mm: filemap: only do access activations on reads

[linux.git] / mm / filemap.c

/*
 *      linux/mm/filemap.c
 *
 * Copyright (C) 1994-1999  Linus Torvalds
 */

/*
 * This file handles the generic file mmap semantics used by
 * most "normal" filesystems (but you don't /have/ to use this:
 * the NFS filesystem used to do this differently, for example)
 */
#include <linux/export.h>
#include <linux/compiler.h>
#include <linux/dax.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include <linux/capability.h>
#include <linux/kernel_stat.h>
#include <linux/gfp.h>
#include <linux/mm.h>
#include <linux/swap.h>
#include <linux/mman.h>
#include <linux/pagemap.h>
#include <linux/file.h>
#include <linux/uio.h>
#include <linux/hash.h>
#include <linux/writeback.h>
#include <linux/backing-dev.h>
#include <linux/pagevec.h>
#include <linux/blkdev.h>
#include <linux/security.h>
#include <linux/cpuset.h>
#include <linux/hardirq.h> /* for BUG_ON(!in_atomic()) only */
#include <linux/hugetlb.h>
#include <linux/memcontrol.h>
#include <linux/cleancache.h>
#include <linux/rmap.h>
#include "internal.h"

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

/*
 * FIXME: remove all knowledge of the buffer layer from the core VM
 */
#include <linux/buffer_head.h> /* for try_to_free_buffers */

#include <asm/mman.h>

/*
 * Shared mappings implemented 30.11.1994. It's not fully working yet,
 * though.
 *
 * Shared mappings now work. 15.8.1995  Bruno.
 *
 * finished 'unifying' the page and buffer cache and SMP-threaded the
 * page-cache, 21.05.1999, Ingo Molnar <[email protected]>
 *
 * SMP-threaded pagemap-LRU 1999, Andrea Arcangeli <[email protected]>
 */

/*
 * Lock ordering:
 *
 *  ->i_mmap_rwsem              (truncate_pagecache)
 *    ->private_lock            (__free_pte->__set_page_dirty_buffers)
 *      ->swap_lock             (exclusive_swap_page, others)
 *        ->mapping->tree_lock
 *
 *  ->i_mutex
 *    ->i_mmap_rwsem            (truncate->unmap_mapping_range)
 *
 *  ->mmap_sem
 *    ->i_mmap_rwsem
 *      ->page_table_lock or pte_lock   (various, mainly in memory.c)
 *        ->mapping->tree_lock  (arch-dependent flush_dcache_mmap_lock)
 *
 *  ->mmap_sem
 *    ->lock_page               (access_process_vm)
 *
 *  ->i_mutex                   (generic_perform_write)
 *    ->mmap_sem                (fault_in_pages_readable->do_page_fault)
 *
 *  bdi->wb.list_lock
 *    sb_lock                   (fs/fs-writeback.c)
 *    ->mapping->tree_lock      (__sync_single_inode)
 *
 *  ->i_mmap_rwsem
 *    ->anon_vma.lock           (vma_adjust)
 *
 *  ->anon_vma.lock
 *    ->page_table_lock or pte_lock     (anon_vma_prepare and various)
 *
 *  ->page_table_lock or pte_lock
 *    ->swap_lock               (try_to_unmap_one)
 *    ->private_lock            (try_to_unmap_one)
 *    ->tree_lock               (try_to_unmap_one)
 *    ->zone.lru_lock           (follow_page->mark_page_accessed)
 *    ->zone.lru_lock           (check_pte_range->isolate_lru_page)
 *    ->private_lock            (page_remove_rmap->set_page_dirty)
 *    ->tree_lock               (page_remove_rmap->set_page_dirty)
 *    bdi.wb->list_lock         (page_remove_rmap->set_page_dirty)
 *    ->inode->i_lock           (page_remove_rmap->set_page_dirty)
 *    ->memcg->move_lock        (page_remove_rmap->lock_page_memcg)
 *    bdi.wb->list_lock         (zap_pte_range->set_page_dirty)
 *    ->inode->i_lock           (zap_pte_range->set_page_dirty)
 *    ->private_lock            (zap_pte_range->__set_page_dirty_buffers)
 *
 * ->i_mmap_rwsem
 *   ->tasklist_lock            (memory_failure, collect_procs_ao)
 */

static void page_cache_tree_delete(struct address_space *mapping,
                                   struct page *page, void *shadow)
{
        struct radix_tree_node *node;
        unsigned long index;
        unsigned int offset;
        unsigned int tag;
        void **slot;

        VM_BUG_ON(!PageLocked(page));

        __radix_tree_lookup(&mapping->page_tree, page->index, &node, &slot);

        if (shadow) {
                mapping->nrexceptional++;
                /*
                 * Make sure the nrexceptional update is committed before
                 * the nrpages update so that final truncate racing
                 * with reclaim does not see both counters 0 at the
                 * same time and miss a shadow entry.
                 */
                smp_wmb();
        }
        mapping->nrpages--;

        if (!node) {
                /* Clear direct pointer tags in root node */
                mapping->page_tree.gfp_mask &= __GFP_BITS_MASK;
                radix_tree_replace_slot(slot, shadow);
                return;
        }

        /* Clear tree tags for the removed page */
        index = page->index;
        offset = index & RADIX_TREE_MAP_MASK;
        for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
                if (test_bit(offset, node->tags[tag]))
                        radix_tree_tag_clear(&mapping->page_tree, index, tag);
        }

        /* Delete page, swap shadow entry */
        radix_tree_replace_slot(slot, shadow);
        workingset_node_pages_dec(node);
        if (shadow)
                workingset_node_shadows_inc(node);
        else
                if (__radix_tree_delete_node(&mapping->page_tree, node))
                        return;

        /*
         * Track node that only contains shadow entries.
         *
         * Avoid acquiring the list_lru lock if already tracked.  The
         * list_empty() test is safe as node->private_list is
         * protected by mapping->tree_lock.
         */
        if (!workingset_node_pages(node) &&
            list_empty(&node->private_list)) {
                node->private_data = mapping;
                list_lru_add(&workingset_shadow_nodes, &node->private_list);
        }
}

/*
 * Delete a page from the page cache and free it. Caller has to make
 * sure the page is locked and that nobody else uses it - or that usage
 * is safe.  The caller must hold the mapping's tree_lock.
 */
void __delete_from_page_cache(struct page *page, void *shadow)
{
        struct address_space *mapping = page->mapping;

        trace_mm_filemap_delete_from_page_cache(page);
        /*
         * if we're uptodate, flush out into the cleancache, otherwise
         * invalidate any existing cleancache entries.  We can't leave
         * stale data around in the cleancache once our page is gone
         */
        if (PageUptodate(page) && PageMappedToDisk(page))
                cleancache_put_page(page);
        else
                cleancache_invalidate_page(mapping, page);

        VM_BUG_ON_PAGE(page_mapped(page), page);
        if (!IS_ENABLED(CONFIG_DEBUG_VM) && unlikely(page_mapped(page))) {
                int mapcount;

                pr_alert("BUG: Bad page cache in process %s  pfn:%05lx\n",
                         current->comm, page_to_pfn(page));
                dump_page(page, "still mapped when deleted");
                dump_stack();
                add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);

                mapcount = page_mapcount(page);
                if (mapping_exiting(mapping) &&
                    page_count(page) >= mapcount + 2) {
                        /*
                         * All vmas have already been torn down, so it's
                         * a good bet that actually the page is unmapped,
                         * and we'd prefer not to leak it: if we're wrong,
                         * some other bad page check should catch it later.
                         */
                        page_mapcount_reset(page);
                        page_ref_sub(page, mapcount);
                }
        }

        page_cache_tree_delete(mapping, page, shadow);

        page->mapping = NULL;
        /* Leave page->index set: truncation lookup relies upon it */

