice(&clean_pages, page_list);
        mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE, -ret);
        return ret;
}

/*
 * Attempt to remove the specified page from its LRU.  Only take this page
 * if it is of the appropriate PageActive status.  Pages which are being
 * freed elsewhere are also ignored.
 *
 * page:        page to consider
 * mode:        one of the LRU isolation modes defined above
 *
 * returns 0 on success, -ve errno on failure.
 */
int __isolate_lru_page(struct page *page, isolate_mode_t mode)
{
        int ret = -EINVAL;

        /* Only take pages on the LRU. */
        if (!PageLRU(page))
                return ret;

        /* Compaction should not handle unevictable pages but CMA can do so */
        if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
                return ret;

        ret = -EBUSY;

        /*
         * To minimise LRU disruption, the caller can indicate that it only
         * wants to isolate pages it will be able to operate on without
         * blocking - clean pages for the most part.
         *
         * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
         * that it is possible to migrate without blocking
         */
        if (mode & ISOLATE_ASYNC_MIGRATE) {
                /* All the caller can do on PageWriteback is block */
                if (PageWriteback(page))
                        return ret;

                if (PageDirty(page)) {
                        struct address_space *mapping;
                        bool migrate_dirty;

                        /*
                         * Only pages without mappings or that have a
                         * ->migratepage callback are possible to migrate
                         * without blocking. However, we can be racing with
                         * truncation so it's necessary to lock the page
                         * to stabilise the mapping as truncation holds
                         * the page lock until after the page is removed
                         * from the page cache.
                         */
                        if (!trylock_page(page))
                                return ret;

                        mapping = page_mapping(page);
                        migrate_dirty = !mapping || mapping->a_ops->migratepage;
                        unlock_page(page);
                        if (!migrate_dirty)
                                return ret;
                }
        }

        if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
                return ret;

        if (likely(get_page_unless_zero(page))) {
                /*
                 * Be careful not to clear PageLRU until after we're
                 * sure the page is not being freed elsewhere -- the
                 * page release code relies on it.
                 */
                ClearPageLRU(page);
                ret = 0;
        }

        return ret;
}


/*
 * Update LRU sizes after isolating pages. The LRU size updates must
 * be complete before mem_cgroup_update_lru_size due to a santity check.
 */
static __always_inline void update_lru_sizes(struct lruvec *lruvec,
                        enum lru_list lru, unsigned long *nr_zone_taken)
{
        int zid;

        for (zid = 0; zid < MAX_NR_ZONES; zid++) {
                if (!nr_zone_taken[zid])
                        continue;

                __update_lru_size(lruvec, lru, zid, -nr_zone_taken[zid]);
#ifdef CONFIG_MEMCG
                mem_cgroup_update_lru_size(lruvec, lru, zid, -nr_zone_taken[zid]);
#endif
        }

}

/**
 * pgdat->lru_lock is heavily contended.  Some of the functions that
 * shrink the lists perform better by taking out a batch of pages
 * and working on them outside the LRU lock.
 *
 * For pagecache intensive workloads, this function is the hottest
 * spot in the kernel (apart from copy_*_user functions).
 *
 * Appropriate locks must be held before calling this function.
 *
 * @nr_to_scan: The number of eligible pages to look through on the list.
 * @lruvec:     The LRU vector to pull pages from.
 * @dst:        The temp list to put pages on to.
 * @nr_scanned: The number of pages that were scanned.
 * @sc:         The scan_control struct for this reclaim session
 * @mode:       One of the LRU isolation modes
 * @lru:        LRU list id for isolating
 *
 * returns how many pages were moved onto *@dst.
 */
static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
                struct lruvec *lruvec, struct list_head *dst,
                unsigned long *nr_scanned, struct scan_control *sc,
                enum lru_list lru)
{
        struct list_head *src = &lruvec->lists[lru];
        unsigned long nr_taken = 0;
        unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
        unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
        unsigned long skipped = 0;
        unsigned long scan, total_scan, nr_pages;
        LIST_HEAD(pages_skipped);
        isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);

        total_scan = 0;
        scan = 0;
        while (scan < nr_to_scan && !list_empty(src)) {
                struct page *page;

                page = lru_to_page(src);
                prefetchw_prev_lru_page(page, src, flags);

                VM_BUG_ON_PAGE(!PageLRU(page), page);

                nr_pages = 1 << compound_order(page);
                total_scan += nr_pages;

                if (page_zonenum(page) > sc->reclaim_idx) {
                        list_move(&page->lru, &pages_skipped);
                        nr_skipped[page_zonenum(page)] += nr_pages;
                        continue;
                }

                /*
                 * Do not count skipped pages because that makes the function
                 * return with no isolated pages if the LRU mostly contains
                 * ineligible pages.  This causes the VM to not reclaim any
                 * pages, triggering a premature OOM.
                 *
                 * Account all tail pages of THP.  This would not cause
                 * premature OOM since __isolate_lru_page() returns -EBUSY
                 * only when the page is being freed somewhere else.
                 */
                scan += nr_pages;
                switch (__isolate_lru_page(page, mode)) {
                case 0:
                        nr_taken += nr_pages;
                        nr_zone_taken[page_zonenum(page)] += nr_pages;
                        list_move(&page->lru, dst);
                        break;

                case -EBUSY:
                        /* else it is being freed elsewhere */
                        list_move(&page->lru, src);
                        continue;

                default:
                        BUG();
                }
        }

        /*
         * Splice any skipped pages to the start of the LRU list. Note that
         * this disrupts the LRU order when reclaiming for lower zones but
         * we cannot splice to the tail. If we did then the SWAP_CLUSTER_MAX
         * scanning would soon rescan the same pages to skip and put the
         * system at risk of premature OOM.
         */
        if (!list_empty(&pages_skipped)) {
                int zid;

                list_splice(&pages_skipped, src);
                for (zid = 0; zid < MAX_NR_ZONES; zid++) {
                        if (!nr_skipped[zid])
                                continue;

                        __count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
                        skipped += nr_skipped[zid];
                }
        }
        *nr_scanned = total_scan;
        trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
                                    total_scan, skipped, nr_taken, mode, lru);
        update_lru_sizes(lruvec, lru, nr_zone_taken);
        return nr_taken;
}

/**
 * isolate_lru_page - tries to isolate a page from its LRU list
 * @page: page to isolate from its LRU list
 *
 * Isolates a @page from an LRU list, clears PageLRU and adjusts the
 * vmstat statistic corresponding to whatever LRU list the page was on.
 *
 * Returns 0 if the page was removed from an LRU list.
 * Returns -EBUSY if the page was not on an LRU list.
 *
 * The returned page will have PageLRU() cleared.  If it was found on
 * the active list, it will have PageActive set.  If it was found on
 * the unevictable list, it will have the PageUnevictable bit set. That flag
 * may need to be cleared by the caller before letting the page go.
 *
 * The vmstat statistic corresponding to the list on which the page was
 * found will be decremented.
 *
 * Restrictions:
 *
 * (1) Must be called with an elevated refcount on the page. This is a
 *     fundamentnal difference from isolate_lru_pages (which is called
 *     without a stable reference).
 * (2) the lru_lock must not be held.
 * (3) interrupts must be enabled.
 */
int isolate_lru_page(struct page *page)
{
        int ret = -EBUSY;

        VM_BUG_ON_PAGE(!page_count(page), page);
        WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");

        if (PageLRU(page)) {
                pg_data_t *pgdat = page_pgdat(page);
                struct lruvec *lruvec;

                spin_lock_irq(&pgdat->lru_lock);
                lruvec = mem_cgroup_page_lruvec(page, pgdat);
                if (PageLRU(page)) {
                        int lru = page_lru(page);
                        get_page(page);
                        ClearPageLRU(page);
                        del_page_from_lru_list(page, lruvec, lru);
                        ret = 0;
                }
                spin_unlock_irq(&pgdat->lru_lock);
        }
        return ret;
}

/*
 * A direct reclaimer may isolate SWAP_CLUSTER_MAX pages from the LRU list and
 * then get resheduled. When there are massive number of tasks doing page
 * allocation, such sleeping direct reclaimers may keep piling up on each CPU,
 * the LRU list will go small and be scanned faster than necessary, leading to
 * unnecessary swapping, thrashing and OOM.
 */
static int too_many_isolated(struct pglist_data *pgdat, int file,
                struct scan_control *sc)
{
        unsigned long inactive, isolated;

        if (current_is_kswapd())
                return 0;

        if (!sane_reclaim(sc))
                return 0;

        if (file) {
                inactive = node_page_state(pgdat, NR_INACTIVE_FILE);
                isolated = node_page_state(pgdat, NR_ISOLATED_FILE);
        } else {
                inactive = node_page_state(pgdat, NR_INACTIVE_ANON);
                isolated = node_page_state(pgdat, NR_ISOLATED_ANON);
        }

        /*
         * GFP_NOIO/GFP_NOFS callers are allowed to isolate more pages, so they
         * won't get blocked by normal direct-reclaimers, forming a circular
         * deadlock.
         */
        if ((sc->gfp_mask & (__GFP_IO | __GFP_FS)) == (__GFP_IO | __GFP_FS))
                inactive >>= 3;

        return isolated > inactive;
}

/*
 * This moves pages from @list to corresponding LRU list.
 *
 * We move them the other way if the page is referenced by one or more
 * processes, from rmap.
 *
 * If the pages are mostly unmapped, the processing is fast and it is
 * appropriate to hold zone_lru_lock across the whole operation.  But if
 * the pages are mapped, the processing is slow (page_referenced()) so we
 * should drop zone_lru_lock around each page.  It's impossible to balance
 * this, so instead we remove the pages from the LRU while processing them.
 * It is safe to rely on PG_active against the non-LRU pages in here because
 * nobody will play with that bit on a non-LRU page.
 *
 * The downside is that we have to touch page->_refcount against each page.
 * But we had to alter page->flags anyway.
 *
 * Returns the number of pages moved to the given lruvec.
 */

static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
                                                     struct list_head *list)
{
        struct pglist_data *pgdat = lruvec_pgdat(lruvec);
        int nr_pages, nr_moved = 0;
        LIST_HEAD(pages_to_free);
        struct page *page;
        enum lru_list lru;

        while (!list_empty(list)) {
                page = lru_to_page(list);
                VM_BUG_ON_PAGE(PageLRU(page), page);
                if (unlikely(!page_evictable(page))) {
                        list_del(&page->lru);
                        spin_unlock_irq(&pgdat->lru_lock);
                        putback_lru_page(page);
                        spin_lock_irq(&pgdat->lru_lock);
                        continue;
                }
                lruvec = mem_cgroup_page_lruvec(page, pgdat);

                SetPageLRU(page);
                lru = page_lru(page);

                nr_pages = hpage_nr_pages(page);
                update_lru_size(lruvec, lru, page_zonenum(page), nr_pages);
                list_move(&page->lru, &lruvec->lists[lru]);

                if (put_page_testzero(page)) {
                        __ClearPageLRU(page);
                        __ClearPageActive(page);
                        del_page_from_lru_list(page, lruvec, lru);

