mm + fs: prepare for non-page entries in page cache radix trees

[linux.git] / mm / vmscan.c

/*
 *  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.
 */

#include <linux/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/prefetch.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 {
        /* 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;

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

        unsigned long hibernation_mode;

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

        int may_writepage;

        /* Can mapped pages be reclaimed? */
        int may_unmap;

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

        int order;

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

        /*
         * 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;

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

#define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))

#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;
unsigned long vm_total_pages;   /* The total number of pages which the VM controls */

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

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

static unsigned long zone_reclaimable_pages(struct zone *zone)
{
        int nr;

        nr = zone_page_state(zone, NR_ACTIVE_FILE) +
             zone_page_state(zone, NR_INACTIVE_FILE);

        if (get_nr_swap_pages() > 0)
                nr += zone_page_state(zone, NR_ACTIVE_ANON) +
                      zone_page_state(zone, NR_INACTIVE_ANON);

        return nr;
}

bool zone_reclaimable(struct zone *zone)
{
        return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
}

static unsigned long get_lru_size(struct lruvec *lruvec, enum lru_list lru)
{
        if (!mem_cgroup_disabled())
                return mem_cgroup_get_lru_size(lruvec, lru);

        return zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru);
}

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

        /*
         * If we only have one possible node in the system anyway, save
         * ourselves the trouble and disable NUMA aware behavior. This way we
         * will save memory and some small loop time later.
         */
        if (nr_node_ids == 1)
                shrinker->flags &= ~SHRINKER_NUMA_AWARE;

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

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

        down_write(&shrinker_rwsem);
        list_add_tail(&shrinker->list, &shrinker_list);
        up_write(&shrinker_rwsem);
        return 0;
}
EXPORT_SYMBOL(register_shrinker);

/*
 * Remove one
 */
void unregister_shrinker(struct shrinker *shrinker)
{
        down_write(&shrinker_rwsem);
        list_del(&shrinker->list);
        up_write(&shrinker_rwsem);
        kfree(shrinker->nr_deferred);
}
EXPORT_SYMBOL(unregister_shrinker);

#define SHRINK_BATCH 128

static unsigned long
shrink_slab_node(struct shrink_control *shrinkctl, struct shrinker *shrinker,
                 unsigned long nr_pages_scanned, unsigned long lru_pages)
{
        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;

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

        /*
         * 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;
        delta = (4 * nr_pages_scanned) / shrinker->seeks;
        delta *= freeable;
        do_div(delta, lru_pages + 1);
        total_scan += delta;
        if (total_scan < 0) {
                printk(KERN_ERR
                "shrink_slab: %pF negative objects to delete nr=%ld\n",
                       shrinker->scan_objects, total_scan);
                total_scan = freeable;
        }

        /*
         * 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,
                                nr_pages_scanned, lru_pages,
                                freeable, delta, total_scan);

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

                count_vm_events(SLABS_SCANNED, nr_to_scan);
                total_scan -= nr_to_scan;

                cond_resched();
        }

        /*
         * 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 (total_scan > 0)
                new_nr = atomic_long_add_return(total_scan,
                                                &shrinker->nr_deferred[nid]);
        else
                new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);

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

/*
 * Call the shrink functions to age shrinkable caches
 *
 * Here we assume it costs one seek to replace a lru page and that it also
 * takes a seek to recreate a cache object.  With this in mind we age equal
 * percentages of the lru and ageable caches.  This should balance the seeks
 * generated by these structures.
 *
 * If the vm encountered mapped pages on the LRU it increase the pressure on
 * slab to avoid swapping.
 *
 * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
 *
 * `lru_pages' represents the number of on-LRU pages in all the zones which
 * are eligible for the caller's allocation attempt.  It is used for balancing
 * slab reclaim versus page reclaim.
 *
 * Returns the number of slab objects which we shrunk.
 */
unsigned long shrink_slab(struct shrink_control *shrinkctl,
                          unsigned long nr_pages_scanned,
                          unsigned long lru_pages)
{
        struct shrinker *shrinker;
        unsigned long freed = 0;

        if (nr_pages_scanned == 0)
                nr_pages_scanned = SWAP_CLUSTER_MAX;

        if (!down_read_trylock(&shrinker_rwsem)) {
                /*
                 * If we would return 0, our callers would understand that we
                 * have nothing else to shrink and give up trying. By returning
                 * 1 we keep it going and assume we'll be able to shrink next
                 * time.
                 */
                freed = 1;
                goto out;
        }

        list_for_each_entry(shrinker, &shrinker_list, list) {
                if (!(shrinker->flags & SHRINKER_NUMA_AWARE)) {
                        shrinkctl->nid = 0;
                        freed += shrink_slab_node(shrinkctl, shrinker,
                                        nr_pages_scanned, lru_pages);
                        continue;
                }

                for_each_node_mask(shrinkctl->nid, shrinkctl->nodes_to_scan) {
                        if (node_online(shrinkctl->nid))
                                freed += shrink_slab_node(shrinkctl, shrinker,
                                                nr_pages_scanned, lru_pages);

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

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 radix tree and
         * optional buffer heads at page->private.
         */
        return page_count(page) - page_has_private(page) == 2;
}

static int may_write_to_queue(struct backing_dev_info *bdi,
                              struct scan_control *sc)
{
        if (current->flags & PF_SWAPWRITE)
                return 1;
        if (!bdi_write_congested(bdi))
                return 1;
        if (bdi == 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_aio_write() 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);
                                printk("%s: orphaned page\n", __func__);
                                return PAGE_CLEAN;
                        }
                }
                return PAGE_KEEP;
        }
        if (mapping->a_ops->writepage == NULL)
                return PAGE_ACTIVATE;
        if (!may_write_to_queue(mapping->backing_dev_info, 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, trace_reclaim_flags(page));
                inc_zone_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)
{
        BUG_ON(!PageLocked(page));
        BUG_ON(mapping != page_mapping(page));

        spin_lock_irq(&mapping->tree_lock);
        /*
         * 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->_count.
         *
         * Note that if SetPageDirty is always performed via set_page_dirty,
         * and thus under tree_lock, then this ordering is not required.
         */
        if (!page_freeze_refs(page, 2))
                goto cannot_free;
        /* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
        if (unlikely(PageDirty(page))) {
                page_unfreeze_refs(page, 2);
                goto cannot_free;
        }

        if (PageSwapCache(page)) {
                swp_entry_t swap = { .val = page_private(page) };
                __delete_from_swap_cache(page);
                spin_unlock_irq(&mapping->tree_lock);
                swapcache_free(swap, page);
        } else {
                void (*freepage)(struct page *);

                freepage = mapping->a_ops->freepage;

                __delete_from_page_cache(page);
                spin_unlock_irq(&mapping->tree_lock);
                mem_cgroup_uncharge_cache_page(page);

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

        return 1;

cannot_free:
        spin_unlock_irq(&mapping->tree_lock);
        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)) {
                /*
                 * Unfreezing the refcount with 1 rather than 2 effectively
                 * drops the pagecache ref for us without requiring another
                 * atomic operation.
                 */
                page_unfreeze_refs(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)
{
        bool is_unevictable;
        int was_unevictable = PageUnevictable(page);

        VM_BUG_ON_PAGE(PageLRU(page), page);

redo:
        ClearPageUnevictable(page);

        if (page_evictable(page)) {
                /*
                 * For evictable pages, we can use the cache.
                 * In event of a race, worst case is we end up with an
                 * unevictable page on [in]active list.
                 * We know how to handle that.
                 */
                is_unevictable = false;
                lru_cache_add(page);
        } else {
                /*
                 * Put unevictable pages directly on zone's unevictable
                 * list.
                 */
                is_unevictable = true;
                add_page_to_unevictable_list(page);
                /*
                 * When racing with an mlock or AS_UNEVICTABLE clearing
                 * (page is unlocked) make sure that if the other thread
                 * does not observe our setting of PG_lru and fails
                 * isolation/check_move_unevictable_pages,
                 * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
                 * the page back to the evictable list.
                 *
                 * The other side is TestClearPageMlocked() or shmem_lock().
                 */
                smp_mb();
        }