        /* hugetlb pages do not participate in page cache accounting. */
        if (!PageHuge(page))
                __dec_zone_page_state(page, NR_FILE_PAGES);
        if (PageSwapBacked(page))
                __dec_zone_page_state(page, NR_SHMEM);

        /*
         * At this point page must be either written or cleaned by truncate.
         * Dirty page here signals a bug and loss of unwritten data.
         *
         * This fixes dirty accounting after removing the page entirely but
         * leaves PageDirty set: it has no effect for truncated page and
         * anyway will be cleared before returning page into buddy allocator.
         */
        if (WARN_ON_ONCE(PageDirty(page)))
                account_page_cleaned(page, mapping, inode_to_wb(mapping->host));
}

/**
 * delete_from_page_cache - delete page from page cache
 * @page: the page which the kernel is trying to remove from page cache
 *
 * This must be called only on pages that have been verified to be in the page
 * cache and locked.  It will never put the page into the free list, the caller
 * has a reference on the page.
 */
void delete_from_page_cache(struct page *page)
{
        struct address_space *mapping = page->mapping;
        unsigned long flags;

        void (*freepage)(struct page *);

        BUG_ON(!PageLocked(page));

        freepage = mapping->a_ops->freepage;

        spin_lock_irqsave(&mapping->tree_lock, flags);
        __delete_from_page_cache(page, NULL);
        spin_unlock_irqrestore(&mapping->tree_lock, flags);

        if (freepage)
                freepage(page);
        put_page(page);
}
EXPORT_SYMBOL(delete_from_page_cache);

static int filemap_check_errors(struct address_space *mapping)
{
        int ret = 0;
        /* Check for outstanding write errors */
        if (test_bit(AS_ENOSPC, &mapping->flags) &&
            test_and_clear_bit(AS_ENOSPC, &mapping->flags))
                ret = -ENOSPC;
        if (test_bit(AS_EIO, &mapping->flags) &&
            test_and_clear_bit(AS_EIO, &mapping->flags))
                ret = -EIO;
        return ret;
}

/**
 * __filemap_fdatawrite_range - start writeback on mapping dirty pages in range
 * @mapping:    address space structure to write
 * @start:      offset in bytes where the range starts
 * @end:        offset in bytes where the range ends (inclusive)
 * @sync_mode:  enable synchronous operation
 *
 * Start writeback against all of a mapping's dirty pages that lie
 * within the byte offsets <start, end> inclusive.
 *
 * If sync_mode is WB_SYNC_ALL then this is a "data integrity" operation, as
 * opposed to a regular memory cleansing writeback.  The difference between
 * these two operations is that if a dirty page/buffer is encountered, it must
 * be waited upon, and not just skipped over.
 */
int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
                                loff_t end, int sync_mode)
{
        int ret;
        struct writeback_control wbc = {
                .sync_mode = sync_mode,
                .nr_to_write = LONG_MAX,
                .range_start = start,
                .range_end = end,
        };

        if (!mapping_cap_writeback_dirty(mapping))
                return 0;

        wbc_attach_fdatawrite_inode(&wbc, mapping->host);
        ret = do_writepages(mapping, &wbc);
        wbc_detach_inode(&wbc);
        return ret;
}

static inline int __filemap_fdatawrite(struct address_space *mapping,
        int sync_mode)
{
        return __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode);
}

int filemap_fdatawrite(struct address_space *mapping)
{
        return __filemap_fdatawrite(mapping, WB_SYNC_ALL);
}
EXPORT_SYMBOL(filemap_fdatawrite);

int filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
                                loff_t end)
{
        return __filemap_fdatawrite_range(mapping, start, end, WB_SYNC_ALL);
}
EXPORT_SYMBOL(filemap_fdatawrite_range);

/**
 * filemap_flush - mostly a non-blocking flush
 * @mapping:    target address_space
 *
 * This is a mostly non-blocking flush.  Not suitable for data-integrity
 * purposes - I/O may not be started against all dirty pages.
 */
int filemap_flush(struct address_space *mapping)
{
        return __filemap_fdatawrite(mapping, WB_SYNC_NONE);
}
EXPORT_SYMBOL(filemap_flush);

static int __filemap_fdatawait_range(struct address_space *mapping,
                                     loff_t start_byte, loff_t end_byte)
{
        pgoff_t index = start_byte >> PAGE_SHIFT;
        pgoff_t end = end_byte >> PAGE_SHIFT;
        struct pagevec pvec;
        int nr_pages;
        int ret = 0;

        if (end_byte < start_byte)
                goto out;

        pagevec_init(&pvec, 0);
        while ((index <= end) &&
                        (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
                        PAGECACHE_TAG_WRITEBACK,
                        min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1)) != 0) {
                unsigned i;

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

                        /* until radix tree lookup accepts end_index */
                        if (page->index > end)
                                continue;

                        wait_on_page_writeback(page);
                        if (TestClearPageError(page))
                                ret = -EIO;
                }
                pagevec_release(&pvec);
                cond_resched();
        }
out:
        return ret;
}

/**
 * filemap_fdatawait_range - wait for writeback to complete
 * @mapping:            address space structure to wait for
 * @start_byte:         offset in bytes where the range starts
 * @end_byte:           offset in bytes where the range ends (inclusive)
 *
 * Walk the list of under-writeback pages of the given address space
 * in the given range and wait for all of them.  Check error status of
 * the address space and return it.
 *
 * Since the error status of the address space is cleared by this function,
 * callers are responsible for checking the return value and handling and/or
 * reporting the error.
 */
int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
                            loff_t end_byte)
{
        int ret, ret2;

        ret = __filemap_fdatawait_range(mapping, start_byte, end_byte);
        ret2 = filemap_check_errors(mapping);
        if (!ret)
                ret = ret2;

        return ret;
}
EXPORT_SYMBOL(filemap_fdatawait_range);

/**
 * filemap_fdatawait_keep_errors - wait for writeback without clearing errors
 * @mapping: address space structure to wait for
 *
 * Walk the list of under-writeback pages of the given address space
 * and wait for all of them.  Unlike filemap_fdatawait(), this function
 * does not clear error status of the address space.
 *
 * Use this function if callers don't handle errors themselves.  Expected
 * call sites are system-wide / filesystem-wide data flushers: e.g. sync(2),
 * fsfreeze(8)
 */
void filemap_fdatawait_keep_errors(struct address_space *mapping)
{
        loff_t i_size = i_size_read(mapping->host);

        if (i_size == 0)
                return;