                        if (unlikely(PageCompound(page))) {
                                spin_unlock_irq(&pgdat->lru_lock);
                                mem_cgroup_uncharge(page);
                                (*get_compound_page_dtor(page))(page);
                                spin_lock_irq(&pgdat->lru_lock);
                        } else
                                list_add(&page->lru, &pages_to_free);
                } else {
                        nr_moved += nr_pages;
                }
        }

        /*
         * To save our caller's stack, now use input list for pages to free.
         */
        list_splice(&pages_to_free, list);

        return nr_moved;
}

/*
 * If a kernel thread (such as nfsd for loop-back mounts) services
 * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
 * In that case we should only throttle if the backing device it is
 * writing to is congested.  In other cases it is safe to throttle.
 */
static int current_may_throttle(void)
{
        return !(current->flags & PF_LESS_THROTTLE) ||
                current->backing_dev_info == NULL ||
                bdi_write_congested(current->backing_dev_info);
}

/*
 * shrink_inactive_list() is a helper for shrink_node().  It returns the number
 * of reclaimed pages
 */
static noinline_for_stack unsigned long
shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
                     struct scan_control *sc, enum lru_list lru)
{
        LIST_HEAD(page_list);
        unsigned long nr_scanned;
        unsigned long nr_reclaimed = 0;
        unsigned long nr_taken;
        struct reclaim_stat stat;
        int file = is_file_lru(lru);
        enum vm_event_item item;
        struct pglist_data *pgdat = lruvec_pgdat(lruvec);
        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
        bool stalled = false;

        while (unlikely(too_many_isolated(pgdat, file, sc))) {
                if (stalled)
                        return 0;

                /* wait a bit for the reclaimer. */
                msleep(100);
                stalled = true;

                /* We are about to die and free our memory. Return now. */
                if (fatal_signal_pending(current))
                        return SWAP_CLUSTER_MAX;
        }

        lru_add_drain();

        spin_lock_irq(&pgdat->lru_lock);

        nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &page_list,
                                     &nr_scanned, sc, lru);

        __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
        reclaim_stat->recent_scanned[file] += nr_taken;

        item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;
        if (global_reclaim(sc))
                __count_vm_events(item, nr_scanned);
        __count_memcg_events(lruvec_memcg(lruvec), item, nr_scanned);
        spin_unlock_irq(&pgdat->lru_lock);

        if (nr_taken == 0)
                return 0;

        nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, 0,
                                &stat, false);

        spin_lock_irq(&pgdat->lru_lock);

        item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;
        if (global_reclaim(sc))
                __count_vm_events(item, nr_reclaimed);
        __count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
        reclaim_stat->recent_rotated[0] += stat.nr_activate[0];
        reclaim_stat->recent_rotated[1] += stat.nr_activate[1];

        move_pages_to_lru(lruvec, &page_list);

        __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);

        spin_unlock_irq(&pgdat->lru_lock);

        mem_cgroup_uncharge_list(&page_list);
        free_unref_page_list(&page_list);

        /*
         * If dirty pages are scanned that are not queued for IO, it
         * implies that flushers are not doing their job. This can
         * happen when memory pressure pushes dirty pages to the end of
         * the LRU before the dirty limits are breached and the dirty
         * data has expired. It can also happen when the proportion of
         * dirty pages grows not through writes but through memory
         * pressure reclaiming all the clean cache. And in some cases,
         * the flushers simply cannot keep up with the allocation
         * rate. Nudge the flusher threads in case they are asleep.
         */
        if (stat.nr_unqueued_dirty == nr_taken)
                wakeup_flusher_threads(WB_REASON_VMSCAN);

        sc->nr.dirty += stat.nr_dirty;
        sc->nr.congested += stat.nr_congested;
        sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
        sc->nr.writeback += stat.nr_writeback;
        sc->nr.immediate += stat.nr_immediate;
        sc->nr.taken += nr_taken;
        if (file)
                sc->nr.file_taken += nr_taken;

        trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
                        nr_scanned, nr_reclaimed, &stat, sc->priority, file);
        return nr_reclaimed;
}

static void shrink_active_list(unsigned long nr_to_scan,
                               struct lruvec *lruvec,
                               struct scan_control *sc,
                               enum lru_list lru)
{
        unsigned long nr_taken;
        unsigned long nr_scanned;
        unsigned long vm_flags;
        LIST_HEAD(l_hold);      /* The pages which were snipped off */
        LIST_HEAD(l_active);
        LIST_HEAD(l_inactive);
        struct page *page;
        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
        unsigned nr_deactivate, nr_activate;
        unsigned nr_rotated = 0;
        int file = is_file_lru(lru);
        struct pglist_data *pgdat = lruvec_pgdat(lruvec);

        lru_add_drain();

        spin_lock_irq(&pgdat->lru_lock);

        nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
                                     &nr_scanned, sc, lru);

        __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
        reclaim_stat->recent_scanned[file] += nr_taken;

        __count_vm_events(PGREFILL, nr_scanned);
        __count_memcg_events(lruvec_memcg(lruvec), PGREFILL, nr_scanned);

        spin_unlock_irq(&pgdat->lru_lock);

        while (!list_empty(&l_hold)) {
                cond_resched();
                page = lru_to_page(&l_hold);
                list_del(&page->lru);

                if (unlikely(!page_evictable(page))) {
                        putback_lru_page(page);
                        continue;
                }

                if (unlikely(buffer_heads_over_limit)) {
                        if (page_has_private(page) && trylock_page(page)) {
                                if (page_has_private(page))
                                        try_to_release_page(page, 0);
                                unlock_page(page);
                        }
                }

                if (page_referenced(page, 0, sc->target_mem_cgroup,
                                    &vm_flags)) {
                        nr_rotated += hpage_nr_pages(page);
                        /*
                         * Identify referenced, file-backed active pages and
                         * give them one more trip around the active list. So
                         * that executable code get better chances to stay in
                         * memory under moderate memory pressure.  Anon pages
                         * are not likely to be evicted by use-once streaming
                         * IO, plus JVM can create lots of anon VM_EXEC pages,
                         * so we ignore them here.
                         */
                        if ((vm_flags & VM_EXEC) && page_is_file_cache(page)) {
                                list_add(&page->lru, &l_active);
                                continue;
                        }
                }

                ClearPageActive(page);  /* we are de-activating */
                SetPageWorkingset(page);
                list_add(&page->lru, &l_inactive);
        }

        /*
         * Move pages back to the lru list.
         */
        spin_lock_irq(&pgdat->lru_lock);
        /*
         * Count referenced pages from currently used mappings as rotated,
         * even though only some of them are actually re-activated.  This
         * helps balance scan pressure between file and anonymous pages in
         * get_scan_count.
         */
        reclaim_stat->recent_rotated[file] += nr_rotated;

        nr_activate = move_pages_to_lru(lruvec, &l_active);
        nr_deactivate = move_pages_to_lru(lruvec, &l_inactive);
        /* Keep all free pages in l_active list */
        list_splice(&l_inactive, &l_active);

        __count_vm_events(PGDEACTIVATE, nr_deactivate);
        __count_memcg_events(lruvec_memcg(lruvec), PGDEACTIVATE, nr_deactivate);

        __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
        spin_unlock_irq(&pgdat->lru_lock);

        mem_cgroup_uncharge_list(&l_active);
        free_unref_page_list(&l_active);
        trace_mm_vmscan_lru_shrink_active(pgdat->node_id, nr_taken, nr_activate,
                        nr_deactivate, nr_rotated, sc->priority, file);
}

/*
 * The inactive anon list should be small enough that the VM never has
 * to do too much work.
 *
 * The inactive file list should be small enough to leave most memory
 * to the established workingset on the scan-resistant active list,
 * but large enough to avoid thrashing the aggregate readahead window.
 *
 * Both inactive lists should also be large enough that each inactive
 * page has a chance to be referenced again before it is reclaimed.
 *
 * If that fails and refaulting is observed, the inactive list grows.
 *
 * The inactive_ratio is the target ratio of ACTIVE to INACTIVE pages
 * on this LRU, maintained by the pageout code. An inactive_ratio
 * of 3 means 3:1 or 25% of the pages are kept on the inactive list.
 *
 * total     target    max
 * memory    ratio     inactive
 * -------------------------------------
 *   10MB       1         5MB
 *  100MB       1        50MB
 *    1GB       3       250MB
 *   10GB      10       0.9GB
 *  100GB      31         3GB
 *    1TB     101        10GB
 *   10TB     320        32GB
 */
static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
                                 struct scan_control *sc, bool trace)
{
        enum lru_list active_lru = file * LRU_FILE + LRU_ACTIVE;
        struct pglist_data *pgdat = lruvec_pgdat(lruvec);
        enum lru_list inactive_lru = file * LRU_FILE;
        unsigned long inactive, active;
        unsigned long inactive_ratio;
        unsigned long refaults;
        unsigned long gb;

        /*
         * If we don't have swap space, anonymous page deactivation
         * is pointless.
         */
        if (!file && !total_swap_pages)
                return false;

        inactive = lruvec_lru_size(lruvec, inactive_lru, sc->reclaim_idx);
        active = lruvec_lru_size(lruvec, active_lru, sc->reclaim_idx);

        /*
         * When refaults are being observed, it means a new workingset
         * is being established. Disable active list protection to get
         * rid of the stale workingset quickly.
         */
        refaults = lruvec_page_state_local(lruvec, WORKINGSET_ACTIVATE);
        if (file && lruvec->refaults != refaults) {
                inactive_ratio = 0;
        } else {
                gb = (inactive + active) >> (30 - PAGE_SHIFT);
                if (gb)
                        inactive_ratio = int_sqrt(10 * gb);
                else
                        inactive_ratio = 1;
        }

        if (trace)
                trace_mm_vmscan_inactive_list_is_low(pgdat->node_id, sc->reclaim_idx,
                        lruvec_lru_size(lruvec, inactive_lru, MAX_NR_ZONES), inactive,
                        lruvec_lru_size(lruvec, active_lru, MAX_NR_ZONES), active,
                        inactive_ratio, file);

        return inactive * inactive_ratio < active;
}

static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
                                 struct lruvec *lruvec, struct scan_control *sc)
{
        if (is_active_lru(lru)) {
                if (inactive_list_is_low(lruvec, is_file_lru(lru), sc, true))
                        shrink_active_list(nr_to_scan, lruvec, sc, lru);
                return 0;
        }

        return shrink_inactive_list(nr_to_scan, lruvec, sc, lru);
}

enum scan_balance {
        SCAN_EQUAL,
        SCAN_FRACT,
        SCAN_ANON,
        SCAN_FILE,
};