        /*
         * page's status can change while we move it among lru. If an evictable
         * page is on unevictable list, it never be freed. To avoid that,
         * check after we added it to the list, again.
         */
        if (is_unevictable && page_evictable(page)) {
                if (!isolate_lru_page(page)) {
                        put_page(page);
                        goto redo;
                }
                /* This means someone else dropped this page from LRU
                 * So, it will be freed or putback to LRU again. There is
                 * nothing to do here.
                 */
        }

        if (was_unevictable && !is_unevictable)
                count_vm_event(UNEVICTABLE_PGRESCUED);
        else if (!was_unevictable && is_unevictable)
                count_vm_event(UNEVICTABLE_PGCULLED);

        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)) {
                *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 zone *zone,
                                      struct scan_control *sc,
                                      enum ttu_flags ttu_flags,
                                      unsigned long *ret_nr_dirty,
                                      unsigned long *ret_nr_unqueued_dirty,
                                      unsigned long *ret_nr_congested,
                                      unsigned long *ret_nr_writeback,
                                      unsigned long *ret_nr_immediate,
                                      bool force_reclaim)
{
        LIST_HEAD(ret_pages);
        LIST_HEAD(free_pages);
        int pgactivate = 0;
        unsigned long nr_unqueued_dirty = 0;
        unsigned long nr_dirty = 0;
        unsigned long nr_congested = 0;
        unsigned long nr_reclaimed = 0;
        unsigned long nr_writeback = 0;
        unsigned long nr_immediate = 0;

        cond_resched();

        mem_cgroup_uncharge_start();
        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;

                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);
                VM_BUG_ON_PAGE(page_zone(page) != zone, page);

                sc->nr_scanned++;

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

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

                /* Double the slab pressure for mapped and swapcache pages */
                if (page_mapped(page) || PageSwapCache(page))
                        sc->nr_scanned++;

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

                /*
                 * The number of dirty pages determines if a zone 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)
                        nr_dirty++;

                if (dirty && !writeback)
                        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 ((mapping && bdi_write_congested(mapping->backing_dev_info)) ||
                    (writeback && PageReclaim(page)))
                        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 reclaim encounters a page, memcg encounters a
                 *    page that is not marked for immediate reclaim or
                 *    the caller does not have __GFP_IO. In this case mark
                 *    the page for immediate reclaim and continue scanning.
                 *
                 *    __GFP_IO is checked  because a loop driver thread 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.
                 *
                 *    Don't require __GFP_FS, since we're not going into the
                 *    FS, just waiting on its writeback completion. Worryingly,
                 *    ext4 gfs2 and xfs allocate pages with
                 *    grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
                 *    may_enter_fs here is liable to OOM on them.
                 *
                 * 3) memcg encounters a page that is not 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.
                 */
                if (PageWriteback(page)) {
                        /* Case 1 above */
                        if (current_is_kswapd() &&
                            PageReclaim(page) &&
                            zone_is_reclaim_writeback(zone)) {
                                nr_immediate++;
                                goto keep_locked;

                        /* Case 2 above */
                        } else if (global_reclaim(sc) ||
                            !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
                                /*
                                 * 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);
                                nr_writeback++;

                                goto keep_locked;

                        /* Case 3 above */
                        } else {
                                wait_on_page_writeback(page);
                        }
                }

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

                switch (references) {
                case PAGEREF_ACTIVATE:
                        goto activate_locked;
                case PAGEREF_KEEP:
                        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.
                 */
                if (PageAnon(page) && !PageSwapCache(page)) {
                        if (!(sc->gfp_mask & __GFP_IO))
                                goto keep_locked;
                        if (!add_to_swap(page, page_list))
                                goto activate_locked;
                        may_enter_fs = 1;

                        /* Adding to swap updated mapping */
                        mapping = page_mapping(page);
                }

                /*
                 * The page is mapped into the page tables of one or more
                 * processes. Try to unmap it here.
                 */
                if (page_mapped(page) && mapping) {
                        switch (try_to_unmap(page, ttu_flags)) {
                        case SWAP_FAIL:
                                goto activate_locked;
                        case SWAP_AGAIN:
                                goto keep_locked;
                        case SWAP_MLOCK:
                                goto cull_mlocked;
                        case SWAP_SUCCESS:
                                ; /* try to free the page below */
                        }
                }

                if (PageDirty(page)) {
                        /*
                         * Only kswapd can writeback filesystem pages to
                         * avoid risk of stack overflow but only writeback
                         * if many dirty pages have been encountered.
                         */
                        if (page_is_file_cache(page) &&
                                        (!current_is_kswapd() ||
                                         !zone_is_reclaim_dirty(zone))) {
                                /*
                                 * Immediately reclaim when written back.
                                 * Similar in principal to deactivate_page()
                                 * except we already have the page isolated
                                 * and know it's dirty
                                 */
                                inc_zone_page_state(page, NR_VMSCAN_IMMEDIATE);
                                SetPageReclaim(page);

                                goto keep_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, try to write it out here */
                        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 (!mapping || !__remove_mapping(mapping, page))
                        goto keep_locked;

                /*
                 * At this point, we have no other references and there is
                 * no way to pick any more up (removed from LRU, removed
                 * from pagecache). Can use non-atomic bitops now (and
                 * we obviously don't have to worry about waking up a process
                 * waiting on the page lock, because there are no references.
                 */
                __clear_page_locked(page);
free_it:
                nr_reclaimed++;

                /*
                 * Is there need to periodically free_page_list? It would
                 * appear not as the counts should be low
                 */
                list_add(&page->lru, &free_pages);
                continue;

cull_mlocked:
                if (PageSwapCache(page))
                        try_to_free_swap(page);
                unlock_page(page);
                putback_lru_page(page);
                continue;

activate_locked:
                /* Not a candidate for swapping, so reclaim swap space. */
                if (PageSwapCache(page) && vm_swap_full())
                        try_to_free_swap(page);
                VM_BUG_ON_PAGE(PageActive(page), page);
                SetPageActive(page);
                pgactivate++;
keep_locked:
                unlock_page(page);
keep:
                list_add(&page->lru, &ret_pages);
                VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
        }

        free_hot_cold_page_list(&free_pages, 1);

        list_splice(&ret_pages, page_list);
        count_vm_events(PGACTIVATE, pgactivate);
        mem_cgroup_uncharge_end();
        *ret_nr_dirty += nr_dirty;
        *ret_nr_congested += nr_congested;
        *ret_nr_unqueued_dirty += nr_unqueued_dirty;
        *ret_nr_writeback += nr_writeback;
        *ret_nr_immediate += nr_immediate;
        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,
        };
        unsigned long ret, dummy1, dummy2, dummy3, dummy4, dummy5;
        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) &&
                    !isolated_balloon_page(page)) {
                        ClearPageActive(page);
                        list_move(&page->lru, &clean_pages);
                }
        }

        ret = shrink_page_list(&clean_pages, zone, &sc,
                        TTU_UNMAP|TTU_IGNORE_ACCESS,
                        &dummy1, &dummy2, &dummy3, &dummy4, &dummy5, true);
        list_splice(&clean_pages, page_list);
        __mod_zone_page_state(zone, 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_CLEAN means that only clean pages should be isolated. This
         * is used by reclaim when it is cannot write to backing storage
         *
         * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
         * that it is possible to migrate without blocking
         */
        if (mode & (ISOLATE_CLEAN|ISOLATE_ASYNC_MIGRATE)) {
                /* All the caller can do on PageWriteback is block */
                if (PageWriteback(page))
                        return ret;

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

                        /* ISOLATE_CLEAN means only clean pages */
                        if (mode & ISOLATE_CLEAN)
                                return ret;

                        /*
                         * Only pages without mappings or that have a
                         * ->migratepage callback are possible to migrate
                         * without blocking
                         */
                        mapping = page_mapping(page);
                        if (mapping && !mapping->a_ops->migratepage)
                                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;
}