        __filemap_fdatawait_range(mapping, 0, i_size - 1);
}

/**
 * filemap_fdatawait - wait for all under-writeback pages to complete
 * @mapping: address space structure to wait for
 *
 * Walk the list of under-writeback pages of the given address space
 * and wait for all of them.  Check error status of the address space
 * and return it.
 *
 * Since the error status of the address space is cleared by this function,
 * callers are responsible for checking the return value and handling and/or
 * reporting the error.
 */
int filemap_fdatawait(struct address_space *mapping)
{
        loff_t i_size = i_size_read(mapping->host);

        if (i_size == 0)
                return 0;

        return filemap_fdatawait_range(mapping, 0, i_size - 1);
}
EXPORT_SYMBOL(filemap_fdatawait);

int filemap_write_and_wait(struct address_space *mapping)
{
        int err = 0;

        if ((!dax_mapping(mapping) && mapping->nrpages) ||
            (dax_mapping(mapping) && mapping->nrexceptional)) {
                err = filemap_fdatawrite(mapping);
                /*
                 * Even if the above returned error, the pages may be
                 * written partially (e.g. -ENOSPC), so we wait for it.
                 * But the -EIO is special case, it may indicate the worst
                 * thing (e.g. bug) happened, so we avoid waiting for it.
                 */
                if (err != -EIO) {
                        int err2 = filemap_fdatawait(mapping);
                        if (!err)
                                err = err2;
                }
        } else {
                err = filemap_check_errors(mapping);
        }
        return err;
}
EXPORT_SYMBOL(filemap_write_and_wait);

/**
 * filemap_write_and_wait_range - write out & wait on a file range
 * @mapping:    the address_space for the pages
 * @lstart:     offset in bytes where the range starts
 * @lend:       offset in bytes where the range ends (inclusive)
 *
 * Write out and wait upon file offsets lstart->lend, inclusive.
 *
 * Note that `lend' is inclusive (describes the last byte to be written) so
 * that this function can be used to write to the very end-of-file (end = -1).
 */
int filemap_write_and_wait_range(struct address_space *mapping,
                                 loff_t lstart, loff_t lend)
{
        int err = 0;

        if ((!dax_mapping(mapping) && mapping->nrpages) ||
            (dax_mapping(mapping) && mapping->nrexceptional)) {
                err = __filemap_fdatawrite_range(mapping, lstart, lend,
                                                 WB_SYNC_ALL);
                /* See comment of filemap_write_and_wait() */
                if (err != -EIO) {
                        int err2 = filemap_fdatawait_range(mapping,
                                                lstart, lend);
                        if (!err)
                                err = err2;
                }
        } else {
                err = filemap_check_errors(mapping);
        }
        return err;
}
EXPORT_SYMBOL(filemap_write_and_wait_range);

/**
 * replace_page_cache_page - replace a pagecache page with a new one
 * @old:        page to be replaced
 * @new:        page to replace with
 * @gfp_mask:   allocation mode
 *
 * This function replaces a page in the pagecache with a new one.  On
 * success it acquires the pagecache reference for the new page and
 * drops it for the old page.  Both the old and new pages must be
 * locked.  This function does not add the new page to the LRU, the
 * caller must do that.
 *
 * The remove + add is atomic.  The only way this function can fail is
 * memory allocation failure.
 */
int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
{
        int error;

        VM_BUG_ON_PAGE(!PageLocked(old), old);
        VM_BUG_ON_PAGE(!PageLocked(new), new);
        VM_BUG_ON_PAGE(new->mapping, new);

        error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
        if (!error) {
                struct address_space *mapping = old->mapping;
                void (*freepage)(struct page *);
                unsigned long flags;

                pgoff_t offset = old->index;
                freepage = mapping->a_ops->freepage;

                get_page(new);
                new->mapping = mapping;
                new->index = offset;

                spin_lock_irqsave(&mapping->tree_lock, flags);
                __delete_from_page_cache(old, NULL);
                error = radix_tree_insert(&mapping->page_tree, offset, new);
                BUG_ON(error);
                mapping->nrpages++;

                /*
                 * hugetlb pages do not participate in page cache accounting.
                 */
                if (!PageHuge(new))
                        __inc_zone_page_state(new, NR_FILE_PAGES);
                if (PageSwapBacked(new))
                        __inc_zone_page_state(new, NR_SHMEM);
                spin_unlock_irqrestore(&mapping->tree_lock, flags);
                mem_cgroup_migrate(old, new);
                radix_tree_preload_end();
                if (freepage)
                        freepage(old);
                put_page(old);
        }

        return error;
}
EXPORT_SYMBOL_GPL(replace_page_cache_page);

static int page_cache_tree_insert(struct address_space *mapping,
                                  struct page *page, void **shadowp)
{
        struct radix_tree_node *node;
        void **slot;
        int error;

        error = __radix_tree_create(&mapping->page_tree, page->index, 0,
                                    &node, &slot);
        if (error)
                return error;
        if (*slot) {
                void *p;

                p = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
                if (!radix_tree_exceptional_entry(p))
                        return -EEXIST;

                if (WARN_ON(dax_mapping(mapping)))
                        return -EINVAL;

                if (shadowp)
                        *shadowp = p;
                mapping->nrexceptional--;
                if (node)
                        workingset_node_shadows_dec(node);
        }
        radix_tree_replace_slot(slot, page);
        mapping->nrpages++;
        if (node) {
                workingset_node_pages_inc(node);
                /*
                 * Don't track node that contains actual pages.
                 *
                 * Avoid acquiring the list_lru lock if already
                 * untracked.  The list_empty() test is safe as
                 * node->private_list is protected by
                 * mapping->tree_lock.
                 */
                if (!list_empty(&node->private_list))
                        list_lru_del(&workingset_shadow_nodes,
                                     &node->private_list);
        }
        return 0;
}

static int __add_to_page_cache_locked(struct page *page,
                                      struct address_space *mapping,
                                      pgoff_t offset, gfp_t gfp_mask,
                                      void **shadowp)
{
        int huge = PageHuge(page);
        struct mem_cgroup *memcg;
        int error;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(PageSwapBacked(page), page);

        if (!huge) {
                error = mem_cgroup_try_charge(page, current->mm,
                                              gfp_mask, &memcg, false);
                if (error)
                        return error;
        }

        error = radix_tree_maybe_preload(gfp_mask & ~__GFP_HIGHMEM);
        if (error) {
                if (!huge)
                        mem_cgroup_cancel_charge(page, memcg, false);
                return error;
        }

        get_page(page);
        page->mapping = mapping;
        page->index = offset;

        spin_lock_irq(&mapping->tree_lock);
        error = page_cache_tree_insert(mapping, page, shadowp);
        radix_tree_preload_end();
        if (unlikely(error))
                goto err_insert;

        /* hugetlb pages do not participate in page cache accounting. */
        if (!huge)
                __inc_zone_page_state(page, NR_FILE_PAGES);
        spin_unlock_irq(&mapping->tree_lock);
        if (!huge)
                mem_cgroup_commit_charge(page, memcg, false, false);
        trace_mm_filemap_add_to_page_cache(page);
        return 0;
err_insert:
        page->mapping = NULL;
        /* Leave page->index set: truncation relies upon it */
        spin_unlock_irq(&mapping->tree_lock);
        if (!huge)
                mem_cgroup_cancel_charge(page, memcg, false);
        put_page(page);
        return error;
}

/**
 * add_to_page_cache_locked - add a locked page to the pagecache
 * @page:       page to add
 * @mapping:    the page's address_space
 * @offset:     page index
 * @gfp_mask:   page allocation mode
 *
 * This function is used to add a page to the pagecache. It must be locked.
 * This function does not add the page to the LRU.  The caller must do that.
 */
int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
                pgoff_t offset, gfp_t gfp_mask)
{
        return __add_to_page_cache_locked(page, mapping, offset,
                                          gfp_mask, NULL);
}
EXPORT_SYMBOL(add_to_page_cache_locked);

int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
                                pgoff_t offset, gfp_t gfp_mask)
{
        void *shadow = NULL;
        int ret;