/*
 * Determine how aggressively the anon and file LRU lists should be
 * scanned.  The relative value of each set of LRU lists is determined
 * by looking at the fraction of the pages scanned we did rotate back
 * onto the active list instead of evict.
 *
 * nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
 * nr[2] = file inactive pages to scan; nr[3] = file active pages to scan
 */
static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
                           struct scan_control *sc, unsigned long *nr,
                           unsigned long *lru_pages)
{
        int swappiness = mem_cgroup_swappiness(memcg);
        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
        u64 fraction[2];
        u64 denominator = 0;    /* gcc */
        struct pglist_data *pgdat = lruvec_pgdat(lruvec);
        unsigned long anon_prio, file_prio;
        enum scan_balance scan_balance;
        unsigned long anon, file;
        unsigned long ap, fp;
        enum lru_list lru;

        /* If we have no swap space, do not bother scanning anon pages. */
        if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
                scan_balance = SCAN_FILE;
                goto out;
        }

        /*
         * Global reclaim will swap to prevent OOM even with no
         * swappiness, but memcg users want to use this knob to
         * disable swapping for individual groups completely when
         * using the memory controller's swap limit feature would be
         * too expensive.
         */
        if (!global_reclaim(sc) && !swappiness) {
                scan_balance = SCAN_FILE;
                goto out;
        }

        /*
         * Do not apply any pressure balancing cleverness when the
         * system is close to OOM, scan both anon and file equally
         * (unless the swappiness setting disagrees with swapping).
         */
        if (!sc->priority && swappiness) {
                scan_balance = SCAN_EQUAL;
                goto out;
        }

        /*
         * Prevent the reclaimer from falling into the cache trap: as
         * cache pages start out inactive, every cache fault will tip
         * the scan balance towards the file LRU.  And as the file LRU
         * shrinks, so does the window for rotation from references.
         * This means we have a runaway feedback loop where a tiny
         * thrashing file LRU becomes infinitely more attractive than
         * anon pages.  Try to detect this based on file LRU size.
         */
        if (global_reclaim(sc)) {
                unsigned long pgdatfile;
                unsigned long pgdatfree;
                int z;
                unsigned long total_high_wmark = 0;

                pgdatfree = sum_zone_node_page_state(pgdat->node_id, NR_FREE_PAGES);
                pgdatfile = node_page_state(pgdat, NR_ACTIVE_FILE) +
                           node_page_state(pgdat, NR_INACTIVE_FILE);

                for (z = 0; z < MAX_NR_ZONES; z++) {
                        struct zone *zone = &pgdat->node_zones[z];
                        if (!managed_zone(zone))
                                continue;

                        total_high_wmark += high_wmark_pages(zone);
                }

                if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
                        /*
                         * Force SCAN_ANON if there are enough inactive
                         * anonymous pages on the LRU in eligible zones.
                         * Otherwise, the small LRU gets thrashed.
                         */
                        if (!inactive_list_is_low(lruvec, false, sc, false) &&
                            lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, sc->reclaim_idx)
                                        >> sc->priority) {
                                scan_balance = SCAN_ANON;
                                goto out;
                        }
                }
        }

        /*
         * If there is enough inactive page cache, i.e. if the size of the
         * inactive list is greater than that of the active list *and* the
         * inactive list actually has some pages to scan on this priority, we
         * do not reclaim anything from the anonymous working set right now.
         * Without the second condition we could end up never scanning an
         * lruvec even if it has plenty of old anonymous pages unless the
         * system is under heavy pressure.
         */
        if (!inactive_list_is_low(lruvec, true, sc, false) &&
            lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, sc->reclaim_idx) >> sc->priority) {
                scan_balance = SCAN_FILE;
                goto out;
        }

        scan_balance = SCAN_FRACT;

        /*
         * With swappiness at 100, anonymous and file have the same priority.
         * This scanning priority is essentially the inverse of IO cost.
         */
        anon_prio = swappiness;
        file_prio = 200 - anon_prio;

        /*
         * OK, so we have swap space and a fair amount of page cache
         * pages.  We use the recently rotated / recently scanned
         * ratios to determine how valuable each cache is.
         *
         * Because workloads change over time (and to avoid overflow)
         * we keep these statistics as a floating average, which ends
         * up weighing recent references more than old ones.
         *
         * anon in [0], file in [1]
         */

        anon  = lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, MAX_NR_ZONES) +
                lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, MAX_NR_ZONES);
        file  = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES) +
                lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES);

        spin_lock_irq(&pgdat->lru_lock);
        if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {
                reclaim_stat->recent_scanned[0] /= 2;
                reclaim_stat->recent_rotated[0] /= 2;
        }

        if (unlikely(reclaim_stat->recent_scanned[1] > file / 4)) {
                reclaim_stat->recent_scanned[1] /= 2;
                reclaim_stat->recent_rotated[1] /= 2;
        }

        /*
         * The amount of pressure on anon vs file pages is inversely
         * proportional to the fraction of recently scanned pages on
         * each list that were recently referenced and in active use.
         */
        ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
        ap /= reclaim_stat->recent_rotated[0] + 1;

        fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
        fp /= reclaim_stat->recent_rotated[1] + 1;
        spin_unlock_irq(&pgdat->lru_lock);

        fraction[0] = ap;
        fraction[1] = fp;
        denominator = ap + fp + 1;
out:
        *lru_pages = 0;
        for_each_evictable_lru(lru) {
                int file = is_file_lru(lru);
                unsigned long size;
                unsigned long scan;

                size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
                scan = size >> sc->priority;
                /*
                 * If the cgroup's already been deleted, make sure to
                 * scrape out the remaining cache.
                 */
                if (!scan && !mem_cgroup_online(memcg))
                        scan = min(size, SWAP_CLUSTER_MAX);

                switch (scan_balance) {
                case SCAN_EQUAL:
                        /* Scan lists relative to size */
                        break;
                case SCAN_FRACT:
                        /*
                         * Scan types proportional to swappiness and
                         * their relative recent reclaim efficiency.
                         * Make sure we don't miss the last page
                         * because of a round-off error.
                         */
                        scan = DIV64_U64_ROUND_UP(scan * fraction[file],
                                                  denominator);
                        break;
                case SCAN_FILE:
                case SCAN_ANON:
                        /* Scan one type exclusively */
                        if ((scan_balance == SCAN_FILE) != file) {
                                size = 0;
                                scan = 0;
                        }
                        break;
                default:
                        /* Look ma, no brain */
                        BUG();
                }

                *lru_pages += size;
                nr[lru] = scan;
        }
}

/*
 * This is a basic per-node page freer.  Used by both kswapd and direct reclaim.
 */
static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memcg,
                              struct scan_control *sc, unsigned long *lru_pages)
{
        struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);
        unsigned long nr[NR_LRU_LISTS];
        unsigned long targets[NR_LRU_LISTS];
        unsigned long nr_to_scan;
        enum lru_list lru;
        unsigned long nr_reclaimed = 0;
        unsigned long nr_to_reclaim = sc->nr_to_reclaim;
        struct blk_plug plug;
        bool scan_adjusted;

        get_scan_count(lruvec, memcg, sc, nr, lru_pages);

        /* Record the original scan target for proportional adjustments later */
        memcpy(targets, nr, sizeof(nr));

        /*
         * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
         * event that can occur when there is little memory pressure e.g.
         * multiple streaming readers/writers. Hence, we do not abort scanning
         * when the requested number of pages are reclaimed when scanning at
         * DEF_PRIORITY on the assumption that the fact we are direct
         * reclaiming implies that kswapd is not keeping up and it is best to
         * do a batch of work at once. For memcg reclaim one check is made to
         * abort proportional reclaim if either the file or anon lru has already
         * dropped to zero at the first pass.
         */
        scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() &&
                         sc->priority == DEF_PRIORITY);

        blk_start_plug(&plug);
        while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
                                        nr[LRU_INACTIVE_FILE]) {
                unsigned long nr_anon, nr_file, percentage;
                unsigned long nr_scanned;

                for_each_evictable_lru(lru) {
                        if (nr[lru]) {
                                nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
                                nr[lru] -= nr_to_scan;

                                nr_reclaimed += shrink_list(lru, nr_to_scan,
                                                            lruvec, sc);
                        }
                }

                cond_resched();

                if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
                        continue;

                /*
                 * For kswapd and memcg, reclaim at least the number of pages
                 * requested. Ensure that the anon and file LRUs are scanned
                 * proportionally what was requested by get_scan_count(). We
                 * stop reclaiming one LRU and reduce the amount scanning
                 * proportional to the original scan target.
                 */
                nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
                nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];

                /*
                 * It's just vindictive to attack the larger once the smaller
                 * has gone to zero.  And given the way we stop scanning the
                 * smaller below, this makes sure that we only make one nudge
                 * towards proportionality once we've got nr_to_reclaim.
                 */
                if (!nr_file || !nr_anon)
                        break;

                if (nr_file > nr_anon) {
                        unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
                                                targets[LRU_ACTIVE_ANON] + 1;
                        lru = LRU_BASE;
                        percentage = nr_anon * 100 / scan_target;
                } else {
                        unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
                                                targets[LRU_ACTIVE_FILE] + 1;
                        lru = LRU_FILE;
                        percentage = nr_file * 100 / scan_target;
                }

                /* Stop scanning the smaller of the LRU */
                nr[lru] = 0;
                nr[lru + LRU_ACTIVE] = 0;

                /*
                 * Recalculate the other LRU scan count based on its original
                 * scan target and the percentage scanning already complete
                 */
                lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
                nr_scanned = targets[lru] - nr[lru];
                nr[lru] = targets[lru] * (100 - percentage) / 100;
                nr[lru] -= min(nr[lru], nr_scanned);

                lru += LRU_ACTIVE;
                nr_scanned = targets[lru] - nr[lru];
                nr[lru] = targets[lru] * (100 - percentage) / 100;
                nr[lru] -= min(nr[lru], nr_scanned);

                scan_adjusted = true;
        }
        blk_finish_plug(&plug);
        sc->nr_reclaimed += nr_reclaimed;

        /*
         * Even if we did not try to evict anon pages at all, we want to
         * rebalance the anon lru active/inactive ratio.
         */
        if (inactive_list_is_low(lruvec, false, sc, true))
                shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                                   sc, LRU_ACTIVE_ANON);
}

/* Use reclaim/compaction for costly allocs or under memory pressure */
static bool in_reclaim_compaction(struct scan_control *sc)
{
        if (IS_ENABLED(CONFIG_COMPACTION) && sc->order &&
                        (sc->order > PAGE_ALLOC_COSTLY_ORDER ||
                         sc->priority < DEF_PRIORITY - 2))
                return true;

        return false;
}

/*
 * Reclaim/compaction is used for high-order allocation requests. It reclaims
 * order-0 pages before compacting the zone. should_continue_reclaim() returns
 * true if more pages should be reclaimed such that when the page allocator
 * calls try_to_compact_zone() that it will have enough free pages to succeed.
 * It will give up earlier than that if there is difficulty reclaiming pages.
 */
static inline bool should_continue_reclaim(struct pglist_data *pgdat,
                                        unsigned long nr_reclaimed,
                                        unsigned long nr_scanned,
                                        struct scan_control *sc)
{
        unsigned long pages_for_compaction;
        unsigned long inactive_lru_pages;
        int z;