/*
 * zone->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 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,
                isolate_mode_t mode, enum lru_list lru)
{
        struct list_head *src = &lruvec->lists[lru];
        unsigned long nr_taken = 0;
        unsigned long scan;

        for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
                struct page *page;
                int nr_pages;

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

                VM_BUG_ON_PAGE(!PageLRU(page), page);

                switch (__isolate_lru_page(page, mode)) {
                case 0:
                        nr_pages = hpage_nr_pages(page);
                        mem_cgroup_update_lru_size(lruvec, lru, -nr_pages);
                        list_move(&page->lru, dst);
                        nr_taken += nr_pages;
                        break;

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

                default:
                        BUG();
                }
        }

        *nr_scanned = scan;
        trace_mm_vmscan_lru_isolate(sc->order, nr_to_scan, scan,
                                    nr_taken, mode, is_file_lru(lru));
        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);

        if (PageLRU(page)) {
                struct zone *zone = page_zone(page);
                struct lruvec *lruvec;

                spin_lock_irq(&zone->lru_lock);
                lruvec = mem_cgroup_page_lruvec(page, zone);
                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(&zone->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 zone *zone, int file,
                struct scan_control *sc)
{
        unsigned long inactive, isolated;

        if (current_is_kswapd())
                return 0;

        if (!global_reclaim(sc))
                return 0;

        if (file) {
                inactive = zone_page_state(zone, NR_INACTIVE_FILE);
                isolated = zone_page_state(zone, NR_ISOLATED_FILE);
        } else {
                inactive = zone_page_state(zone, NR_INACTIVE_ANON);
                isolated = zone_page_state(zone, 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_IOFS) == GFP_IOFS)
                inactive >>= 3;

        return isolated > inactive;
}

static noinline_for_stack void
putback_inactive_pages(struct lruvec *lruvec, struct list_head *page_list)
{
        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
        struct zone *zone = lruvec_zone(lruvec);
        LIST_HEAD(pages_to_free);

        /*
         * Put back any unfreeable pages.
         */
        while (!list_empty(page_list)) {
                struct page *page = lru_to_page(page_list);
                int lru;

                VM_BUG_ON_PAGE(PageLRU(page), page);
                list_del(&page->lru);
                if (unlikely(!page_evictable(page))) {
                        spin_unlock_irq(&zone->lru_lock);
                        putback_lru_page(page);
                        spin_lock_irq(&zone->lru_lock);
                        continue;
                }

                lruvec = mem_cgroup_page_lruvec(page, zone);

                SetPageLRU(page);
                lru = page_lru(page);
                add_page_to_lru_list(page, lruvec, lru);

                if (is_active_lru(lru)) {
                        int file = is_file_lru(lru);
                        int numpages = hpage_nr_pages(page);
                        reclaim_stat->recent_rotated[file] += numpages;
                }
                if (put_page_testzero(page)) {
                        __ClearPageLRU(page);
                        __ClearPageActive(page);
                        del_page_from_lru_list(page, lruvec, lru);

                        if (unlikely(PageCompound(page))) {
                                spin_unlock_irq(&zone->lru_lock);
                                (*get_compound_page_dtor(page))(page);
                                spin_lock_irq(&zone->lru_lock);
                        } else
                                list_add(&page->lru, &pages_to_free);
                }
        }

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

/*
 * shrink_inactive_list() is a helper for shrink_zone().  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;
        unsigned long nr_dirty = 0;
        unsigned long nr_congested = 0;
        unsigned long nr_unqueued_dirty = 0;
        unsigned long nr_writeback = 0;
        unsigned long nr_immediate = 0;
        isolate_mode_t isolate_mode = 0;
        int file = is_file_lru(lru);
        struct zone *zone = lruvec_zone(lruvec);
        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;

        while (unlikely(too_many_isolated(zone, file, sc))) {
                congestion_wait(BLK_RW_ASYNC, HZ/10);

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

        lru_add_drain();

        if (!sc->may_unmap)
                isolate_mode |= ISOLATE_UNMAPPED;
        if (!sc->may_writepage)
                isolate_mode |= ISOLATE_CLEAN;

        spin_lock_irq(&zone->lru_lock);

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

        __mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
        __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);

        if (global_reclaim(sc)) {
                zone->pages_scanned += nr_scanned;
                if (current_is_kswapd())
                        __count_zone_vm_events(PGSCAN_KSWAPD, zone, nr_scanned);
                else
                        __count_zone_vm_events(PGSCAN_DIRECT, zone, nr_scanned);
        }
        spin_unlock_irq(&zone->lru_lock);

        if (nr_taken == 0)
                return 0;

        nr_reclaimed = shrink_page_list(&page_list, zone, sc, TTU_UNMAP,
                                &nr_dirty, &nr_unqueued_dirty, &nr_congested,
                                &nr_writeback, &nr_immediate,
                                false);

        spin_lock_irq(&zone->lru_lock);

        reclaim_stat->recent_scanned[file] += nr_taken;

        if (global_reclaim(sc)) {
                if (current_is_kswapd())
                        __count_zone_vm_events(PGSTEAL_KSWAPD, zone,
                                               nr_reclaimed);
                else
                        __count_zone_vm_events(PGSTEAL_DIRECT, zone,
                                               nr_reclaimed);
        }

        putback_inactive_pages(lruvec, &page_list);

        __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);

        spin_unlock_irq(&zone->lru_lock);

        free_hot_cold_page_list(&page_list, 1);

        /*
         * 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 zone is flagged ZONE_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 (nr_writeback && nr_writeback == nr_taken)
                zone_set_flag(zone, ZONE_WRITEBACK);

        /*
         * memcg will stall in page writeback so only consider forcibly
         * stalling for global reclaim
         */
        if (global_reclaim(sc)) {
                /*
                 * Tag a zone as congested if all the dirty pages scanned were
                 * backed by a congested BDI and wait_iff_congested will stall.
                 */
                if (nr_dirty && nr_dirty == nr_congested)
                        zone_set_flag(zone, ZONE_CONGESTED);

                /*
                 * If dirty pages are scanned that are not queued for IO, it
                 * implies that flushers are not keeping up. In this case, flag
                 * the zone ZONE_TAIL_LRU_DIRTY and kswapd will start writing
                 * pages from reclaim context. It will forcibly stall in the
                 * next check.
                 */
                if (nr_unqueued_dirty == nr_taken)
                        zone_set_flag(zone, ZONE_TAIL_LRU_DIRTY);

                /*
                 * In addition, 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 (nr_unqueued_dirty == nr_taken || nr_immediate)
                        congestion_wait(BLK_RW_ASYNC, HZ/10);
        }

        /*
         * Stall direct reclaim for IO completions if underlying BDIs or zone
         * 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())
                wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);

        trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
                zone_idx(zone),
                nr_scanned, nr_reclaimed,
                sc->priority,
                trace_shrink_flags(file));
        return nr_reclaimed;
}

/*
 * This moves pages from the active list to the inactive 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->_count against each page.
 * But we had to alter page->flags anyway.
 */

static void move_active_pages_to_lru(struct lruvec *lruvec,
                                     struct list_head *list,
                                     struct list_head *pages_to_free,
                                     enum lru_list lru)
{
        struct zone *zone = lruvec_zone(lruvec);
        unsigned long pgmoved = 0;
        struct page *page;
        int nr_pages;

        while (!list_empty(list)) {
                page = lru_to_page(list);
                lruvec = mem_cgroup_page_lruvec(page, zone);

                VM_BUG_ON_PAGE(PageLRU(page), page);
                SetPageLRU(page);

                nr_pages = hpage_nr_pages(page);
                mem_cgroup_update_lru_size(lruvec, lru, nr_pages);
                list_move(&page->lru, &lruvec->lists[lru]);
                pgmoved += nr_pages;