        __SetPageLocked(page);
        ret = __add_to_page_cache_locked(page, mapping, offset,
                                         gfp_mask, &shadow);
        if (unlikely(ret))
                __ClearPageLocked(page);
        else {
                /*
                 * The page might have been evicted from cache only
                 * recently, in which case it should be activated like
                 * any other repeatedly accessed page.
                 * The exception is pages getting rewritten; evicting other
                 * data from the working set, only to cache data that will
                 * get overwritten with something else, is a waste of memory.
                 */
                if (!(gfp_mask & __GFP_WRITE) &&
                    shadow && workingset_refault(shadow)) {
                        SetPageActive(page);
                        workingset_activation(page);
                } else
                        ClearPageActive(page);
                lru_cache_add(page);
        }
        return ret;
}
EXPORT_SYMBOL_GPL(add_to_page_cache_lru);

#ifdef CONFIG_NUMA
struct page *__page_cache_alloc(gfp_t gfp)
{
        int n;
        struct page *page;

        if (cpuset_do_page_mem_spread()) {
                unsigned int cpuset_mems_cookie;
                do {
                        cpuset_mems_cookie = read_mems_allowed_begin();
                        n = cpuset_mem_spread_node();
                        page = __alloc_pages_node(n, gfp, 0);
                } while (!page && read_mems_allowed_retry(cpuset_mems_cookie));

                return page;
        }
        return alloc_pages(gfp, 0);
}
EXPORT_SYMBOL(__page_cache_alloc);
#endif

/*
 * In order to wait for pages to become available there must be
 * waitqueues associated with pages. By using a hash table of
 * waitqueues where the bucket discipline is to maintain all
 * waiters on the same queue and wake all when any of the pages
 * become available, and for the woken contexts to check to be
 * sure the appropriate page became available, this saves space
 * at a cost of "thundering herd" phenomena during rare hash
 * collisions.
 */
wait_queue_head_t *page_waitqueue(struct page *page)
{
        const struct zone *zone = page_zone(page);

        return &zone->wait_table[hash_ptr(page, zone->wait_table_bits)];
}
EXPORT_SYMBOL(page_waitqueue);

void wait_on_page_bit(struct page *page, int bit_nr)
{
        DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);

        if (test_bit(bit_nr, &page->flags))
                __wait_on_bit(page_waitqueue(page), &wait, bit_wait_io,
                                                        TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(wait_on_page_bit);

int wait_on_page_bit_killable(struct page *page, int bit_nr)
{
        DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);

        if (!test_bit(bit_nr, &page->flags))
                return 0;

        return __wait_on_bit(page_waitqueue(page), &wait,
                             bit_wait_io, TASK_KILLABLE);
}

int wait_on_page_bit_killable_timeout(struct page *page,
                                       int bit_nr, unsigned long timeout)
{
        DEFINE_WAIT_BIT(wait, &page->flags, bit_nr);

        wait.key.timeout = jiffies + timeout;
        if (!test_bit(bit_nr, &page->flags))
                return 0;
        return __wait_on_bit(page_waitqueue(page), &wait,
                             bit_wait_io_timeout, TASK_KILLABLE);
}
EXPORT_SYMBOL_GPL(wait_on_page_bit_killable_timeout);

/**
 * add_page_wait_queue - Add an arbitrary waiter to a page's wait queue
 * @page: Page defining the wait queue of interest
 * @waiter: Waiter to add to the queue
 *
 * Add an arbitrary @waiter to the wait queue for the nominated @page.
 */
void add_page_wait_queue(struct page *page, wait_queue_t *waiter)
{
        wait_queue_head_t *q = page_waitqueue(page);
        unsigned long flags;

        spin_lock_irqsave(&q->lock, flags);
        __add_wait_queue(q, waiter);
        spin_unlock_irqrestore(&q->lock, flags);
}
EXPORT_SYMBOL_GPL(add_page_wait_queue);

/**
 * unlock_page - unlock a locked page
 * @page: the page
 *
 * Unlocks the page and wakes up sleepers in ___wait_on_page_locked().
 * Also wakes sleepers in wait_on_page_writeback() because the wakeup
 * mechanism between PageLocked pages and PageWriteback pages is shared.
 * But that's OK - sleepers in wait_on_page_writeback() just go back to sleep.
 *
 * The mb is necessary to enforce ordering between the clear_bit and the read
 * of the waitqueue (to avoid SMP races with a parallel wait_on_page_locked()).
 */
void unlock_page(struct page *page)
{
        page = compound_head(page);
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        clear_bit_unlock(PG_locked, &page->flags);
        smp_mb__after_atomic();
        wake_up_page(page, PG_locked);
}
EXPORT_SYMBOL(unlock_page);

/**
 * end_page_writeback - end writeback against a page
 * @page: the page
 */
void end_page_writeback(struct page *page)
{
        /*
         * TestClearPageReclaim could be used here but it is an atomic
         * operation and overkill in this particular case. Failing to
         * shuffle a page marked for immediate reclaim is too mild to
         * justify taking an atomic operation penalty at the end of
         * ever page writeback.
         */
        if (PageReclaim(page)) {
                ClearPageReclaim(page);
                rotate_reclaimable_page(page);
        }

        if (!test_clear_page_writeback(page))
                BUG();

        smp_mb__after_atomic();
        wake_up_page(page, PG_writeback);
}
EXPORT_SYMBOL(end_page_writeback);

/*
 * After completing I/O on a page, call this routine to update the page
 * flags appropriately
 */
void page_endio(struct page *page, int rw, int err)
{
        if (rw == READ) {
                if (!err) {
                        SetPageUptodate(page);
                } else {
                        ClearPageUptodate(page);
                        SetPageError(page);
                }
                unlock_page(page);
        } else { /* rw == WRITE */
                if (err) {
                        SetPageError(page);
                        if (page->mapping)
                                mapping_set_error(page->mapping, err);
                }
                end_page_writeback(page);
        }
}
EXPORT_SYMBOL_GPL(page_endio);

/**
 * __lock_page - get a lock on the page, assuming we need to sleep to get it
 * @page: the page to lock
 */
void __lock_page(struct page *page)
{
        struct page *page_head = compound_head(page);
        DEFINE_WAIT_BIT(wait, &page_head->flags, PG_locked);

        __wait_on_bit_lock(page_waitqueue(page_head), &wait, bit_wait_io,
                                                        TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(__lock_page);

int __lock_page_killable(struct page *page)
{
        struct page *page_head = compound_head(page);
        DEFINE_WAIT_BIT(wait, &page_head->flags, PG_locked);

        return __wait_on_bit_lock(page_waitqueue(page_head), &wait,
                                        bit_wait_io, TASK_KILLABLE);
}
EXPORT_SYMBOL_GPL(__lock_page_killable);

/*
 * Return values:
 * 1 - page is locked; mmap_sem is still held.
 * 0 - page is not locked.
 *     mmap_sem has been released (up_read()), unless flags had both
 *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
 *     which case mmap_sem is still held.
 *
 * If neither ALLOW_RETRY nor KILLABLE are set, will always return 1
 * with the page locked and the mmap_sem unperturbed.
 */
int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
                         unsigned int flags)
{
        if (flags & FAULT_FLAG_ALLOW_RETRY) {
                /*
                 * CAUTION! In this case, mmap_sem is not released
                 * even though return 0.
                 */
                if (flags & FAULT_FLAG_RETRY_NOWAIT)
                        return 0;

                up_read(&mm->mmap_sem);
                if (flags & FAULT_FLAG_KILLABLE)
                        wait_on_page_locked_killable(page);
                else
                        wait_on_page_locked(page);
                return 0;
        } else {
                if (flags & FAULT_FLAG_KILLABLE) {
                        int ret;