        /* If not in reclaim/compaction mode, stop */
        if (!in_reclaim_compaction(sc))
                return false;

        /* Consider stopping depending on scan and reclaim activity */
        if (sc->gfp_mask & __GFP_RETRY_MAYFAIL) {
                /*
                 * For __GFP_RETRY_MAYFAIL allocations, stop reclaiming if the
                 * full LRU list has been scanned and we are still failing
                 * to reclaim pages. This full LRU scan is potentially
                 * expensive but a __GFP_RETRY_MAYFAIL caller really wants to succeed
                 */
                if (!nr_reclaimed && !nr_scanned)
                        return false;
        } else {
                /*
                 * For non-__GFP_RETRY_MAYFAIL allocations which can presumably
                 * fail without consequence, stop if we failed to reclaim
                 * any pages from the last SWAP_CLUSTER_MAX number of
                 * pages that were scanned. This will return to the
                 * caller faster at the risk reclaim/compaction and
                 * the resulting allocation attempt fails
                 */
                if (!nr_reclaimed)
                        return false;
        }

        /*
         * If we have not reclaimed enough pages for compaction and the
         * inactive lists are large enough, continue reclaiming
         */
        pages_for_compaction = compact_gap(sc->order);
        inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
        if (get_nr_swap_pages() > 0)
                inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);
        if (sc->nr_reclaimed < pages_for_compaction &&
                        inactive_lru_pages > pages_for_compaction)
                return true;

        /* If compaction would go ahead or the allocation would succeed, stop */
        for (z = 0; z <= sc->reclaim_idx; z++) {
                struct zone *zone = &pgdat->node_zones[z];
                if (!managed_zone(zone))
                        continue;

                switch (compaction_suitable(zone, sc->order, 0, sc->reclaim_idx)) {
                case COMPACT_SUCCESS:
                case COMPACT_CONTINUE:
                        return false;
                default:
                        /* check next zone */
                        ;
                }
        }
        return true;
}

static bool pgdat_memcg_congested(pg_data_t *pgdat, struct mem_cgroup *memcg)
{
        return test_bit(PGDAT_CONGESTED, &pgdat->flags) ||
                (memcg && memcg_congested(pgdat, memcg));
}

static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
{
        struct reclaim_state *reclaim_state = current->reclaim_state;
        unsigned long nr_reclaimed, nr_scanned;
        bool reclaimable = false;

        do {
                struct mem_cgroup *root = sc->target_mem_cgroup;
                struct mem_cgroup_reclaim_cookie reclaim = {
                        .pgdat = pgdat,
                        .priority = sc->priority,
                };
                unsigned long node_lru_pages = 0;
                struct mem_cgroup *memcg;

                memset(&sc->nr, 0, sizeof(sc->nr));

                nr_reclaimed = sc->nr_reclaimed;
                nr_scanned = sc->nr_scanned;

                memcg = mem_cgroup_iter(root, NULL, &reclaim);
                do {
                        unsigned long lru_pages;
                        unsigned long reclaimed;
                        unsigned long scanned;

                        switch (mem_cgroup_protected(root, memcg)) {
                        case MEMCG_PROT_MIN:
                                /*
                                 * Hard protection.
                                 * If there is no reclaimable memory, OOM.
                                 */
                                continue;
                        case MEMCG_PROT_LOW:
                                /*
                                 * Soft protection.
                                 * Respect the protection only as long as
                                 * there is an unprotected supply
                                 * of reclaimable memory from other cgroups.
                                 */
                                if (!sc->memcg_low_reclaim) {
                                        sc->memcg_low_skipped = 1;
                                        continue;
                                }
                                memcg_memory_event(memcg, MEMCG_LOW);
                                break;
                        case MEMCG_PROT_NONE:
                                break;
                        }

                        reclaimed = sc->nr_reclaimed;
                        scanned = sc->nr_scanned;
                        shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
                        node_lru_pages += lru_pages;

                        if (sc->may_shrinkslab) {
                                shrink_slab(sc->gfp_mask, pgdat->node_id,
                                    memcg, sc->priority);
                        }

                        /* Record the group's reclaim efficiency */
                        vmpressure(sc->gfp_mask, memcg, false,
                                   sc->nr_scanned - scanned,
                                   sc->nr_reclaimed - reclaimed);

                        /*
                         * Kswapd have to scan all memory cgroups to fulfill
                         * the overall scan target for the node.
                         *
                         * Limit reclaim, on the other hand, only cares about
                         * nr_to_reclaim pages to be reclaimed and it will
                         * retry with decreasing priority if one round over the
                         * whole hierarchy is not sufficient.
                         */
                        if (!current_is_kswapd() &&
                                        sc->nr_reclaimed >= sc->nr_to_reclaim) {
                                mem_cgroup_iter_break(root, memcg);
                                break;
                        }
                } while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));

                if (reclaim_state) {
                        sc->nr_reclaimed += reclaim_state->reclaimed_slab;
                        reclaim_state->reclaimed_slab = 0;
                }

                /* Record the subtree's reclaim efficiency */
                vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
                           sc->nr_scanned - nr_scanned,
                           sc->nr_reclaimed - nr_reclaimed);

                if (sc->nr_reclaimed - nr_reclaimed)
                        reclaimable = true;

                if (current_is_kswapd()) {
                        /*
                         * If reclaim is isolating dirty pages under writeback,
                         * it implies that the long-lived page allocation rate
                         * is exceeding the page laundering rate. Either the
                         * global limits are not being effective at throttling
                         * processes due to the page distribution throughout
                         * zones or there is heavy usage of a slow backing
                         * device. The only option is to throttle from reclaim
                         * context which is not ideal as there is no guarantee
                         * the dirtying process is throttled in the same way
                         * balance_dirty_pages() manages.
                         *
                         * Once a node is flagged PGDAT_WRITEBACK, kswapd will
                         * count the number of pages under pages flagged for
                         * immediate reclaim and stall if any are encountered
                         * in the nr_immediate check below.
                         */
                        if (sc->nr.writeback && sc->nr.writeback == sc->nr.taken)
                                set_bit(PGDAT_WRITEBACK, &pgdat->flags);

                        /*
                         * Tag a node as congested if all the dirty pages
                         * scanned were backed by a congested BDI and
                         * wait_iff_congested will stall.
                         */
                        if (sc->nr.dirty && sc->nr.dirty == sc->nr.congested)
                                set_bit(PGDAT_CONGESTED, &pgdat->flags);

                        /* Allow kswapd to start writing pages during reclaim.*/
                        if (sc->nr.unqueued_dirty == sc->nr.file_taken)
                                set_bit(PGDAT_DIRTY, &pgdat->flags);

                        /*
                         * If kswapd scans pages marked marked for immediate
                         * reclaim and under writeback (nr_immediate), it
                         * implies that pages are cycling through the LRU
                         * faster than they are written so also forcibly stall.
                         */
                        if (sc->nr.immediate)
                                congestion_wait(BLK_RW_ASYNC, HZ/10);
                }

                /*
                 * Legacy memcg will stall in page writeback so avoid forcibly
                 * stalling in wait_iff_congested().
                 */
                if (!global_reclaim(sc) && sane_reclaim(sc) &&
                    sc->nr.dirty && sc->nr.dirty == sc->nr.congested)
                        set_memcg_congestion(pgdat, root, true);

                /*
                 * Stall direct reclaim for IO completions if underlying BDIs
                 * and node is congested. Allow kswapd to continue until it
                 * starts encountering unqueued dirty pages or cycling through
                 * the LRU too quickly.
                 */
                if (!sc->hibernation_mode && !current_is_kswapd() &&
                   current_may_throttle() && pgdat_memcg_congested(pgdat, root))
                        wait_iff_congested(BLK_RW_ASYNC, HZ/10);

        } while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
                                         sc->nr_scanned - nr_scanned, sc));

        /*
         * Kswapd gives up on balancing particular nodes after too
         * many failures to reclaim anything from them and goes to
         * sleep. On reclaim progress, reset the failure counter. A
         * successful direct reclaim run will revive a dormant kswapd.
         */
        if (reclaimable)
                pgdat->kswapd_failures = 0;

        return reclaimable;
}

/*
 * Returns true if compaction should go ahead for a costly-order request, or
 * the allocation would already succeed without compaction. Return false if we
 * should reclaim first.
 */
static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
{
        unsigned long watermark;
        enum compact_result suitable;

        suitable = compaction_suitable(zone, sc->order, 0, sc->reclaim_idx);
        if (suitable == COMPACT_SUCCESS)
                /* Allocation should succeed already. Don't reclaim. */
                return true;
        if (suitable == COMPACT_SKIPPED)
                /* Compaction cannot yet proceed. Do reclaim. */
                return false;

        /*
         * Compaction is already possible, but it takes time to run and there
         * are potentially other callers using the pages just freed. So proceed
         * with reclaim to make a buffer of free pages available to give
         * compaction a reasonable chance of completing and allocating the page.
         * Note that we won't actually reclaim the whole buffer in one attempt
         * as the target watermark in should_continue_reclaim() is lower. But if
         * we are already above the high+gap watermark, don't reclaim at all.
         */
        watermark = high_wmark_pages(zone) + compact_gap(sc->order);

        return zone_watermark_ok_safe(zone, 0, watermark, sc->reclaim_idx);
}

/*
 * This is the direct reclaim path, for page-allocating processes.  We only
 * try to reclaim pages from zones which will satisfy the caller's allocation
 * request.
 *
 * If a zone is deemed to be full of pinned pages then just give it a light
 * scan then give up on it.
 */
static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
{
        struct zoneref *z;
        struct zone *zone;
        unsigned long nr_soft_reclaimed;
        unsigned long nr_soft_scanned;
        gfp_t orig_mask;
        pg_data_t *last_pgdat = NULL;

        /*
         * If the number of buffer_heads in the machine exceeds the maximum
         * allowed level, force direct reclaim to scan the highmem zone as
         * highmem pages could be pinning lowmem pages storing buffer_heads
         */
        orig_mask = sc->gfp_mask;
        if (buffer_heads_over_limit) {
                sc->gfp_mask |= __GFP_HIGHMEM;
                sc->reclaim_idx = gfp_zone(sc->gfp_mask);
        }

        for_each_zone_zonelist_nodemask(zone, z, zonelist,
                                        sc->reclaim_idx, sc->nodemask) {
                /*
                 * Take care memory controller reclaiming has small influence
                 * to global LRU.
                 */
                if (global_reclaim(sc)) {
                        if (!cpuset_zone_allowed(zone,
                                                 GFP_KERNEL | __GFP_HARDWALL))
                                continue;