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

                        if (unlikely(PageCompound(page))) {
                                spin_unlock_irq(&zone->lru_lock);
                                (*get_compound_page_dtor(page))(page);
                                spin_lock_irq(&zone->lru_lock);
                        } else
                                list_add(&page->lru, pages_to_free);
                }
        }
        __mod_zone_page_state(zone, NR_LRU_BASE + lru, pgmoved);
        if (!is_active_lru(lru))
                __count_vm_events(PGDEACTIVATE, pgmoved);
}

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 long nr_rotated = 0;
        isolate_mode_t isolate_mode = 0;
        int file = is_file_lru(lru);
        struct zone *zone = lruvec_zone(lruvec);

        lru_add_drain();

        if (!sc->may_unmap)
                isolate_mode |= ISOLATE_UNMAPPED;
        if (!sc->may_writepage)
                isolate_mode |= ISOLATE_CLEAN;

        spin_lock_irq(&zone->lru_lock);

        nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
                                     &nr_scanned, sc, isolate_mode, lru);
        if (global_reclaim(sc))
                zone->pages_scanned += nr_scanned;

        reclaim_stat->recent_scanned[file] += nr_taken;

        __count_zone_vm_events(PGREFILL, zone, nr_scanned);
        __mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
        __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);
        spin_unlock_irq(&zone->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 */
                list_add(&page->lru, &l_inactive);
        }

        /*
         * Move pages back to the lru list.
         */
        spin_lock_irq(&zone->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_ratio.
         */
        reclaim_stat->recent_rotated[file] += nr_rotated;

        move_active_pages_to_lru(lruvec, &l_active, &l_hold, lru);
        move_active_pages_to_lru(lruvec, &l_inactive, &l_hold, lru - LRU_ACTIVE);
        __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
        spin_unlock_irq(&zone->lru_lock);

        free_hot_cold_page_list(&l_hold, 1);
}

#ifdef CONFIG_SWAP
static int inactive_anon_is_low_global(struct zone *zone)
{
        unsigned long active, inactive;

        active = zone_page_state(zone, NR_ACTIVE_ANON);
        inactive = zone_page_state(zone, NR_INACTIVE_ANON);

        if (inactive * zone->inactive_ratio < active)
                return 1;

        return 0;
}

/**
 * inactive_anon_is_low - check if anonymous pages need to be deactivated
 * @lruvec: LRU vector to check
 *
 * Returns true if the zone does not have enough inactive anon pages,
 * meaning some active anon pages need to be deactivated.
 */
static int inactive_anon_is_low(struct lruvec *lruvec)
{
        /*
         * If we don't have swap space, anonymous page deactivation
         * is pointless.
         */
        if (!total_swap_pages)
                return 0;

        if (!mem_cgroup_disabled())
                return mem_cgroup_inactive_anon_is_low(lruvec);

        return inactive_anon_is_low_global(lruvec_zone(lruvec));
}
#else
static inline int inactive_anon_is_low(struct lruvec *lruvec)
{
        return 0;
}
#endif

/**
 * inactive_file_is_low - check if file pages need to be deactivated
 * @lruvec: LRU vector to check
 *
 * When the system is doing streaming IO, memory pressure here
 * ensures that active file pages get deactivated, until more
 * than half of the file pages are on the inactive list.
 *
 * Once we get to that situation, protect the system's working
 * set from being evicted by disabling active file page aging.
 *
 * This uses a different ratio than the anonymous pages, because
 * the page cache uses a use-once replacement algorithm.
 */
static int inactive_file_is_low(struct lruvec *lruvec)
{
        unsigned long inactive;
        unsigned long active;

        inactive = get_lru_size(lruvec, LRU_INACTIVE_FILE);
        active = get_lru_size(lruvec, LRU_ACTIVE_FILE);

        return active > inactive;
}

static int inactive_list_is_low(struct lruvec *lruvec, enum lru_list lru)
{
        if (is_file_lru(lru))
                return inactive_file_is_low(lruvec);
        else
                return inactive_anon_is_low(lruvec);
}

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, lru))
                        shrink_active_list(nr_to_scan, lruvec, sc, lru);
                return 0;
        }

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

static int vmscan_swappiness(struct scan_control *sc)
{
        if (global_reclaim(sc))
                return vm_swappiness;
        return mem_cgroup_swappiness(sc->target_mem_cgroup);
}

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 scan_control *sc,
                           unsigned long *nr)
{
        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
        u64 fraction[2];
        u64 denominator = 0;    /* gcc */
        struct zone *zone = lruvec_zone(lruvec);
        unsigned long anon_prio, file_prio;
        enum scan_balance scan_balance;
        unsigned long anon, file, free;
        bool force_scan = false;
        unsigned long ap, fp;
        enum lru_list lru;

        /*
         * If the zone or memcg is small, nr[l] can be 0.  This
         * results in no scanning on this priority and a potential
         * priority drop.  Global direct reclaim can go to the next
         * zone and tends to have no problems. Global kswapd is for
         * zone balancing and it needs to scan a minimum amount. When
         * reclaiming for a memcg, a priority drop can cause high
         * latencies, so it's better to scan a minimum amount there as
         * well.
         */
        if (current_is_kswapd() && !zone_reclaimable(zone))
                force_scan = true;
        if (!global_reclaim(sc))
                force_scan = true;

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

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

        /*
         * If it's foreseeable that reclaiming the file cache won't be
         * enough to get the zone back into a desirable shape, we have
         * to swap.  Better start now and leave the - probably heavily
         * thrashing - remaining file pages alone.
         */
        if (global_reclaim(sc)) {
                free = zone_page_state(zone, NR_FREE_PAGES);
                if (unlikely(file + free <= high_wmark_pages(zone))) {
                        scan_balance = SCAN_ANON;
                        goto out;
                }
        }

        /*
         * There is enough inactive page cache, do not reclaim
         * anything from the anonymous working set right now.
         */
        if (!inactive_file_is_low(lruvec)) {
                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 = vmscan_swappiness(sc);
        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]
         */
        spin_lock_irq(&zone->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(&zone->lru_lock);

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

                size = get_lru_size(lruvec, lru);
                scan = size >> sc->priority;

                if (!scan && force_scan)
                        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.
                         */
                        scan = div64_u64(scan * fraction[file], denominator);
                        break;
                case SCAN_FILE:
                case SCAN_ANON:
                        /* Scan one type exclusively */
                        if ((scan_balance == SCAN_FILE) != file)
                                scan = 0;
                        break;
                default:
                        /* Look ma, no brain */
                        BUG();
                }
                nr[lru] = scan;
        }
}

/*
 * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
 */
static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
{
        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 = false;

        get_scan_count(lruvec, sc, nr);

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

        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);
                        }
                }

                if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
                        continue;

                /*
                 * For global direct reclaim, reclaim only the number of pages
                 * requested. Less care is taken to scan proportionally as it
                 * is more important to minimise direct reclaim stall latency
                 * than it is to properly age the LRU lists.
                 */
                if (global_reclaim(sc) && !current_is_kswapd())
                        break;

                /*
                 * For kswapd and memcg, reclaim at least the number of pages
                 * requested. Ensure that the anon and file LRUs shrink
                 * 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];

                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_anon_is_low(lruvec))
                shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                                   sc, LRU_ACTIVE_ANON);

        throttle_vm_writeout(sc->gfp_mask);
}

/* 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 zone *zone,
                                        unsigned long nr_reclaimed,
                                        unsigned long nr_scanned,
                                        struct scan_control *sc)
{
        unsigned long pages_for_compaction;
        unsigned long inactive_lru_pages;