                        ret = __lock_page_killable(page);
                        if (ret) {
                                up_read(&mm->mmap_sem);
                                return 0;
                        }
                } else
                        __lock_page(page);
                return 1;
        }
}

/**
 * page_cache_next_hole - find the next hole (not-present entry)
 * @mapping: mapping
 * @index: index
 * @max_scan: maximum range to search
 *
 * Search the set [index, min(index+max_scan-1, MAX_INDEX)] for the
 * lowest indexed hole.
 *
 * Returns: the index of the hole if found, otherwise returns an index
 * outside of the set specified (in which case 'return - index >=
 * max_scan' will be true). In rare cases of index wrap-around, 0 will
 * be returned.
 *
 * page_cache_next_hole may be called under rcu_read_lock. However,
 * like radix_tree_gang_lookup, this will not atomically search a
 * snapshot of the tree at a single point in time. For example, if a
 * hole is created at index 5, then subsequently a hole is created at
 * index 10, page_cache_next_hole covering both indexes may return 10
 * if called under rcu_read_lock.
 */
pgoff_t page_cache_next_hole(struct address_space *mapping,
                             pgoff_t index, unsigned long max_scan)
{
        unsigned long i;

        for (i = 0; i < max_scan; i++) {
                struct page *page;

                page = radix_tree_lookup(&mapping->page_tree, index);
                if (!page || radix_tree_exceptional_entry(page))
                        break;
                index++;
                if (index == 0)
                        break;
        }

        return index;
}
EXPORT_SYMBOL(page_cache_next_hole);

/**
 * page_cache_prev_hole - find the prev hole (not-present entry)
 * @mapping: mapping
 * @index: index
 * @max_scan: maximum range to search
 *
 * Search backwards in the range [max(index-max_scan+1, 0), index] for
 * the first hole.
 *
 * Returns: the index of the hole if found, otherwise returns an index
 * outside of the set specified (in which case 'index - return >=
 * max_scan' will be true). In rare cases of wrap-around, ULONG_MAX
 * will be returned.
 *
 * page_cache_prev_hole may be called under rcu_read_lock. However,
 * like radix_tree_gang_lookup, this will not atomically search a
 * snapshot of the tree at a single point in time. For example, if a
 * hole is created at index 10, then subsequently a hole is created at
 * index 5, page_cache_prev_hole covering both indexes may return 5 if
 * called under rcu_read_lock.
 */
pgoff_t page_cache_prev_hole(struct address_space *mapping,
                             pgoff_t index, unsigned long max_scan)
{
        unsigned long i;

        for (i = 0; i < max_scan; i++) {
                struct page *page;

                page = radix_tree_lookup(&mapping->page_tree, index);
                if (!page || radix_tree_exceptional_entry(page))
                        break;
                index--;
                if (index == ULONG_MAX)
                        break;
        }

        return index;
}
EXPORT_SYMBOL(page_cache_prev_hole);

/**
 * find_get_entry - find and get a page cache entry
 * @mapping: the address_space to search
 * @offset: the page cache index
 *
 * Looks up the page cache slot at @mapping & @offset.  If there is a
 * page cache page, it is returned with an increased refcount.
 *
 * If the slot holds a shadow entry of a previously evicted page, or a
 * swap entry from shmem/tmpfs, it is returned.
 *
 * Otherwise, %NULL is returned.
 */
struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
{
        void **pagep;
        struct page *page;

        rcu_read_lock();
repeat:
        page = NULL;
        pagep = radix_tree_lookup_slot(&mapping->page_tree, offset);
        if (pagep) {
                page = radix_tree_deref_slot(pagep);
                if (unlikely(!page))
                        goto out;
                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page))
                                goto repeat;
                        /*
                         * A shadow entry of a recently evicted page,
                         * or a swap entry from shmem/tmpfs.  Return
                         * it without attempting to raise page count.
                         */
                        goto out;
                }
                if (!page_cache_get_speculative(page))
                        goto repeat;

                /*
                 * Has the page moved?
                 * This is part of the lockless pagecache protocol. See
                 * include/linux/pagemap.h for details.
                 */
                if (unlikely(page != *pagep)) {
                        put_page(page);
                        goto repeat;
                }
        }
out:
        rcu_read_unlock();

        return page;
}
EXPORT_SYMBOL(find_get_entry);

/**
 * find_lock_entry - locate, pin and lock a page cache entry
 * @mapping: the address_space to search
 * @offset: the page cache index
 *
 * Looks up the page cache slot at @mapping & @offset.  If there is a
 * page cache page, it is returned locked and with an increased
 * refcount.
 *
 * If the slot holds a shadow entry of a previously evicted page, or a
 * swap entry from shmem/tmpfs, it is returned.
 *
 * Otherwise, %NULL is returned.
 *
 * find_lock_entry() may sleep.
 */
struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset)
{
        struct page *page;

repeat:
        page = find_get_entry(mapping, offset);
        if (page && !radix_tree_exception(page)) {
                lock_page(page);
                /* Has the page been truncated? */
                if (unlikely(page->mapping != mapping)) {
                        unlock_page(page);
                        put_page(page);
                        goto repeat;
                }
                VM_BUG_ON_PAGE(page->index != offset, page);
        }
        return page;
}
EXPORT_SYMBOL(find_lock_entry);

/**
 * pagecache_get_page - find and get a page reference
 * @mapping: the address_space to search
 * @offset: the page index
 * @fgp_flags: PCG flags
 * @gfp_mask: gfp mask to use for the page cache data page allocation
 *
 * Looks up the page cache slot at @mapping & @offset.
 *
 * PCG flags modify how the page is returned.
 *
 * FGP_ACCESSED: the page will be marked accessed
 * FGP_LOCK: Page is return locked
 * FGP_CREAT: If page is not present then a new page is allocated using
 *              @gfp_mask and added to the page cache and the VM's LRU
 *              list. The page is returned locked and with an increased
 *              refcount. Otherwise, %NULL is returned.
 *
 * If FGP_LOCK or FGP_CREAT are specified then the function may sleep even
 * if the GFP flags specified for FGP_CREAT are atomic.
 *
 * If there is a page cache page, it is returned with an increased refcount.
 */
struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
        int fgp_flags, gfp_t gfp_mask)
{
        struct page *page;

repeat:
        page = find_get_entry(mapping, offset);
        if (radix_tree_exceptional_entry(page))
                page = NULL;
        if (!page)
                goto no_page;

        if (fgp_flags & FGP_LOCK) {
                if (fgp_flags & FGP_NOWAIT) {
                        if (!trylock_page(page)) {
                                put_page(page);
                                return NULL;
                        }
                } else {
                        lock_page(page);
                }

                /* Has the page been truncated? */
                if (unlikely(page->mapping != mapping)) {
                        unlock_page(page);
                        put_page(page);
                        goto repeat;
                }
                VM_BUG_ON_PAGE(page->index != offset, page);
        }

        if (page && (fgp_flags & FGP_ACCESSED))
                mark_page_accessed(page);

no_page:
        if (!page && (fgp_flags & FGP_CREAT)) {
                int err;
                if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))
                        gfp_mask |= __GFP_WRITE;
                if (fgp_flags & FGP_NOFS)
                        gfp_mask &= ~__GFP_FS;

                page = __page_cache_alloc(gfp_mask);
                if (!page)
                        return NULL;

                if (WARN_ON_ONCE(!(fgp_flags & FGP_LOCK)))
                        fgp_flags |= FGP_LOCK;