                        /*
                         * If we already have plenty of memory free for
                         * compaction in this zone, don't free any more.
                         * Even though compaction is invoked for any
                         * non-zero order, only frequent costly order
                         * reclamation is disruptive enough to become a
                         * noticeable problem, like transparent huge
                         * page allocations.
                         */
                        if (IS_ENABLED(CONFIG_COMPACTION) &&
                            sc->order > PAGE_ALLOC_COSTLY_ORDER &&
                            compaction_ready(zone, sc)) {
                                sc->compaction_ready = true;
                                continue;
                        }

                        /*
                         * Shrink each node in the zonelist once. If the
                         * zonelist is ordered by zone (not the default) then a
                         * node may be shrunk multiple times but in that case
                         * the user prefers lower zones being preserved.
                         */
                        if (zone->zone_pgdat == last_pgdat)
                                continue;

                        /*
                         * This steals pages from memory cgroups over softlimit
                         * and returns the number of reclaimed pages and
                         * scanned pages. This works for global memory pressure
                         * and balancing, not for a memcg's limit.
                         */
                        nr_soft_scanned = 0;
                        nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone->zone_pgdat,
                                                sc->order, sc->gfp_mask,
                                                &nr_soft_scanned);
                        sc->nr_reclaimed += nr_soft_reclaimed;
                        sc->nr_scanned += nr_soft_scanned;
                        /* need some check for avoid more shrink_zone() */
                }

                /* See comment about same check for global reclaim above */
                if (zone->zone_pgdat == last_pgdat)
                        continue;
                last_pgdat = zone->zone_pgdat;
                shrink_node(zone->zone_pgdat, sc);
        }

        /*
         * Restore to original mask to avoid the impact on the caller if we
         * promoted it to __GFP_HIGHMEM.
         */
        sc->gfp_mask = orig_mask;
}

static void snapshot_refaults(struct mem_cgroup *root_memcg, pg_data_t *pgdat)
{
        struct mem_cgroup *memcg;

        memcg = mem_cgroup_iter(root_memcg, NULL, NULL);
        do {
                unsigned long refaults;
                struct lruvec *lruvec;

                lruvec = mem_cgroup_lruvec(pgdat, memcg);
                refaults = lruvec_page_state_local(lruvec, WORKINGSET_ACTIVATE);
                lruvec->refaults = refaults;
        } while ((memcg = mem_cgroup_iter(root_memcg, memcg, NULL)));
}

/*
 * This is the main entry point to direct page reclaim.
 *
 * If a full scan of the inactive list fails to free enough memory then we
 * are "out of memory" and something needs to be killed.
 *
 * If the caller is !__GFP_FS then the probability of a failure is reasonably
 * high - the zone may be full of dirty or under-writeback pages, which this
 * caller can't do much about.  We kick the writeback threads and take explicit
 * naps in the hope that some of these pages can be written.  But if the
 * allocating task holds filesystem locks which prevent writeout this might not
 * work, and the allocation attempt will fail.
 *
 * returns:     0, if no pages reclaimed
 *              else, the number of pages reclaimed
 */
static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
                                          struct scan_control *sc)
{
        int initial_priority = sc->priority;
        pg_data_t *last_pgdat;
        struct zoneref *z;
        struct zone *zone;
retry:
        delayacct_freepages_start();

        if (global_reclaim(sc))
                __count_zid_vm_events(ALLOCSTALL, sc->reclaim_idx, 1);

        do {
                vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
                                sc->priority);
                sc->nr_scanned = 0;
                shrink_zones(zonelist, sc);

                if (sc->nr_reclaimed >= sc->nr_to_reclaim)
                        break;

                if (sc->compaction_ready)
                        break;

                /*
                 * If we're getting trouble reclaiming, start doing
                 * writepage even in laptop mode.
                 */
                if (sc->priority < DEF_PRIORITY - 2)
                        sc->may_writepage = 1;
        } while (--sc->priority >= 0);

        last_pgdat = NULL;
        for_each_zone_zonelist_nodemask(zone, z, zonelist, sc->reclaim_idx,
                                        sc->nodemask) {
                if (zone->zone_pgdat == last_pgdat)
                        continue;
                last_pgdat = zone->zone_pgdat;
                snapshot_refaults(sc->target_mem_cgroup, zone->zone_pgdat);
                set_memcg_congestion(last_pgdat, sc->target_mem_cgroup, false);
        }

        delayacct_freepages_end();

        if (sc->nr_reclaimed)
                return sc->nr_reclaimed;

        /* Aborted reclaim to try compaction? don't OOM, then */
        if (sc->compaction_ready)
                return 1;

        /* Untapped cgroup reserves?  Don't OOM, retry. */
        if (sc->memcg_low_skipped) {
                sc->priority = initial_priority;
                sc->memcg_low_reclaim = 1;
                sc->memcg_low_skipped = 0;
                goto retry;
        }

        return 0;
}

static bool allow_direct_reclaim(pg_data_t *pgdat)
{
        struct zone *zone;
        unsigned long pfmemalloc_reserve = 0;
        unsigned long free_pages = 0;
        int i;
        bool wmark_ok;

        if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
                return true;

        for (i = 0; i <= ZONE_NORMAL; i++) {
                zone = &pgdat->node_zones[i];
                if (!managed_zone(zone))
                        continue;

                if (!zone_reclaimable_pages(zone))
                        continue;

                pfmemalloc_reserve += min_wmark_pages(zone);
                free_pages += zone_page_state(zone, NR_FREE_PAGES);
        }

        /* If there are no reserves (unexpected config) then do not throttle */
        if (!pfmemalloc_reserve)
                return true;

        wmark_ok = free_pages > pfmemalloc_reserve / 2;

        /* kswapd must be awake if processes are being throttled */
        if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
                pgdat->kswapd_classzone_idx = min(pgdat->kswapd_classzone_idx,
                                                (enum zone_type)ZONE_NORMAL);
                wake_up_interruptible(&pgdat->kswapd_wait);
        }

        return wmark_ok;
}

/*
 * Throttle direct reclaimers if backing storage is backed by the network
 * and the PFMEMALLOC reserve for the preferred node is getting dangerously
 * depleted. kswapd will continue to make progress and wake the processes
 * when the low watermark is reached.
 *
 * Returns true if a fatal signal was delivered during throttling. If this
 * happens, the page allocator should not consider triggering the OOM killer.
 */
static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
                                        nodemask_t *nodemask)
{
        struct zoneref *z;
        struct zone *zone;
        pg_data_t *pgdat = NULL;

        /*
         * Kernel threads should not be throttled as they may be indirectly
         * responsible for cleaning pages necessary for reclaim to make forward
         * progress. kjournald for example may enter direct reclaim while
         * committing a transaction where throttling it could forcing other
         * processes to block on log_wait_commit().
         */
        if (current->flags & PF_KTHREAD)
                goto out;

        /*
         * If a fatal signal is pending, this process should not throttle.
         * It should return quickly so it can exit and free its memory
         */
        if (fatal_signal_pending(current))
                goto out;

        /*
         * Check if the pfmemalloc reserves are ok by finding the first node
         * with a usable ZONE_NORMAL or lower zone. The expectation is that
         * GFP_KERNEL will be required for allocating network buffers when
         * swapping over the network so ZONE_HIGHMEM is unusable.
         *
         * Throttling is based on the first usable node and throttled processes
         * wait on a queue until kswapd makes progress and wakes them. There
         * is an affinity then between processes waking up and where reclaim
         * progress has been made assuming the process wakes on the same node.
         * More importantly, processes running on remote nodes will not compete
         * for remote pfmemalloc reserves and processes on different nodes
         * should make reasonable progress.
         */
        for_each_zone_zonelist_nodemask(zone, z, zonelist,
                                        gfp_zone(gfp_mask), nodemask) {
                if (zone_idx(zone) > ZONE_NORMAL)
                        continue;

                /* Throttle based on the first usable node */
                pgdat = zone->zone_pgdat;
                if (allow_direct_reclaim(pgdat))
                        goto out;
                break;
        }

        /* If no zone was usable by the allocation flags then do not throttle */
        if (!pgdat)
                goto out;

        /* Account for the throttling */
        count_vm_event(PGSCAN_DIRECT_THROTTLE);

        /*
         * If the caller cannot enter the filesystem, it's possible that it
         * is due to the caller holding an FS lock or performing a journal
         * transaction in the case of a filesystem like ext[3|4]. In this case,
         * it is not safe to block on pfmemalloc_wait as kswapd could be
         * blocked waiting on the same lock. Instead, throttle for up to a
         * second before continuing.
         */
        if (!(gfp_mask & __GFP_FS)) {
                wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
                        allow_direct_reclaim(pgdat), HZ);

                goto check_pending;
        }

        /* Throttle until kswapd wakes the process */
        wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
                allow_direct_reclaim(pgdat));

check_pending:
        if (fatal_signal_pending(current))
                return true;

out:
        return false;
}

unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
                                gfp_t gfp_mask, nodemask_t *nodemask)
{
        unsigned long nr_reclaimed;
        struct scan_control sc = {
                .nr_to_reclaim = SWAP_CLUSTER_MAX,
                .gfp_mask = current_gfp_context(gfp_mask),
                .reclaim_idx = gfp_zone(gfp_mask),
                .order = order,
                .nodemask = nodemask,
                .priority = DEF_PRIORITY,
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .may_swap = 1,
                .may_shrinkslab = 1,
        };

        /*
         * scan_control uses s8 fields for order, priority, and reclaim_idx.
         * Confirm they are large enough for max values.
         */
        BUILD_BUG_ON(MAX_ORDER > S8_MAX);
        BUILD_BUG_ON(DEF_PRIORITY > S8_MAX);
        BUILD_BUG_ON(MAX_NR_ZONES > S8_MAX);

        /*
         * Do not enter reclaim if fatal signal was delivered while throttled.
         * 1 is returned so that the page allocator does not OOM kill at this
         * point.
         */
        if (throttle_direct_reclaim(sc.gfp_mask, zonelist, nodemask))
                return 1;

        trace_mm_vmscan_direct_reclaim_begin(order, sc.gfp_mask);

        nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

        trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);

        return nr_reclaimed;
}

#ifdef CONFIG_MEMCG

unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
                                                gfp_t gfp_mask, bool noswap,
                                                pg_data_t *pgdat,
                                                unsigned long *nr_scanned)
{
        struct scan_control sc = {
                .nr_to_reclaim = SWAP_CLUSTER_MAX,
                .target_mem_cgroup = memcg,
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .reclaim_idx = MAX_NR_ZONES - 1,
                .may_swap = !noswap,
                .may_shrinkslab = 1,
        };
        unsigned long lru_pages;

        sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
                        (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);

        trace_mm_vmscan_memcg_softlimit_reclaim_begin(sc.order,
                                                      sc.gfp_mask);

        /*
         * NOTE: Although we can get the priority field, using it
         * here is not a good idea, since it limits the pages we can scan.
         * if we don't reclaim here, the shrink_node from balance_pgdat
         * will pick up pages from other mem cgroup's as well. We hack
         * the priority and make it zero.
         */
        shrink_node_memcg(pgdat, memcg, &sc, &lru_pages);

        trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);