        /* 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_REPEAT) {
                /*
                 * For __GFP_REPEAT 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_REPEAT caller really wants to succeed
                 */
                if (!nr_reclaimed && !nr_scanned)
                        return false;
        } else {
                /*
                 * For non-__GFP_REPEAT 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 = (2UL << sc->order);
        inactive_lru_pages = zone_page_state(zone, NR_INACTIVE_FILE);
        if (get_nr_swap_pages() > 0)
                inactive_lru_pages += zone_page_state(zone, 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 */
        switch (compaction_suitable(zone, sc->order)) {
        case COMPACT_PARTIAL:
        case COMPACT_CONTINUE:
                return false;
        default:
                return true;
        }
}

static void shrink_zone(struct zone *zone, struct scan_control *sc)
{
        unsigned long nr_reclaimed, nr_scanned;

        do {
                struct mem_cgroup *root = sc->target_mem_cgroup;
                struct mem_cgroup_reclaim_cookie reclaim = {
                        .zone = zone,
                        .priority = sc->priority,
                };
                struct mem_cgroup *memcg;

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

                memcg = mem_cgroup_iter(root, NULL, &reclaim);
                do {
                        struct lruvec *lruvec;

                        lruvec = mem_cgroup_zone_lruvec(zone, memcg);

                        shrink_lruvec(lruvec, sc);

                        /*
                         * Direct reclaim and kswapd have to scan all memory
                         * cgroups to fulfill the overall scan target for the
                         * zone.
                         *
                         * 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 (!global_reclaim(sc) &&
                                        sc->nr_reclaimed >= sc->nr_to_reclaim) {
                                mem_cgroup_iter_break(root, memcg);
                                break;
                        }
                        memcg = mem_cgroup_iter(root, memcg, &reclaim);
                } while (memcg);

                vmpressure(sc->gfp_mask, sc->target_mem_cgroup,
                           sc->nr_scanned - nr_scanned,
                           sc->nr_reclaimed - nr_reclaimed);

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

/* Returns true if compaction should go ahead for a high-order request */
static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
{
        unsigned long balance_gap, watermark;
        bool watermark_ok;

        /* Do not consider compaction for orders reclaim is meant to satisfy */
        if (sc->order <= PAGE_ALLOC_COSTLY_ORDER)
                return false;

        /*
         * Compaction takes time to run and there are potentially other
         * callers using the pages just freed. Continue reclaiming until
         * there is a buffer of free pages available to give compaction
         * a reasonable chance of completing and allocating the page
         */
        balance_gap = min(low_wmark_pages(zone),
                (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
                        KSWAPD_ZONE_BALANCE_GAP_RATIO);
        watermark = high_wmark_pages(zone) + balance_gap + (2UL << sc->order);
        watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, 0, 0);

        /*
         * If compaction is deferred, reclaim up to a point where
         * compaction will have a chance of success when re-enabled
         */
        if (compaction_deferred(zone, sc->order))
                return watermark_ok;

        /* If compaction is not ready to start, keep reclaiming */
        if (!compaction_suitable(zone, sc->order))
                return false;

        return watermark_ok;
}

/*
 * 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.
 *
 * We reclaim from a zone even if that zone is over high_wmark_pages(zone).
 * Because:
 * a) The caller may be trying to free *extra* pages to satisfy a higher-order
 *    allocation or
 * b) The target zone may be at high_wmark_pages(zone) but the lower zones
 *    must go *over* high_wmark_pages(zone) to satisfy the `incremental min'
 *    zone defense algorithm.
 *
 * If a zone is deemed to be full of pinned pages then just give it a light
 * scan then give up on it.
 *
 * This function returns true if a zone is being reclaimed for a costly
 * high-order allocation and compaction is ready to begin. This indicates to
 * the caller that it should consider retrying the allocation instead of
 * further reclaim.
 */
static bool 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;
        unsigned long lru_pages = 0;
        bool aborted_reclaim = false;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        struct shrink_control shrink = {
                .gfp_mask = sc->gfp_mask,
        };

        /*
         * 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
         */
        if (buffer_heads_over_limit)
                sc->gfp_mask |= __GFP_HIGHMEM;

        nodes_clear(shrink.nodes_to_scan);

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

                        lru_pages += zone_reclaimable_pages(zone);
                        node_set(zone_to_nid(zone), shrink.nodes_to_scan);

                        if (sc->priority != DEF_PRIORITY &&
                            !zone_reclaimable(zone))
                                continue;       /* Let kswapd poll it */
                        if (IS_ENABLED(CONFIG_COMPACTION)) {
                                /*
                                 * 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 (compaction_ready(zone, sc)) {
                                        aborted_reclaim = true;
                                        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,
                                                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() */
                }

                shrink_zone(zone, sc);
        }

        /*
         * Don't shrink slabs when reclaiming memory from over limit cgroups
         * but do shrink slab at least once when aborting reclaim for
         * compaction to avoid unevenly scanning file/anon LRU pages over slab
         * pages.
         */
        if (global_reclaim(sc)) {
                shrink_slab(&shrink, sc->nr_scanned, lru_pages);
                if (reclaim_state) {
                        sc->nr_reclaimed += reclaim_state->reclaimed_slab;
                        reclaim_state->reclaimed_slab = 0;
                }
        }

        return aborted_reclaim;
}

/* All zones in zonelist are unreclaimable? */
static bool all_unreclaimable(struct zonelist *zonelist,
                struct scan_control *sc)
{
        struct zoneref *z;
        struct zone *zone;

        for_each_zone_zonelist_nodemask(zone, z, zonelist,
                        gfp_zone(sc->gfp_mask), sc->nodemask) {
                if (!populated_zone(zone))
                        continue;
                if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                        continue;
                if (zone_reclaimable(zone))
                        return false;
        }

        return true;
}

/*
 * 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)
{
        unsigned long total_scanned = 0;
        unsigned long writeback_threshold;
        bool aborted_reclaim;

        delayacct_freepages_start();

        if (global_reclaim(sc))
                count_vm_event(ALLOCSTALL);

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

                total_scanned += sc->nr_scanned;
                if (sc->nr_reclaimed >= sc->nr_to_reclaim)
                        goto out;

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

                /*
                 * Try to write back as many pages as we just scanned.  This
                 * tends to cause slow streaming writers to write data to the
                 * disk smoothly, at the dirtying rate, which is nice.   But
                 * that's undesirable in laptop mode, where we *want* lumpy
                 * writeout.  So in laptop mode, write out the whole world.
                 */
                writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
                if (total_scanned > writeback_threshold) {
                        wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
                                                WB_REASON_TRY_TO_FREE_PAGES);
                        sc->may_writepage = 1;
                }
        } while (--sc->priority >= 0 && !aborted_reclaim);

out:
        delayacct_freepages_end();

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

        /*
         * As hibernation is going on, kswapd is freezed so that it can't mark
         * the zone into all_unreclaimable. Thus bypassing all_unreclaimable
         * check.
         */
        if (oom_killer_disabled)
                return 0;

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

        /* top priority shrink_zones still had more to do? don't OOM, then */
        if (global_reclaim(sc) && !all_unreclaimable(zonelist, sc))
                return 1;

        return 0;
}

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

        for (i = 0; i <= ZONE_NORMAL; i++) {
                zone = &pgdat->node_zones[i];
                pfmemalloc_reserve += min_wmark_pages(zone);
                free_pages += zone_page_state(zone, NR_FREE_PAGES);
        }

        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->classzone_idx = min(pgdat->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 zone *zone;
        int high_zoneidx = gfp_zone(gfp_mask);
        pg_data_t *pgdat;

        /*
         * 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 */
        first_zones_zonelist(zonelist, high_zoneidx, NULL, &zone);
        pgdat = zone->zone_pgdat;
        if (pfmemalloc_watermark_ok(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,
                        pfmemalloc_watermark_ok(pgdat), HZ);

                goto check_pending;
        }

        /* Throttle until kswapd wakes the process */
        wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
                pfmemalloc_watermark_ok(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 = {
                .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
                .may_writepage = !laptop_mode,
                .nr_to_reclaim = SWAP_CLUSTER_MAX,
                .may_unmap = 1,
                .may_swap = 1,
                .order = order,
                .priority = DEF_PRIORITY,
                .target_mem_cgroup = NULL,
                .nodemask = nodemask,
        };