                /* Init accessed so avoid atomic mark_page_accessed later */
                if (fgp_flags & FGP_ACCESSED)
                        __SetPageReferenced(page);

                err = add_to_page_cache_lru(page, mapping, offset,
                                gfp_mask & GFP_RECLAIM_MASK);
                if (unlikely(err)) {
                        put_page(page);
                        page = NULL;
                        if (err == -EEXIST)
                                goto repeat;
                }
        }

        return page;
}
EXPORT_SYMBOL(pagecache_get_page);

/**
 * find_get_entries - gang pagecache lookup
 * @mapping:    The address_space to search
 * @start:      The starting page cache index
 * @nr_entries: The maximum number of entries
 * @entries:    Where the resulting entries are placed
 * @indices:    The cache indices corresponding to the entries in @entries
 *
 * find_get_entries() will search for and return a group of up to
 * @nr_entries entries in the mapping.  The entries are placed at
 * @entries.  find_get_entries() takes a reference against any actual
 * pages it returns.
 *
 * The search returns a group of mapping-contiguous page cache entries
 * with ascending indexes.  There may be holes in the indices due to
 * not-present pages.
 *
 * Any shadow entries of evicted pages, or swap entries from
 * shmem/tmpfs, are included in the returned array.
 *
 * find_get_entries() returns the number of pages and shadow entries
 * which were found.
 */
unsigned find_get_entries(struct address_space *mapping,
                          pgoff_t start, unsigned int nr_entries,
                          struct page **entries, pgoff_t *indices)
{
        void **slot;
        unsigned int ret = 0;
        struct radix_tree_iter iter;

        if (!nr_entries)
                return 0;

        rcu_read_lock();
        radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
                struct page *page;
repeat:
                page = radix_tree_deref_slot(slot);
                if (unlikely(!page))
                        continue;
                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page)) {
                                slot = radix_tree_iter_retry(&iter);
                                continue;
                        }
                        /*
                         * A shadow entry of a recently evicted page, a swap
                         * entry from shmem/tmpfs or a DAX entry.  Return it
                         * without attempting to raise page count.
                         */
                        goto export;
                }
                if (!page_cache_get_speculative(page))
                        goto repeat;

                /* Has the page moved? */
                if (unlikely(page != *slot)) {
                        put_page(page);
                        goto repeat;
                }
export:
                indices[ret] = iter.index;
                entries[ret] = page;
                if (++ret == nr_entries)
                        break;
        }
        rcu_read_unlock();
        return ret;
}

/**
 * find_get_pages - gang pagecache lookup
 * @mapping:    The address_space to search
 * @start:      The starting page index
 * @nr_pages:   The maximum number of pages
 * @pages:      Where the resulting pages are placed
 *
 * find_get_pages() will search for and return a group of up to
 * @nr_pages pages in the mapping.  The pages are placed at @pages.
 * find_get_pages() takes a reference against the returned pages.
 *
 * The search returns a group of mapping-contiguous pages with ascending
 * indexes.  There may be holes in the indices due to not-present pages.
 *
 * find_get_pages() returns the number of pages which were found.
 */
unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
                            unsigned int nr_pages, struct page **pages)
{
        struct radix_tree_iter iter;
        void **slot;
        unsigned ret = 0;

        if (unlikely(!nr_pages))
                return 0;

        rcu_read_lock();
        radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
                struct page *page;
repeat:
                page = radix_tree_deref_slot(slot);
                if (unlikely(!page))
                        continue;

                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page)) {
                                slot = radix_tree_iter_retry(&iter);
                                continue;
                        }
                        /*
                         * A shadow entry of a recently evicted page,
                         * or a swap entry from shmem/tmpfs.  Skip
                         * over it.
                         */
                        continue;
                }

                if (!page_cache_get_speculative(page))
                        goto repeat;

                /* Has the page moved? */
                if (unlikely(page != *slot)) {
                        put_page(page);
                        goto repeat;
                }

                pages[ret] = page;
                if (++ret == nr_pages)
                        break;
        }

        rcu_read_unlock();
        return ret;
}

/**
 * find_get_pages_contig - gang contiguous pagecache lookup
 * @mapping:    The address_space to search
 * @index:      The starting page index
 * @nr_pages:   The maximum number of pages
 * @pages:      Where the resulting pages are placed
 *
 * find_get_pages_contig() works exactly like find_get_pages(), except
 * that the returned number of pages are guaranteed to be contiguous.
 *
 * find_get_pages_contig() returns the number of pages which were found.
 */
unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
                               unsigned int nr_pages, struct page **pages)
{
        struct radix_tree_iter iter;
        void **slot;
        unsigned int ret = 0;

        if (unlikely(!nr_pages))
                return 0;

        rcu_read_lock();
        radix_tree_for_each_contig(slot, &mapping->page_tree, &iter, index) {
                struct page *page;
repeat:
                page = radix_tree_deref_slot(slot);
                /* The hole, there no reason to continue */
                if (unlikely(!page))
                        break;

                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page)) {
                                slot = radix_tree_iter_retry(&iter);
                                continue;
                        }
                        /*
                         * A shadow entry of a recently evicted page,
                         * or a swap entry from shmem/tmpfs.  Stop
                         * looking for contiguous pages.
                         */
                        break;
                }

                if (!page_cache_get_speculative(page))
                        goto repeat;

                /* Has the page moved? */
                if (unlikely(page != *slot)) {
                        put_page(page);
                        goto repeat;
                }

                /*
                 * must check mapping and index after taking the ref.
                 * otherwise we can get both false positives and false
                 * negatives, which is just confusing to the caller.
                 */
                if (page->mapping == NULL || page->index != iter.index) {
                        put_page(page);
                        break;
                }

                pages[ret] = page;
                if (++ret == nr_pages)
                        break;
        }
        rcu_read_unlock();
        return ret;
}
EXPORT_SYMBOL(find_get_pages_contig);

/**
 * find_get_pages_tag - find and return pages that match @tag
 * @mapping:    the address_space to search
 * @index:      the starting page index
 * @tag:        the tag index
 * @nr_pages:   the maximum number of pages
 * @pages:      where the resulting pages are placed
 *
 * Like find_get_pages, except we only return pages which are tagged with
 * @tag.   We update @index to index the next page for the traversal.
 */
unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
                        int tag, unsigned int nr_pages, struct page **pages)
{
        struct radix_tree_iter iter;
        void **slot;
        unsigned ret = 0;

        if (unlikely(!nr_pages))
                return 0;

        rcu_read_lock();
        radix_tree_for_each_tagged(slot, &mapping->page_tree,
                                   &iter, *index, tag) {
                struct page *page;
repeat:
                page = radix_tree_deref_slot(slot);
                if (unlikely(!page))
                        continue;

                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page)) {
                                slot = radix_tree_iter_retry(&iter);
                                continue;
                        }
                        /*
                         * A shadow entry of a recently evicted page.
                         *
                         * Those entries should never be tagged, but
                         * this tree walk is lockless and the tags are
                         * looked up in bulk, one radix tree node at a
                         * time, so there is a sizable window for page
                         * reclaim to evict a page we saw tagged.
                         *
                         * Skip over it.
                         */
                        continue;
                }

                if (!page_cache_get_speculative(page))
                        goto repeat;