        *nr_scanned = sc.nr_scanned;
        return sc.nr_reclaimed;
}

unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
                                           unsigned long nr_pages,
                                           gfp_t gfp_mask,
                                           bool may_swap)
{
        struct zonelist *zonelist;
        unsigned long nr_reclaimed;
        unsigned long pflags;
        int nid;
        unsigned int noreclaim_flag;
        struct scan_control sc = {
                .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
                .gfp_mask = (current_gfp_context(gfp_mask) & GFP_RECLAIM_MASK) |
                                (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
                .reclaim_idx = MAX_NR_ZONES - 1,
                .target_mem_cgroup = memcg,
                .priority = DEF_PRIORITY,
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .may_swap = may_swap,
                .may_shrinkslab = 1,
        };

        /*
         * Unlike direct reclaim via alloc_pages(), memcg's reclaim doesn't
         * take care of from where we get pages. So the node where we start the
         * scan does not need to be the current node.
         */
        nid = mem_cgroup_select_victim_node(memcg);

        zonelist = &NODE_DATA(nid)->node_zonelists[ZONELIST_FALLBACK];

        trace_mm_vmscan_memcg_reclaim_begin(0, sc.gfp_mask);

        psi_memstall_enter(&pflags);
        noreclaim_flag = memalloc_noreclaim_save();

        nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

        memalloc_noreclaim_restore(noreclaim_flag);
        psi_memstall_leave(&pflags);

        trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);

        return nr_reclaimed;
}
#endif

static void age_active_anon(struct pglist_data *pgdat,
                                struct scan_control *sc)
{
        struct mem_cgroup *memcg;

        if (!total_swap_pages)
                return;

        memcg = mem_cgroup_iter(NULL, NULL, NULL);
        do {
                struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);

                if (inactive_list_is_low(lruvec, false, sc, true))
                        shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                                           sc, LRU_ACTIVE_ANON);

                memcg = mem_cgroup_iter(NULL, memcg, NULL);
        } while (memcg);
}

static bool pgdat_watermark_boosted(pg_data_t *pgdat, int classzone_idx)
{
        int i;
        struct zone *zone;

        /*
         * Check for watermark boosts top-down as the higher zones
         * are more likely to be boosted. Both watermarks and boosts
         * should not be checked at the time time as reclaim would
         * start prematurely when there is no boosting and a lower
         * zone is balanced.
         */
        for (i = classzone_idx; i >= 0; i--) {
                zone = pgdat->node_zones + i;
                if (!managed_zone(zone))
                        continue;

                if (zone->watermark_boost)
                        return true;
        }

        return false;
}

/*
 * Returns true if there is an eligible zone balanced for the request order
 * and classzone_idx
 */
static bool pgdat_balanced(pg_data_t *pgdat, int order, int classzone_idx)
{
        int i;
        unsigned long mark = -1;
        struct zone *zone;

        /*
         * Check watermarks bottom-up as lower zones are more likely to
         * meet watermarks.
         */
        for (i = 0; i <= classzone_idx; i++) {
                zone = pgdat->node_zones + i;

                if (!managed_zone(zone))
                        continue;

                mark = high_wmark_pages(zone);
                if (zone_watermark_ok_safe(zone, order, mark, classzone_idx))
                        return true;
        }

        /*
         * If a node has no populated zone within classzone_idx, it does not
         * need balancing by definition. This can happen if a zone-restricted
         * allocation tries to wake a remote kswapd.
         */
        if (mark == -1)
                return true;

        return false;
}

/* Clear pgdat state for congested, dirty or under writeback. */
static void clear_pgdat_congested(pg_data_t *pgdat)
{
        clear_bit(PGDAT_CONGESTED, &pgdat->flags);
        clear_bit(PGDAT_DIRTY, &pgdat->flags);
        clear_bit(PGDAT_WRITEBACK, &pgdat->flags);
}

/*
 * Prepare kswapd for sleeping. This verifies that there are no processes
 * waiting in throttle_direct_reclaim() and that watermarks have been met.
 *
 * Returns true if kswapd is ready to sleep
 */
static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
{
        /*
         * The throttled processes are normally woken up in balance_pgdat() as
         * soon as allow_direct_reclaim() is true. But there is a potential
         * race between when kswapd checks the watermarks and a process gets
         * throttled. There is also a potential race if processes get
         * throttled, kswapd wakes, a large process exits thereby balancing the
         * zones, which causes kswapd to exit balance_pgdat() before reaching
         * the wake up checks. If kswapd is going to sleep, no process should
         * be sleeping on pfmemalloc_wait, so wake them now if necessary. If
         * the wake up is premature, processes will wake kswapd and get
         * throttled again. The difference from wake ups in balance_pgdat() is
         * that here we are under prepare_to_wait().
         */
        if (waitqueue_active(&pgdat->pfmemalloc_wait))
                wake_up_all(&pgdat->pfmemalloc_wait);

        /* Hopeless node, leave it to direct reclaim */
        if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
                return true;

        if (pgdat_balanced(pgdat, order, classzone_idx)) {
                clear_pgdat_congested(pgdat);
                return true;
        }

        return false;
}

/*
 * kswapd shrinks a node of pages that are at or below the highest usable
 * zone that is currently unbalanced.
 *
 * Returns true if kswapd scanned at least the requested number of pages to
 * reclaim or if the lack of progress was due to pages under writeback.
 * This is used to determine if the scanning priority needs to be raised.
 */
static bool kswapd_shrink_node(pg_data_t *pgdat,
                               struct scan_control *sc)
{
        struct zone *zone;
        int z;

        /* Reclaim a number of pages proportional to the number of zones */
        sc->nr_to_reclaim = 0;
        for (z = 0; z <= sc->reclaim_idx; z++) {
                zone = pgdat->node_zones + z;
                if (!managed_zone(zone))
                        continue;

                sc->nr_to_reclaim += max(high_wmark_pages(zone), SWAP_CLUSTER_MAX);
        }

        /*
         * Historically care was taken to put equal pressure on all zones but
         * now pressure is applied based on node LRU order.
         */
        shrink_node(pgdat, sc);

        /*
         * Fragmentation may mean that the system cannot be rebalanced for
         * high-order allocations. If twice the allocation size has been
         * reclaimed then recheck watermarks only at order-0 to prevent
         * excessive reclaim. Assume that a process requested a high-order
         * can direct reclaim/compact.
         */
        if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order))
                sc->order = 0;

        return sc->nr_scanned >= sc->nr_to_reclaim;
}

/*
 * For kswapd, balance_pgdat() will reclaim pages across a node from zones
 * that are eligible for use by the caller until at least one zone is
 * balanced.
 *
 * Returns the order kswapd finished reclaiming at.
 *
 * kswapd scans the zones in the highmem->normal->dma direction.  It skips
 * zones which have free_pages > high_wmark_pages(zone), but once a zone is
 * found to have free_pages <= high_wmark_pages(zone), any page in that zone
 * or lower is eligible for reclaim until at least one usable zone is
 * balanced.
 */
static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
{
        int i;
        unsigned long nr_soft_reclaimed;
        unsigned long nr_soft_scanned;
        unsigned long pflags;
        unsigned long nr_boost_reclaim;
        unsigned long zone_boosts[MAX_NR_ZONES] = { 0, };
        bool boosted;
        struct zone *zone;
        struct scan_control sc = {
                .gfp_mask = GFP_KERNEL,
                .order = order,
                .may_unmap = 1,
        };

        psi_memstall_enter(&pflags);
        __fs_reclaim_acquire();

        count_vm_event(PAGEOUTRUN);

        /*
         * Account for the reclaim boost. Note that the zone boost is left in
         * place so that parallel allocations that are near the watermark will
         * stall or direct reclaim until kswapd is finished.
         */
        nr_boost_reclaim = 0;
        for (i = 0; i <= classzone_idx; i++) {
                zone = pgdat->node_zones + i;
                if (!managed_zone(zone))
                        continue;

                nr_boost_reclaim += zone->watermark_boost;
                zone_boosts[i] = zone->watermark_boost;
        }
        boosted = nr_boost_reclaim;

restart:
        sc.priority = DEF_PRIORITY;
        do {
                unsigned long nr_reclaimed = sc.nr_reclaimed;
                bool raise_priority = true;
                bool balanced;
                bool ret;

                sc.reclaim_idx = classzone_idx;

                /*
                 * If the number of buffer_heads exceeds the maximum allowed
                 * then consider reclaiming from all zones. This has a dual
                 * purpose -- on 64-bit systems it is expected that
                 * buffer_heads are stripped during active rotation. On 32-bit
                 * systems, highmem pages can pin lowmem memory and shrinking
                 * buffers can relieve lowmem pressure. Reclaim may still not
                 * go ahead if all eligible zones for the original allocation
                 * request are balanced to avoid excessive reclaim from kswapd.
                 */
                if (buffer_heads_over_limit) {
                        for (i = MAX_NR_ZONES - 1; i >= 0; i--) {
                                zone = pgdat->node_zones + i;
                                if (!managed_zone(zone))
                                        continue;

                                sc.reclaim_idx = i;
                                break;
                        }
                }

                /*
                 * If the pgdat is imbalanced then ignore boosting and preserve
                 * the watermarks for a later time and restart. Note that the
                 * zone watermarks will be still reset at the end of balancing
                 * on the grounds that the normal reclaim should be enough to
                 * re-evaluate if boosting is required when kswapd next wakes.
                 */
                balanced = pgdat_balanced(pgdat, sc.order, classzone_idx);
                if (!balanced && nr_boost_reclaim) {
                        nr_boost_reclaim = 0;
                        goto restart;
                }

                /*
                 * If boosting is not active then only reclaim if there are no
                 * eligible zones. Note that sc.reclaim_idx is not used as
                 * buffer_heads_over_limit may have adjusted it.
                 */
                if (!nr_boost_reclaim && balanced)
                        goto out;

                /* Limit the priority of boosting to avoid reclaim writeback */
                if (nr_boost_reclaim && sc.priority == DEF_PRIORITY - 2)
                        raise_priority = false;

                /*
                 * Do not writeback or swap pages for boosted reclaim. The
                 * intent is to relieve pressure not issue sub-optimal IO
                 * from reclaim context. If no pages are reclaimed, the
                 * reclaim will be aborted.
                 */
                sc.may_writepage = !laptop_mode && !nr_boost_reclaim;
                sc.may_swap = !nr_boost_reclaim;
                sc.may_shrinkslab = !nr_boost_reclaim;

                /*
                 * Do some background aging of the anon list, to give
                 * pages a chance to be referenced before reclaiming. All
                 * pages are rotated regardless of classzone as this is
                 * about consistent aging.
                 */
                age_active_anon(pgdat, &sc);