        /*
         * 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(gfp_mask, zonelist, nodemask))
                return 1;

        trace_mm_vmscan_direct_reclaim_begin(order,
                                sc.may_writepage,
                                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_zone(struct mem_cgroup *memcg,
                                                gfp_t gfp_mask, bool noswap,
                                                struct zone *zone,
                                                unsigned long *nr_scanned)
{
        struct scan_control sc = {
                .nr_scanned = 0,
                .nr_to_reclaim = SWAP_CLUSTER_MAX,
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .may_swap = !noswap,
                .order = 0,
                .priority = 0,
                .target_mem_cgroup = memcg,
        };
        struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);

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

        trace_mm_vmscan_memcg_softlimit_reclaim_begin(sc.order,
                                                      sc.may_writepage,
                                                      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_zone from balance_pgdat
         * will pick up pages from other mem cgroup's as well. We hack
         * the priority and make it zero.
         */
        shrink_lruvec(lruvec, &sc);

        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,
                                           gfp_t gfp_mask,
                                           bool noswap)
{
        struct zonelist *zonelist;
        unsigned long nr_reclaimed;
        int nid;
        struct scan_control sc = {
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .may_swap = !noswap,
                .nr_to_reclaim = SWAP_CLUSTER_MAX,
                .order = 0,
                .priority = DEF_PRIORITY,
                .target_mem_cgroup = memcg,
                .nodemask = NULL, /* we don't care the placement */
                .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
                                (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
        };

        /*
         * 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;

        trace_mm_vmscan_memcg_reclaim_begin(0,
                                            sc.may_writepage,
                                            sc.gfp_mask);

        nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

        trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);

        return nr_reclaimed;
}
#endif

static void age_active_anon(struct zone *zone, 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_zone_lruvec(zone, memcg);

                if (inactive_anon_is_low(lruvec))
                        shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                                           sc, LRU_ACTIVE_ANON);

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

static bool zone_balanced(struct zone *zone, int order,
                          unsigned long balance_gap, int classzone_idx)
{
        if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone) +
                                    balance_gap, classzone_idx, 0))
                return false;

        if (IS_ENABLED(CONFIG_COMPACTION) && order &&
            !compaction_suitable(zone, order))
                return false;

        return true;
}

/*
 * pgdat_balanced() is used when checking if a node is balanced.
 *
 * For order-0, all zones must be balanced!
 *
 * For high-order allocations only zones that meet watermarks and are in a
 * zone allowed by the callers classzone_idx are added to balanced_pages. The
 * total of balanced pages must be at least 25% of the zones allowed by
 * classzone_idx for the node to be considered balanced. Forcing all zones to
 * be balanced for high orders can cause excessive reclaim when there are
 * imbalanced zones.
 * The choice of 25% is due to
 *   o a 16M DMA zone that is balanced will not balance a zone on any
 *     reasonable sized machine
 *   o On all other machines, the top zone must be at least a reasonable
 *     percentage of the middle zones. For example, on 32-bit x86, highmem
 *     would need to be at least 256M for it to be balance a whole node.
 *     Similarly, on x86-64 the Normal zone would need to be at least 1G
 *     to balance a node on its own. These seemed like reasonable ratios.
 */
static bool pgdat_balanced(pg_data_t *pgdat, int order, int classzone_idx)
{
        unsigned long managed_pages = 0;
        unsigned long balanced_pages = 0;
        int i;

        /* Check the watermark levels */
        for (i = 0; i <= classzone_idx; i++) {
                struct zone *zone = pgdat->node_zones + i;

                if (!populated_zone(zone))
                        continue;

                managed_pages += zone->managed_pages;

                /*
                 * A special case here:
                 *
                 * balance_pgdat() skips over all_unreclaimable after
                 * DEF_PRIORITY. Effectively, it considers them balanced so
                 * they must be considered balanced here as well!
                 */
                if (!zone_reclaimable(zone)) {
                        balanced_pages += zone->managed_pages;
                        continue;
                }

                if (zone_balanced(zone, order, 0, i))
                        balanced_pages += zone->managed_pages;
                else if (!order)
                        return false;
        }

        if (order)
                return balanced_pages >= (managed_pages >> 2);
        else
                return true;
}

/*
 * 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, long remaining,
                                        int classzone_idx)
{
        /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
        if (remaining)
                return false;

        /*
         * There is a potential race between when kswapd checks its watermarks
         * and a process gets throttled. There is also a potential race if
         * processes get throttled, kswapd wakes, a large process exits therby
         * balancing the zones that causes kswapd to miss a wakeup. If kswapd
         * is going to sleep, no process should be sleeping on pfmemalloc_wait
         * so wake them now if necessary. If necessary, processes will wake
         * kswapd and get throttled again
         */
        if (waitqueue_active(&pgdat->pfmemalloc_wait)) {
                wake_up(&pgdat->pfmemalloc_wait);
                return false;
        }

        return pgdat_balanced(pgdat, order, classzone_idx);
}

/*
 * kswapd shrinks the zone by the number of pages required to reach
 * the high watermark.
 *
 * 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_zone(struct zone *zone,
                               int classzone_idx,
                               struct scan_control *sc,
                               unsigned long lru_pages,
                               unsigned long *nr_attempted)
{
        int testorder = sc->order;
        unsigned long balance_gap;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        struct shrink_control shrink = {
                .gfp_mask = sc->gfp_mask,
        };
        bool lowmem_pressure;

        /* Reclaim above the high watermark. */
        sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));

        /*
         * Kswapd reclaims only single pages with compaction enabled. Trying
         * too hard to reclaim until contiguous free pages have become
         * available can hurt performance by evicting too much useful data
         * from memory. Do not reclaim more than needed for compaction.
         */
        if (IS_ENABLED(CONFIG_COMPACTION) && sc->order &&
                        compaction_suitable(zone, sc->order) !=
                                COMPACT_SKIPPED)
                testorder = 0;

        /*
         * We put equal pressure on every zone, unless one zone has way too
         * many pages free already. The "too many pages" is defined as the
         * high wmark plus a "gap" where the gap is either the low
         * watermark or 1% of the zone, whichever is smaller.
         */
        balance_gap = min(low_wmark_pages(zone),
                (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
                KSWAPD_ZONE_BALANCE_GAP_RATIO);

        /*
         * If there is no low memory pressure or the zone is balanced then no
         * reclaim is necessary
         */
        lowmem_pressure = (buffer_heads_over_limit && is_highmem(zone));
        if (!lowmem_pressure && zone_balanced(zone, testorder,
                                                balance_gap, classzone_idx))
                return true;

        shrink_zone(zone, sc);
        nodes_clear(shrink.nodes_to_scan);
        node_set(zone_to_nid(zone), shrink.nodes_to_scan);

        reclaim_state->reclaimed_slab = 0;
        shrink_slab(&shrink, sc->nr_scanned, lru_pages);
        sc->nr_reclaimed += reclaim_state->reclaimed_slab;

        /* Account for the number of pages attempted to reclaim */
        *nr_attempted += sc->nr_to_reclaim;

        zone_clear_flag(zone, ZONE_WRITEBACK);

        /*
         * If a zone reaches its high watermark, consider it to be no longer
         * congested. It's possible there are dirty pages backed by congested
         * BDIs but as pressure is relieved, speculatively avoid congestion
         * waits.
         */
        if (zone_reclaimable(zone) &&
            zone_balanced(zone, testorder, 0, classzone_idx)) {
                zone_clear_flag(zone, ZONE_CONGESTED);
                zone_clear_flag(zone, ZONE_TAIL_LRU_DIRTY);
        }