                /* Has the page moved? */
                if (unlikely(page != *slot)) {
                        put_page(page);
                        goto repeat;
                }

                pages[ret] = page;
                if (++ret == nr_pages)
                        break;
        }

        rcu_read_unlock();

        if (ret)
                *index = pages[ret - 1]->index + 1;

        return ret;
}
EXPORT_SYMBOL(find_get_pages_tag);

/**
 * find_get_entries_tag - find and return entries that match @tag
 * @mapping:    the address_space to search
 * @start:      the starting page cache index
 * @tag:        the tag index
 * @nr_entries: the maximum number of entries
 * @entries:    where the resulting entries are placed
 * @indices:    the cache indices corresponding to the entries in @entries
 *
 * Like find_get_entries, except we only return entries which are tagged with
 * @tag.
 */
unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
                        int tag, unsigned int nr_entries,
                        struct page **entries, pgoff_t *indices)
{
        void **slot;
        unsigned int ret = 0;
        struct radix_tree_iter iter;

        if (!nr_entries)
                return 0;

        rcu_read_lock();
        radix_tree_for_each_tagged(slot, &mapping->page_tree,
                                   &iter, start, tag) {
                struct page *page;
repeat:
                page = radix_tree_deref_slot(slot);
                if (unlikely(!page))
                        continue;
                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page)) {
                                slot = radix_tree_iter_retry(&iter);
                                continue;
                        }

                        /*
                         * A shadow entry of a recently evicted page, a swap
                         * entry from shmem/tmpfs or a DAX entry.  Return it
                         * without attempting to raise page count.
                         */
                        goto export;
                }
                if (!page_cache_get_speculative(page))
                        goto repeat;

                /* Has the page moved? */
                if (unlikely(page != *slot)) {
                        put_page(page);
                        goto repeat;
                }
export:
                indices[ret] = iter.index;
                entries[ret] = page;
                if (++ret == nr_entries)
                        break;
        }
        rcu_read_unlock();
        return ret;
}
EXPORT_SYMBOL(find_get_entries_tag);

/*
 * CD/DVDs are error prone. When a medium error occurs, the driver may fail
 * a _large_ part of the i/o request. Imagine the worst scenario:
 *
 *      ---R__________________________________________B__________
 *         ^ reading here                             ^ bad block(assume 4k)
 *
 * read(R) => miss => readahead(R...B) => media error => frustrating retries
 * => failing the whole request => read(R) => read(R+1) =>
 * readahead(R+1...B+1) => bang => read(R+2) => read(R+3) =>
 * readahead(R+3...B+2) => bang => read(R+3) => read(R+4) =>
 * readahead(R+4...B+3) => bang => read(R+4) => read(R+5) => ......
 *
 * It is going insane. Fix it by quickly scaling down the readahead size.
 */
static void shrink_readahead_size_eio(struct file *filp,
                                        struct file_ra_state *ra)
{
        ra->ra_pages /= 4;
}

/**
 * do_generic_file_read - generic file read routine
 * @filp:       the file to read
 * @ppos:       current file position
 * @iter:       data destination
 * @written:    already copied
 *
 * This is a generic file read routine, and uses the
 * mapping->a_ops->readpage() function for the actual low-level stuff.
 *
 * This is really ugly. But the goto's actually try to clarify some
 * of the logic when it comes to error handling etc.
 */
static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
                struct iov_iter *iter, ssize_t written)
{
        struct address_space *mapping = filp->f_mapping;
        struct inode *inode = mapping->host;
        struct file_ra_state *ra = &filp->f_ra;
        pgoff_t index;
        pgoff_t last_index;
        pgoff_t prev_index;
        unsigned long offset;      /* offset into pagecache page */
        unsigned int prev_offset;
        int error = 0;

        index = *ppos >> PAGE_SHIFT;
        prev_index = ra->prev_pos >> PAGE_SHIFT;
        prev_offset = ra->prev_pos & (PAGE_SIZE-1);
        last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
        offset = *ppos & ~PAGE_MASK;

        for (;;) {
                struct page *page;
                pgoff_t end_index;
                loff_t isize;
                unsigned long nr, ret;

                cond_resched();
find_page:
                page = find_get_page(mapping, index);
                if (!page) {
                        page_cache_sync_readahead(mapping,
                                        ra, filp,
                                        index, last_index - index);
                        page = find_get_page(mapping, index);
                        if (unlikely(page == NULL))
                                goto no_cached_page;
                }
                if (PageReadahead(page)) {
                        page_cache_async_readahead(mapping,
                                        ra, filp, page,
                                        index, last_index - index);
                }
                if (!PageUptodate(page)) {
                        /*
                         * See comment in do_read_cache_page on why
                         * wait_on_page_locked is used to avoid unnecessarily
                         * serialisations and why it's safe.
                         */
                        wait_on_page_locked_killable(page);
                        if (PageUptodate(page))
                                goto page_ok;

                        if (inode->i_blkbits == PAGE_SHIFT ||
                                        !mapping->a_ops->is_partially_uptodate)
                                goto page_not_up_to_date;
                        if (!trylock_page(page))
                                goto page_not_up_to_date;
                        /* Did it get truncated before we got the lock? */
                        if (!page->mapping)
                                goto page_not_up_to_date_locked;
                        if (!mapping->a_ops->is_partially_uptodate(page,
                                                        offset, iter->count))
                                goto page_not_up_to_date_locked;
                        unlock_page(page);
                }
page_ok:
                /*
                 * i_size must be checked after we know the page is Uptodate.
                 *
                 * Checking i_size after the check allows us to calculate
                 * the correct value for "nr", which means the zero-filled
                 * part of the page is not copied back to userspace (unless
                 * another truncate extends the file - this is desired though).
                 */

                isize = i_size_read(inode);
                end_index = (isize - 1) >> PAGE_SHIFT;
                if (unlikely(!isize || index > end_index)) {
                        put_page(page);
                        goto out;
                }

                /* nr is the maximum number of bytes to copy from this page */
                nr = PAGE_SIZE;
                if (index == end_index) {
                        nr = ((isize - 1) & ~PAGE_MASK) + 1;
                        if (nr <= offset) {
                                put_page(page);
                                goto out;
                        }
                }
                nr = nr - offset;

                /* If users can be writing to this page using arbitrary
                 * virtual addresses, take care about potential aliasing
                 * before reading the page on the kernel side.
                 */
                if (mapping_writably_mapped(mapping))
                        flush_dcache_page(page);

                /*
                 * When a sequential read accesses a page several times,
                 * only mark it as accessed the first time.
                 */
                if (prev_index != index || offset != prev_offset)
                        mark_page_accessed(page);
                prev_index = index;

                /*
                 * Ok, we have the page, and it's up-to-date, so
                 * now we can copy it to user space...
                 */

                ret = copy_page_to_iter(page, offset, nr, iter);
                offset += ret;
                index += offset >> PAGE_SHIFT;
                offset &= ~PAGE_MASK;
                prev_offset = offset;

                put_page(page);
                written += ret;
                if (!iov_iter_count(iter))
                        goto out;
                if (ret < nr) {
                        error = -EFAULT;
                        goto out;
                }
                continue;

page_not_up_to_date:
                /* Get exclusive access to the page ... */
                error = lock_page_killable(page);
                if (unlikely(error))
                        goto readpage_error;

page_not_up_to_date_locked:
                /* Did it get truncated before we got the lock? */
                if (!page->mapping) {
                        unlock_page(page);
                        put_page(page);
                        continue;
                }