                /*
                 * If we're getting trouble reclaiming, start doing writepage
                 * even in laptop mode.
                 */
                if (sc.priority < DEF_PRIORITY - 2)
                        sc.may_writepage = 1;

                /* Call soft limit reclaim before calling shrink_node. */
                sc.nr_scanned = 0;
                nr_soft_scanned = 0;
                nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(pgdat, sc.order,
                                                sc.gfp_mask, &nr_soft_scanned);
                sc.nr_reclaimed += nr_soft_reclaimed;

                /*
                 * There should be no need to raise the scanning priority if
                 * enough pages are already being scanned that that high
                 * watermark would be met at 100% efficiency.
                 */
                if (kswapd_shrink_node(pgdat, &sc))
                        raise_priority = false;

                /*
                 * If the low watermark is met there is no need for processes
                 * to be throttled on pfmemalloc_wait as they should not be
                 * able to safely make forward progress. Wake them
                 */
                if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
                                allow_direct_reclaim(pgdat))
                        wake_up_all(&pgdat->pfmemalloc_wait);

                /* Check if kswapd should be suspending */
                __fs_reclaim_release();
                ret = try_to_freeze();
                __fs_reclaim_acquire();
                if (ret || kthread_should_stop())
                        break;

                /*
                 * Raise priority if scanning rate is too low or there was no
                 * progress in reclaiming pages
                 */
                nr_reclaimed = sc.nr_reclaimed - nr_reclaimed;
                nr_boost_reclaim -= min(nr_boost_reclaim, nr_reclaimed);

                /*
                 * If reclaim made no progress for a boost, stop reclaim as
                 * IO cannot be queued and it could be an infinite loop in
                 * extreme circumstances.
                 */
                if (nr_boost_reclaim && !nr_reclaimed)
                        break;

                if (raise_priority || !nr_reclaimed)
                        sc.priority--;
        } while (sc.priority >= 1);

        if (!sc.nr_reclaimed)
                pgdat->kswapd_failures++;

out:
        /* If reclaim was boosted, account for the reclaim done in this pass */
        if (boosted) {
                unsigned long flags;

                for (i = 0; i <= classzone_idx; i++) {
                        if (!zone_boosts[i])
                                continue;

                        /* Increments are under the zone lock */
                        zone = pgdat->node_zones + i;
                        spin_lock_irqsave(&zone->lock, flags);
                        zone->watermark_boost -= min(zone->watermark_boost, zone_boosts[i]);
                        spin_unlock_irqrestore(&zone->lock, flags);
                }

                /*
                 * As there is now likely space, wakeup kcompact to defragment
                 * pageblocks.
                 */
                wakeup_kcompactd(pgdat, pageblock_order, classzone_idx);
        }

        snapshot_refaults(NULL, pgdat);
        __fs_reclaim_release();
        psi_memstall_leave(&pflags);
        /*
         * Return the order kswapd stopped reclaiming at as
         * prepare_kswapd_sleep() takes it into account. If another caller
         * entered the allocator slow path while kswapd was awake, order will
         * remain at the higher level.
         */
        return sc.order;
}

/*
 * The pgdat->kswapd_classzone_idx is used to pass the highest zone index to be
 * reclaimed by kswapd from the waker. If the value is MAX_NR_ZONES which is not
 * a valid index then either kswapd runs for first time or kswapd couldn't sleep
 * after previous reclaim attempt (node is still unbalanced). In that case
 * return the zone index of the previous kswapd reclaim cycle.
 */
static enum zone_type kswapd_classzone_idx(pg_data_t *pgdat,
                                           enum zone_type prev_classzone_idx)
{
        if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
                return prev_classzone_idx;
        return pgdat->kswapd_classzone_idx;
}

static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
                                unsigned int classzone_idx)
{
        long remaining = 0;
        DEFINE_WAIT(wait);

        if (freezing(current) || kthread_should_stop())
                return;

        prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);

        /*
         * Try to sleep for a short interval. Note that kcompactd will only be
         * woken if it is possible to sleep for a short interval. This is
         * deliberate on the assumption that if reclaim cannot keep an
         * eligible zone balanced that it's also unlikely that compaction will
         * succeed.
         */
        if (prepare_kswapd_sleep(pgdat, reclaim_order, classzone_idx)) {
                /*
                 * Compaction records what page blocks it recently failed to
                 * isolate pages from and skips them in the future scanning.
                 * When kswapd is going to sleep, it is reasonable to assume
                 * that pages and compaction may succeed so reset the cache.
                 */
                reset_isolation_suitable(pgdat);

                /*
                 * We have freed the memory, now we should compact it to make
                 * allocation of the requested order possible.
                 */
                wakeup_kcompactd(pgdat, alloc_order, classzone_idx);

                remaining = schedule_timeout(HZ/10);

                /*
                 * If woken prematurely then reset kswapd_classzone_idx and
                 * order. The values will either be from a wakeup request or
                 * the previous request that slept prematurely.
                 */
                if (remaining) {
                        pgdat->kswapd_classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
                        pgdat->kswapd_order = max(pgdat->kswapd_order, reclaim_order);
                }

                finish_wait(&pgdat->kswapd_wait, &wait);
                prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
        }

        /*
         * After a short sleep, check if it was a premature sleep. If not, then
         * go fully to sleep until explicitly woken up.
         */
        if (!remaining &&
            prepare_kswapd_sleep(pgdat, reclaim_order, classzone_idx)) {
                trace_mm_vmscan_kswapd_sleep(pgdat->node_id);

                /*
                 * vmstat counters are not perfectly accurate and the estimated
                 * value for counters such as NR_FREE_PAGES can deviate from the
                 * true value by nr_online_cpus * threshold. To avoid the zone
                 * watermarks being breached while under pressure, we reduce the
                 * per-cpu vmstat threshold while kswapd is awake and restore
                 * them before going back to sleep.
                 */
                set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);

                if (!kthread_should_stop())
                        schedule();

                set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
        } else {
                if (remaining)
                        count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
                else
                        count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
        }
        finish_wait(&pgdat->kswapd_wait, &wait);
}

/*
 * The background pageout daemon, started as a kernel thread
 * from the init process.
 *
 * This basically trickles out pages so that we have _some_
 * free memory available even if there is no other activity
 * that frees anything up. This is needed for things like routing
 * etc, where we otherwise might have all activity going on in
 * asynchronous contexts that cannot page things out.
 *
 * If there are applications that are active memory-allocators
 * (most normal use), this basically shouldn't matter.
 */
static int kswapd(void *p)
{
        unsigned int alloc_order, reclaim_order;
        unsigned int classzone_idx = MAX_NR_ZONES - 1;
        pg_data_t *pgdat = (pg_data_t*)p;
        struct task_struct *tsk = current;

        struct reclaim_state reclaim_state = {
                .reclaimed_slab = 0,
        };
        const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);

        if (!cpumask_empty(cpumask))
                set_cpus_allowed_ptr(tsk, cpumask);
        current->reclaim_state = &reclaim_state;

        /*
         * Tell the memory management that we're a "memory allocator",
         * and that if we need more memory we should get access to it
         * regardless (see "__alloc_pages()"). "kswapd" should
         * never get caught in the normal page freeing logic.
         *
         * (Kswapd normally doesn't need memory anyway, but sometimes
         * you need a small amount of memory in order to be able to
         * page out something else, and this flag essentially protects
         * us from recursively trying to free more memory as we're
         * trying to free the first piece of memory in the first place).
         */
        tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
        set_freezable();

        pgdat->kswapd_order = 0;
        pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
        for ( ; ; ) {
                bool ret;

                alloc_order = reclaim_order = pgdat->kswapd_order;
                classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);

kswapd_try_sleep:
                kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
                                        classzone_idx);

                /* Read the new order and classzone_idx */
                alloc_order = reclaim_order = pgdat->kswapd_order;
                classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
                pgdat->kswapd_order = 0;
                pgdat->kswapd_classzone_idx = MAX_NR_ZONES;

                ret = try_to_freeze();
                if (kthread_should_stop())
                        break;

                /*
                 * We can speed up thawing tasks if we don't call balance_pgdat
                 * after returning from the refrigerator
                 */
                if (ret)
                        continue;

                /*
                 * Reclaim begins at the requested order but if a high-order
                 * reclaim fails then kswapd falls back to reclaiming for
                 * order-0. If that happens, kswapd will consider sleeping
                 * for the order it finished reclaiming at (reclaim_order)
                 * but kcompactd is woken to compact for the original
                 * request (alloc_order).
                 */
                trace_mm_vmscan_kswapd_wake(pgdat->node_id, classzone_idx,
                                                alloc_order);
                reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
                if (reclaim_order < alloc_order)
                        goto kswapd_try_sleep;
        }

        tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
        current->reclaim_state = NULL;

        return 0;
}

/*
 * A zone is low on free memory or too fragmented for high-order memory.  If
 * kswapd should reclaim (direct reclaim is deferred), wake it up for the zone's
 * pgdat.  It will wake up kcompactd after reclaiming memory.  If kswapd reclaim
 * has failed or is not needed, still wake up kcompactd if only compaction is
 * needed.
 */
void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
                   enum zone_type classzone_idx)
{
        pg_data_t *pgdat;

        if (!managed_zone(zone))
                return;

        if (!cpuset_zone_allowed(zone, gfp_flags))
                return;
        pgdat = zone->zone_pgdat;

        if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
                pgdat->kswapd_classzone_idx = classzone_idx;
        else
                pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx,
                                                  classzone_idx);
        pgdat->kswapd_order = max(pgdat->kswapd_order, order);
        if (!waitqueue_active(&pgdat->kswapd_wait))
                return;

        /* Hopeless node, leave it to direct reclaim if possible */
        if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ||
            (pgdat_balanced(pgdat, order, classzone_idx) &&
             !pgdat_watermark_boosted(pgdat, classzone_idx))) {
                /*
                 * There may be plenty of free memory available, but it's too
                 * fragmented for high-order allocations.  Wake up kcompactd
                 * and rely on compaction_suitable() to determine if it's
                 * needed.  If it fails, it will defer subsequent attempts to
                 * ratelimit its work.
                 */
                if (!(gfp_flags & __GFP_DIRECT_RECLAIM))
                        wakeup_kcompactd(pgdat, order, classzone_idx);
                return;
        }

        trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, classzone_idx, order,
                                      gfp_flags);
        wake_up_interruptible(&pgdat->kswapd_wait);
}