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

/*
 * For kswapd, balance_pgdat() will work across all this node's zones until
 * they are all at high_wmark_pages(zone).
 *
 * Returns the final order kswapd was reclaiming at
 *
 * There is special handling here for zones which are full of pinned pages.
 * This can happen if the pages are all mlocked, or if they are all used by
 * device drivers (say, ZONE_DMA).  Or if they are all in use by hugetlb.
 * What we do is to detect the case where all pages in the zone have been
 * scanned twice and there has been zero successful reclaim.  Mark the zone as
 * dead and from now on, only perform a short scan.  Basically we're polling
 * the zone for when the problem goes away.
 *
 * 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), we scan that zone and the
 * lower zones regardless of the number of free pages in the lower zones. This
 * interoperates with the page allocator fallback scheme to ensure that aging
 * of pages is balanced across the zones.
 */
static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
                                                        int *classzone_idx)
{
        int i;
        int end_zone = 0;       /* Inclusive.  0 = ZONE_DMA */
        unsigned long nr_soft_reclaimed;
        unsigned long nr_soft_scanned;
        struct scan_control sc = {
                .gfp_mask = GFP_KERNEL,
                .priority = DEF_PRIORITY,
                .may_unmap = 1,
                .may_swap = 1,
                .may_writepage = !laptop_mode,
                .order = order,
                .target_mem_cgroup = NULL,
        };
        count_vm_event(PAGEOUTRUN);

        do {
                unsigned long lru_pages = 0;
                unsigned long nr_attempted = 0;
                bool raise_priority = true;
                bool pgdat_needs_compaction = (order > 0);

                sc.nr_reclaimed = 0;

                /*
                 * Scan in the highmem->dma direction for the highest
                 * zone which needs scanning
                 */
                for (i = pgdat->nr_zones - 1; i >= 0; i--) {
                        struct zone *zone = pgdat->node_zones + i;

                        if (!populated_zone(zone))
                                continue;

                        if (sc.priority != DEF_PRIORITY &&
                            !zone_reclaimable(zone))
                                continue;

                        /*
                         * Do some background aging of the anon list, to give
                         * pages a chance to be referenced before reclaiming.
                         */
                        age_active_anon(zone, &sc);

                        /*
                         * If the number of buffer_heads in the machine
                         * exceeds the maximum allowed level and this node
                         * has a highmem zone, force kswapd to reclaim from
                         * it to relieve lowmem pressure.
                         */
                        if (buffer_heads_over_limit && is_highmem_idx(i)) {
                                end_zone = i;
                                break;
                        }

                        if (!zone_balanced(zone, order, 0, 0)) {
                                end_zone = i;
                                break;
                        } else {
                                /*
                                 * If balanced, clear the dirty and congested
                                 * flags
                                 */
                                zone_clear_flag(zone, ZONE_CONGESTED);
                                zone_clear_flag(zone, ZONE_TAIL_LRU_DIRTY);
                        }
                }

                if (i < 0)
                        goto out;

                for (i = 0; i <= end_zone; i++) {
                        struct zone *zone = pgdat->node_zones + i;

                        if (!populated_zone(zone))
                                continue;

                        lru_pages += zone_reclaimable_pages(zone);

                        /*
                         * If any zone is currently balanced then kswapd will
                         * not call compaction as it is expected that the
                         * necessary pages are already available.
                         */
                        if (pgdat_needs_compaction &&
                                        zone_watermark_ok(zone, order,
                                                low_wmark_pages(zone),
                                                *classzone_idx, 0))
                                pgdat_needs_compaction = false;
                }

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

                /*
                 * Now scan the zone in the dma->highmem direction, stopping
                 * at the last zone which needs scanning.
                 *
                 * We do this because the page allocator works in the opposite
                 * direction.  This prevents the page allocator from allocating
                 * pages behind kswapd's direction of progress, which would
                 * cause too much scanning of the lower zones.
                 */
                for (i = 0; i <= end_zone; i++) {
                        struct zone *zone = pgdat->node_zones + i;

                        if (!populated_zone(zone))
                                continue;

                        if (sc.priority != DEF_PRIORITY &&
                            !zone_reclaimable(zone))
                                continue;

                        sc.nr_scanned = 0;

                        nr_soft_scanned = 0;
                        /*
                         * Call soft limit reclaim before calling shrink_zone.
                         */
                        nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
                                                        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_zone(zone, end_zone, &sc,
                                        lru_pages, &nr_attempted))
                                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) &&
                                pfmemalloc_watermark_ok(pgdat))
                        wake_up(&pgdat->pfmemalloc_wait);

                /*
                 * Fragmentation may mean that the system cannot be rebalanced
                 * for high-order allocations in all zones. If twice the
                 * allocation size has been reclaimed and the zones are still
                 * not balanced then recheck the watermarks at order-0 to
                 * prevent kswapd reclaiming excessively. Assume that a
                 * process requested a high-order can direct reclaim/compact.
                 */
                if (order && sc.nr_reclaimed >= 2UL << order)
                        order = sc.order = 0;

                /* Check if kswapd should be suspending */
                if (try_to_freeze() || kthread_should_stop())
                        break;

                /*
                 * Compact if necessary and kswapd is reclaiming at least the
                 * high watermark number of pages as requsted
                 */
                if (pgdat_needs_compaction && sc.nr_reclaimed > nr_attempted)
                        compact_pgdat(pgdat, order);

                /*
                 * Raise priority if scanning rate is too low or there was no
                 * progress in reclaiming pages
                 */
                if (raise_priority || !sc.nr_reclaimed)
                        sc.priority--;
        } while (sc.priority >= 1 &&
                 !pgdat_balanced(pgdat, order, *classzone_idx));

out:
        /*
         * Return the order we were reclaiming at so prepare_kswapd_sleep()
         * makes a decision on the order we were last reclaiming at. However,
         * if another caller entered the allocator slow path while kswapd
         * was awake, order will remain at the higher level
         */
        *classzone_idx = end_zone;
        return order;
}

static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, 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 */
        if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
                remaining = schedule_timeout(HZ/10);
                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 (prepare_kswapd_sleep(pgdat, order, remaining, 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);

                /*
                 * 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);

                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 long order, new_order;
        unsigned balanced_order;
        int classzone_idx, new_classzone_idx;
        int balanced_classzone_idx;
        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);

        lockdep_set_current_reclaim_state(GFP_KERNEL);

        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();

        order = new_order = 0;
        balanced_order = 0;
        classzone_idx = new_classzone_idx = pgdat->nr_zones - 1;
        balanced_classzone_idx = classzone_idx;
        for ( ; ; ) {
                bool ret;

                /*
                 * If the last balance_pgdat was unsuccessful it's unlikely a
                 * new request of a similar or harder type will succeed soon
                 * so consider going to sleep on the basis we reclaimed at
                 */
                if (balanced_classzone_idx >= new_classzone_idx &&
                                        balanced_order == new_order) {
                        new_order = pgdat->kswapd_max_order;
                        new_classzone_idx = pgdat->classzone_idx;
                        pgdat->kswapd_max_order =  0;
                        pgdat->classzone_idx = pgdat->nr_zones - 1;
                }

                if (order < new_order || classzone_idx > new_classzone_idx) {
                        /*
                         * Don't sleep if someone wants a larger 'order'
                         * allocation or has tigher zone constraints
                         */
                        order = new_order;
                        classzone_idx = new_classzone_idx;
                } else {
                        kswapd_try_to_sleep(pgdat, balanced_order,
                                                balanced_classzone_idx);
                        order = pgdat->kswapd_max_order;
                        classzone_idx = pgdat->classzone_idx;
                        new_order = order;
                        new_classzone_idx = classzone_idx;
                        pgdat->kswapd_max_order = 0;
                        pgdat->classzone_idx = pgdat->nr_zones - 1;
                }