#ifdef CONFIG_HIBERNATION
/*
 * Try to free `nr_to_reclaim' of memory, system-wide, and return the number of
 * freed pages.
 *
 * Rather than trying to age LRUs the aim is to preserve the overall
 * LRU order by reclaiming preferentially
 * inactive > active > active referenced > active mapped
 */
unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
{
        struct reclaim_state reclaim_state;
        struct scan_control sc = {
                .nr_to_reclaim = nr_to_reclaim,
                .gfp_mask = GFP_HIGHUSER_MOVABLE,
                .reclaim_idx = MAX_NR_ZONES - 1,
                .priority = DEF_PRIORITY,
                .may_writepage = 1,
                .may_unmap = 1,
                .may_swap = 1,
                .hibernation_mode = 1,
        };
        struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
        struct task_struct *p = current;
        unsigned long nr_reclaimed;
        unsigned int noreclaim_flag;

        fs_reclaim_acquire(sc.gfp_mask);
        noreclaim_flag = memalloc_noreclaim_save();
        reclaim_state.reclaimed_slab = 0;
        p->reclaim_state = &reclaim_state;

        nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

        p->reclaim_state = NULL;
        memalloc_noreclaim_restore(noreclaim_flag);
        fs_reclaim_release(sc.gfp_mask);

        return nr_reclaimed;
}
#endif /* CONFIG_HIBERNATION */

/* It's optimal to keep kswapds on the same CPUs as their memory, but
   not required for correctness.  So if the last cpu in a node goes
   away, we get changed to run anywhere: as the first one comes back,
   restore their cpu bindings. */
static int kswapd_cpu_online(unsigned int cpu)
{
        int nid;

        for_each_node_state(nid, N_MEMORY) {
                pg_data_t *pgdat = NODE_DATA(nid);
                const struct cpumask *mask;

                mask = cpumask_of_node(pgdat->node_id);

                if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
                        /* One of our CPUs online: restore mask */
                        set_cpus_allowed_ptr(pgdat->kswapd, mask);
        }
        return 0;
}

/*
 * This kswapd start function will be called by init and node-hot-add.
 * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
 */
int kswapd_run(int nid)
{
        pg_data_t *pgdat = NODE_DATA(nid);
        int ret = 0;

        if (pgdat->kswapd)
                return 0;

        pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
        if (IS_ERR(pgdat->kswapd)) {
                /* failure at boot is fatal */
                BUG_ON(system_state < SYSTEM_RUNNING);
                pr_err("Failed to start kswapd on node %d\n", nid);
                ret = PTR_ERR(pgdat->kswapd);
                pgdat->kswapd = NULL;
        }
        return ret;
}

/*
 * Called by memory hotplug when all memory in a node is offlined.  Caller must
 * hold mem_hotplug_begin/end().
 */
void kswapd_stop(int nid)
{
        struct task_struct *kswapd = NODE_DATA(nid)->kswapd;

        if (kswapd) {
                kthread_stop(kswapd);
                NODE_DATA(nid)->kswapd = NULL;
        }
}

static int __init kswapd_init(void)
{
        int nid, ret;

        swap_setup();
        for_each_node_state(nid, N_MEMORY)
                kswapd_run(nid);
        ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
                                        "mm/vmscan:online", kswapd_cpu_online,
                                        NULL);
        WARN_ON(ret < 0);
        return 0;
}

module_init(kswapd_init)

#ifdef CONFIG_NUMA
/*
 * Node reclaim mode
 *
 * If non-zero call node_reclaim when the number of free pages falls below
 * the watermarks.
 */
int node_reclaim_mode __read_mostly;

#define RECLAIM_OFF 0
#define RECLAIM_ZONE (1<<0)     /* Run shrink_inactive_list on the zone */
#define RECLAIM_WRITE (1<<1)    /* Writeout pages during reclaim */
#define RECLAIM_UNMAP (1<<2)    /* Unmap pages during reclaim */

/*
 * Priority for NODE_RECLAIM. This determines the fraction of pages
 * of a node considered for each zone_reclaim. 4 scans 1/16th of
 * a zone.
 */
#define NODE_RECLAIM_PRIORITY 4

/*
 * Percentage of pages in a zone that must be unmapped for node_reclaim to
 * occur.
 */
int sysctl_min_unmapped_ratio = 1;

/*
 * If the number of slab pages in a zone grows beyond this percentage then
 * slab reclaim needs to occur.
 */
int sysctl_min_slab_ratio = 5;

static inline unsigned long node_unmapped_file_pages(struct pglist_data *pgdat)
{
        unsigned long file_mapped = node_page_state(pgdat, NR_FILE_MAPPED);
        unsigned long file_lru = node_page_state(pgdat, NR_INACTIVE_FILE) +
                node_page_state(pgdat, NR_ACTIVE_FILE);

        /*
         * It's possible for there to be more file mapped pages than
         * accounted for by the pages on the file LRU lists because
         * tmpfs pages accounted for as ANON can also be FILE_MAPPED
         */
        return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
}

/* Work out how many page cache pages we can reclaim in this reclaim_mode */
static unsigned long node_pagecache_reclaimable(struct pglist_data *pgdat)
{
        unsigned long nr_pagecache_reclaimable;
        unsigned long delta = 0;

        /*
         * If RECLAIM_UNMAP is set, then all file pages are considered
         * potentially reclaimable. Otherwise, we have to worry about
         * pages like swapcache and node_unmapped_file_pages() provides
         * a better estimate
         */
        if (node_reclaim_mode & RECLAIM_UNMAP)
                nr_pagecache_reclaimable = node_page_state(pgdat, NR_FILE_PAGES);
        else
                nr_pagecache_reclaimable = node_unmapped_file_pages(pgdat);

        /* If we can't clean pages, remove dirty pages from consideration */
        if (!(node_reclaim_mode & RECLAIM_WRITE))
                delta += node_page_state(pgdat, NR_FILE_DIRTY);

        /* Watch for any possible underflows due to delta */
        if (unlikely(delta > nr_pagecache_reclaimable))
                delta = nr_pagecache_reclaimable;

        return nr_pagecache_reclaimable - delta;
}

/*
 * Try to free up some pages from this node through reclaim.
 */
static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
{
        /* Minimum pages needed in order to stay on node */
        const unsigned long nr_pages = 1 << order;
        struct task_struct *p = current;
        struct reclaim_state reclaim_state;
        unsigned int noreclaim_flag;
        struct scan_control sc = {
                .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
                .gfp_mask = current_gfp_context(gfp_mask),
                .order = order,
                .priority = NODE_RECLAIM_PRIORITY,
                .may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
                .may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
                .may_swap = 1,
                .reclaim_idx = gfp_zone(gfp_mask),
        };

        trace_mm_vmscan_node_reclaim_begin(pgdat->node_id, order,
                                           sc.gfp_mask);

        cond_resched();
        fs_reclaim_acquire(sc.gfp_mask);
        /*
         * We need to be able to allocate from the reserves for RECLAIM_UNMAP
         * and we also need to be able to write out pages for RECLAIM_WRITE
         * and RECLAIM_UNMAP.
         */
        noreclaim_flag = memalloc_noreclaim_save();
        p->flags |= PF_SWAPWRITE;
        reclaim_state.reclaimed_slab = 0;
        p->reclaim_state = &reclaim_state;

        if (node_pagecache_reclaimable(pgdat) > pgdat->min_unmapped_pages) {
                /*
                 * Free memory by calling shrink node with increasing
                 * priorities until we have enough memory freed.
                 */
                do {
                        shrink_node(pgdat, &sc);
                } while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
        }

        p->reclaim_state = NULL;
        current->flags &= ~PF_SWAPWRITE;
        memalloc_noreclaim_restore(noreclaim_flag);
        fs_reclaim_release(sc.gfp_mask);

        trace_mm_vmscan_node_reclaim_end(sc.nr_reclaimed);

        return sc.nr_reclaimed >= nr_pages;
}

int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
{
        int ret;

        /*
         * Node reclaim reclaims unmapped file backed pages and
         * slab pages if we are over the defined limits.
         *
         * A small portion of unmapped file backed pages is needed for
         * file I/O otherwise pages read by file I/O will be immediately
         * thrown out if the node is overallocated. So we do not reclaim
         * if less than a specified percentage of the node is used by
         * unmapped file backed pages.
         */
        if (node_pagecache_reclaimable(pgdat) <= pgdat->min_unmapped_pages &&
            node_page_state(pgdat, NR_SLAB_RECLAIMABLE) <= pgdat->min_slab_pages)
                return NODE_RECLAIM_FULL;

        /*
         * Do not scan if the allocation should not be delayed.
         */
        if (!gfpflags_allow_blocking(gfp_mask) || (current->flags & PF_MEMALLOC))
                return NODE_RECLAIM_NOSCAN;

        /*
         * Only run node reclaim on the local node or on nodes that do not
         * have associated processors. This will favor the local processor
         * over remote processors and spread off node memory allocations
         * as wide as possible.
         */
        if (node_state(pgdat->node_id, N_CPU) && pgdat->node_id != numa_node_id())
                return NODE_RECLAIM_NOSCAN;

        if (test_and_set_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags))
                return NODE_RECLAIM_NOSCAN;

        ret = __node_reclaim(pgdat, gfp_mask, order);
        clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags);

        if (!ret)
                count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);

        return ret;
}
#endif

/*
 * page_evictable - test whether a page is evictable
 * @page: the page to test
 *
 * Test whether page is evictable--i.e., should be placed on active/inactive
 * lists vs unevictable list.
 *
 * Reasons page might not be evictable:
 * (1) page's mapping marked unevictable
 * (2) page is part of an mlocked VMA
 *
 */
int page_evictable(struct page *page)
{
        int ret;

        /* Prevent address_space of inode and swap cache from being freed */
        rcu_read_lock();
        ret = !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
        rcu_read_unlock();
        return ret;
}

/**
 * check_move_unevictable_pages - check pages for evictability and move to
 * appropriate zone lru list
 * @pvec: pagevec with lru pages to check
 *
 * Checks pages for evictability, if an evictable page is in the unevictable
 * lru list, moves it to the appropriate evictable lru list. This function
 * should be only used for lru pages.
 */
void check_move_unevictable_pages(struct pagevec *pvec)
{
        struct lruvec *lruvec;
        struct pglist_data *pgdat = NULL;
        int pgscanned = 0;
        int pgrescued = 0;
        int i;

        for (i = 0; i < pvec->nr; i++) {
                struct page *page = pvec->pages[i];
                struct pglist_data *pagepgdat = page_pgdat(page);

                pgscanned++;
                if (pagepgdat != pgdat) {
                        if (pgdat)
                                spin_unlock_irq(&pgdat->lru_lock);
                        pgdat = pagepgdat;
                        spin_lock_irq(&pgdat->lru_lock);
                }
                lruvec = mem_cgroup_page_lruvec(page, pgdat);

                if (!PageLRU(page) || !PageUnevictable(page))
                        continue;

                if (page_evictable(page)) {
                        enum lru_list lru = page_lru_base_type(page);

                        VM_BUG_ON_PAGE(PageActive(page), page);
                        ClearPageUnevictable(page);
                        del_page_from_lru_list(page, lruvec, LRU_UNEVICTABLE);
                        add_page_to_lru_list(page, lruvec, lru);
                        pgrescued++;
                }
        }

        if (pgdat) {
                __count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
                __count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
                spin_unlock_irq(&pgdat->lru_lock);
        }
}
EXPORT_SYMBOL_GPL(check_move_unevictable_pages);