                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) {
                        trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
                        balanced_classzone_idx = classzone_idx;
                        balanced_order = balance_pgdat(pgdat, order,
                                                &balanced_classzone_idx);
                }
        }

        current->reclaim_state = NULL;
        return 0;
}

/*
 * A zone is low on free memory, so wake its kswapd task to service it.
 */
void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
{
        pg_data_t *pgdat;

        if (!populated_zone(zone))
                return;

        if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                return;
        pgdat = zone->zone_pgdat;
        if (pgdat->kswapd_max_order < order) {
                pgdat->kswapd_max_order = order;
                pgdat->classzone_idx = min(pgdat->classzone_idx, classzone_idx);
        }
        if (!waitqueue_active(&pgdat->kswapd_wait))
                return;
        if (zone_balanced(zone, order, 0, 0))
                return;

        trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
        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 = {
                .gfp_mask = GFP_HIGHUSER_MOVABLE,
                .may_swap = 1,
                .may_unmap = 1,
                .may_writepage = 1,
                .nr_to_reclaim = nr_to_reclaim,
                .hibernation_mode = 1,
                .order = 0,
                .priority = DEF_PRIORITY,
        };
        struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
        struct task_struct *p = current;
        unsigned long nr_reclaimed;

        p->flags |= PF_MEMALLOC;
        lockdep_set_current_reclaim_state(sc.gfp_mask);
        reclaim_state.reclaimed_slab = 0;
        p->reclaim_state = &reclaim_state;

        nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

        p->reclaim_state = NULL;
        lockdep_clear_current_reclaim_state();
        p->flags &= ~PF_MEMALLOC;

        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 cpu_callback(struct notifier_block *nfb, unsigned long action,
                        void *hcpu)
{
        int nid;

        if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN) {
                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 NOTIFY_OK;
}

/*
 * 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_BOOTING);
                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 lock_memory_hotplug().
 */
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;

        swap_setup();
        for_each_node_state(nid, N_MEMORY)
                kswapd_run(nid);
        hotcpu_notifier(cpu_callback, 0);
        return 0;
}

module_init(kswapd_init)

#ifdef CONFIG_NUMA
/*
 * Zone reclaim mode
 *
 * If non-zero call zone_reclaim when the number of free pages falls below
 * the watermarks.
 */
int zone_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_SWAP (1<<2)     /* Swap pages out during reclaim */

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

/*
 * Percentage of pages in a zone that must be unmapped for zone_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 zone_unmapped_file_pages(struct zone *zone)
{
        unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);
        unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE) +
                zone_page_state(zone, 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 long zone_pagecache_reclaimable(struct zone *zone)
{
        long nr_pagecache_reclaimable;
        long delta = 0;

        /*
         * If RECLAIM_SWAP is set, then all file pages are considered
         * potentially reclaimable. Otherwise, we have to worry about
         * pages like swapcache and zone_unmapped_file_pages() provides
         * a better estimate
         */
        if (zone_reclaim_mode & RECLAIM_SWAP)
                nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
        else
                nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);

        /* If we can't clean pages, remove dirty pages from consideration */
        if (!(zone_reclaim_mode & RECLAIM_WRITE))
                delta += zone_page_state(zone, 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 zone through reclaim.
 */
static int __zone_reclaim(struct zone *zone, 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;
        struct scan_control sc = {
                .may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
                .may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
                .may_swap = 1,
                .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
                .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
                .order = order,
                .priority = ZONE_RECLAIM_PRIORITY,
        };
        struct shrink_control shrink = {
                .gfp_mask = sc.gfp_mask,
        };
        unsigned long nr_slab_pages0, nr_slab_pages1;

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

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

        nr_slab_pages0 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
        if (nr_slab_pages0 > zone->min_slab_pages) {
                /*
                 * shrink_slab() does not currently allow us to determine how
                 * many pages were freed in this zone. So we take the current
                 * number of slab pages and shake the slab until it is reduced
                 * by the same nr_pages that we used for reclaiming unmapped
                 * pages.
                 */
                nodes_clear(shrink.nodes_to_scan);
                node_set(zone_to_nid(zone), shrink.nodes_to_scan);
                for (;;) {
                        unsigned long lru_pages = zone_reclaimable_pages(zone);

                        /* No reclaimable slab or very low memory pressure */
                        if (!shrink_slab(&shrink, sc.nr_scanned, lru_pages))
                                break;

                        /* Freed enough memory */
                        nr_slab_pages1 = zone_page_state(zone,
                                                        NR_SLAB_RECLAIMABLE);
                        if (nr_slab_pages1 + nr_pages <= nr_slab_pages0)
                                break;
                }

                /*
                 * Update nr_reclaimed by the number of slab pages we
                 * reclaimed from this zone.
                 */
                nr_slab_pages1 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
                if (nr_slab_pages1 < nr_slab_pages0)
                        sc.nr_reclaimed += nr_slab_pages0 - nr_slab_pages1;
        }

        p->reclaim_state = NULL;
        current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);
        lockdep_clear_current_reclaim_state();
        return sc.nr_reclaimed >= nr_pages;
}

int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
{
        int node_id;
        int ret;

        /*
         * Zone 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 zone is overallocated. So we do not reclaim
         * if less than a specified percentage of the zone is used by
         * unmapped file backed pages.
         */
        if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
            zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
                return ZONE_RECLAIM_FULL;

        if (!zone_reclaimable(zone))
                return ZONE_RECLAIM_FULL;

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

        /*
         * Only run zone reclaim on the local zone or on zones 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.
         */
        node_id = zone_to_nid(zone);
        if (node_state(node_id, N_CPU) && node_id != numa_node_id())
                return ZONE_RECLAIM_NOSCAN;

        if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
                return ZONE_RECLAIM_NOSCAN;

        ret = __zone_reclaim(zone, gfp_mask, order);
        zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);

        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)
{
        return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
}

#ifdef CONFIG_SHMEM
/**
 * check_move_unevictable_pages - check pages for evictability and move to appropriate zone lru list
 * @pages:      array of pages to check
 * @nr_pages:   number of pages to check
 *
 * Checks pages for evictability and moves them to the appropriate lru list.
 *
 * This function is only used for SysV IPC SHM_UNLOCK.
 */
void check_move_unevictable_pages(struct page **pages, int nr_pages)
{
        struct lruvec *lruvec;
        struct zone *zone = NULL;
        int pgscanned = 0;
        int pgrescued = 0;
        int i;

        for (i = 0; i < nr_pages; i++) {
                struct page *page = pages[i];
                struct zone *pagezone;

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

                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 (zone) {
                __count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
                __count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
                spin_unlock_irq(&zone->lru_lock);
        }
}
#endif /* CONFIG_SHMEM */

static void warn_scan_unevictable_pages(void)
{
        printk_once(KERN_WARNING
                    "%s: The scan_unevictable_pages sysctl/node-interface has been "
                    "disabled for lack of a legitimate use case.  If you have "
                    "one, please send an email to [email protected].\n",
                    current->comm);
}

/*
 * scan_unevictable_pages [vm] sysctl handler.  On demand re-scan of
 * all nodes' unevictable lists for evictable pages
 */
unsigned long scan_unevictable_pages;

int scan_unevictable_handler(struct ctl_table *table, int write,
                           void __user *buffer,
                           size_t *length, loff_t *ppos)
{
        warn_scan_unevictable_pages();
        proc_doulongvec_minmax(table, write, buffer, length, ppos);
        scan_unevictable_pages = 0;
        return 0;
}

#ifdef CONFIG_NUMA
/*
 * per node 'scan_unevictable_pages' attribute.  On demand re-scan of
 * a specified node's per zone unevictable lists for evictable pages.
 */

static ssize_t read_scan_unevictable_node(struct device *dev,
                                          struct device_attribute *attr,
                                          char *buf)
{
        warn_scan_unevictable_pages();
        return sprintf(buf, "0\n");     /* always zero; should fit... */
}

static ssize_t write_scan_unevictable_node(struct device *dev,
                                           struct device_attribute *attr,
                                        const char *buf, size_t count)
{
        warn_scan_unevictable_pages();
        return 1;
}


static DEVICE_ATTR(scan_unevictable_pages, S_IRUGO | S_IWUSR,
                        read_scan_unevictable_node,
                        write_scan_unevictable_node);

int scan_unevictable_register_node(struct node *node)
{
        return device_create_file(&node->dev, &dev_attr_scan_unevictable_pages);
}

void scan_unevictable_unregister_node(struct node *node)
{
        device_remove_file(&node->dev, &dev_attr_scan_unevictable_pages);
}
#endif