                /* Did somebody else fill it already? */
                if (PageUptodate(page)) {
                        unlock_page(page);
                        goto page_ok;
                }

readpage:
                /*
                 * A previous I/O error may have been due to temporary
                 * failures, eg. multipath errors.
                 * PG_error will be set again if readpage fails.
                 */
                ClearPageError(page);
                /* Start the actual read. The read will unlock the page. */
                error = mapping->a_ops->readpage(filp, page);

                if (unlikely(error)) {
                        if (error == AOP_TRUNCATED_PAGE) {
                                put_page(page);
                                error = 0;
                                goto find_page;
                        }
                        goto readpage_error;
                }

                if (!PageUptodate(page)) {
                        error = lock_page_killable(page);
                        if (unlikely(error))
                                goto readpage_error;
                        if (!PageUptodate(page)) {
                                if (page->mapping == NULL) {
                                        /*
                                         * invalidate_mapping_pages got it
                                         */
                                        unlock_page(page);
                                        put_page(page);
                                        goto find_page;
                                }
                                unlock_page(page);
                                shrink_readahead_size_eio(filp, ra);
                                error = -EIO;
                                goto readpage_error;
                        }
                        unlock_page(page);
                }

                goto page_ok;

readpage_error:
                /* UHHUH! A synchronous read error occurred. Report it */
                put_page(page);
                goto out;

no_cached_page:
                /*
                 * Ok, it wasn't cached, so we need to create a new
                 * page..
                 */
                page = page_cache_alloc_cold(mapping);
                if (!page) {
                        error = -ENOMEM;
                        goto out;
                }
                error = add_to_page_cache_lru(page, mapping, index,
                                mapping_gfp_constraint(mapping, GFP_KERNEL));
                if (error) {
                        put_page(page);
                        if (error == -EEXIST) {
                                error = 0;
                                goto find_page;
                        }
                        goto out;
                }
                goto readpage;
        }

out:
        ra->prev_pos = prev_index;
        ra->prev_pos <<= PAGE_SHIFT;
        ra->prev_pos |= prev_offset;

        *ppos = ((loff_t)index << PAGE_SHIFT) + offset;
        file_accessed(filp);
        return written ? written : error;
}

/**
 * generic_file_read_iter - generic filesystem read routine
 * @iocb:       kernel I/O control block
 * @iter:       destination for the data read
 *
 * This is the "read_iter()" routine for all filesystems
 * that can use the page cache directly.
 */
ssize_t
generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
{
        struct file *file = iocb->ki_filp;
        ssize_t retval = 0;
        size_t count = iov_iter_count(iter);

        if (!count)
                goto out; /* skip atime */

        if (iocb->ki_flags & IOCB_DIRECT) {
                struct address_space *mapping = file->f_mapping;
                struct inode *inode = mapping->host;
                loff_t size;

                size = i_size_read(inode);
                retval = filemap_write_and_wait_range(mapping, iocb->ki_pos,
                                        iocb->ki_pos + count - 1);
                if (!retval) {
                        struct iov_iter data = *iter;
                        retval = mapping->a_ops->direct_IO(iocb, &data);
                }

                if (retval > 0) {
                        iocb->ki_pos += retval;
                        iov_iter_advance(iter, retval);
                }

                /*
                 * Btrfs can have a short DIO read if we encounter
                 * compressed extents, so if there was an error, or if
                 * we've already read everything we wanted to, or if
                 * there was a short read because we hit EOF, go ahead
                 * and return.  Otherwise fallthrough to buffered io for
                 * the rest of the read.  Buffered reads will not work for
                 * DAX files, so don't bother trying.
                 */
                if (retval < 0 || !iov_iter_count(iter) || iocb->ki_pos >= size ||
                    IS_DAX(inode)) {
                        file_accessed(file);
                        goto out;
                }
        }

        retval = do_generic_file_read(file, &iocb->ki_pos, iter, retval);
out:
        return retval;
}
EXPORT_SYMBOL(generic_file_read_iter);

#ifdef CONFIG_MMU
/**
 * page_cache_read - adds requested page to the page cache if not already there
 * @file:       file to read
 * @offset:     page index
 * @gfp_mask:   memory allocation flags
 *
 * This adds the requested page to the page cache if it isn't already there,
 * and schedules an I/O to read in its contents from disk.
 */
static int page_cache_read(struct file *file, pgoff_t offset, gfp_t gfp_mask)
{
        struct address_space *mapping = file->f_mapping;
        struct page *page;
        int ret;

        do {
                page = __page_cache_alloc(gfp_mask|__GFP_COLD);
                if (!page)
                        return -ENOMEM;

                ret = add_to_page_cache_lru(page, mapping, offset, gfp_mask & GFP_KERNEL);
                if (ret == 0)
                        ret = mapping->a_ops->readpage(file, page);
                else if (ret == -EEXIST)
                        ret = 0; /* losing race to add is OK */

                put_page(page);

        } while (ret == AOP_TRUNCATED_PAGE);

        return ret;
}

#define MMAP_LOTSAMISS  (100)

/*
 * Synchronous readahead happens when we don't even find
 * a page in the page cache at all.
 */
static void do_sync_mmap_readahead(struct vm_area_struct *vma,
                                   struct file_ra_state *ra,
                                   struct file *file,
                                   pgoff_t offset)
{
        struct address_space *mapping = file->f_mapping;

        /* If we don't want any read-ahead, don't bother */
        if (vma->vm_flags & VM_RAND_READ)
                return;
        if (!ra->ra_pages)
                return;

        if (vma->vm_flags & VM_SEQ_READ) {
                page_cache_sync_readahead(mapping, ra, file, offset,
                                          ra->ra_pages);
                return;
        }

        /* Avoid banging the cache line if not needed */
        if (ra->mmap_miss < MMAP_LOTSAMISS * 10)
                ra->mmap_miss++;

        /*
         * Do we miss much more than hit in this file? If so,
         * stop bothering with read-ahead. It will only hurt.
         */
        if (ra->mmap_miss > MMAP_LOTSAMISS)
                return;

        /*
         * mmap read-around
         */
        ra->start = max_t(long, 0, offset - ra->ra_pages / 2);
        ra->size = ra->ra_pages;
        ra->async_size = ra->ra_pages / 4;
        ra_submit(ra, mapping, file);
}

/*
 * Asynchronous readahead happens when we find the page and PG_readahead,
 * so we want to possibly extend the readahead further..
 */
static void do_async_mmap_readahead(struct vm_area_struct *vma,
                                    struct file_ra_state *ra,
                                    struct file *file,
                                    struct page *page,
                                    pgoff_t offset)
{
        struct address_space *mapping = file->f_mapping;

        /* If we don't want any read-ahead, don't bother */
        if (vma->vm_flags & VM_RAND_READ)
                return;
        if (ra->mmap_miss > 0)
                ra->mmap_miss--;
        if (PageReadahead(page))
                page_cache_async_readahead(mapping, ra, file,
                                           page, offset, ra->ra_pages);
}

/**
 * filemap_fault - read in file data for page fault handling
 * @vma:        vma in which the fault was taken
 * @vmf:        struct vm_fault containing details of the fault
 *
 * filemap_fault() is invoked via the vma operations vector for a
 * mapped memory region to read in file data during a page fault.
 *
 * The goto's are kind of ugly, but this streamlines the normal case of having
 * it in the page cache, and handles the special cases reasonably without
 * having a lot of duplicated code.
 *
 * vma->vm_mm->mmap_sem must be held on entry.
 *
 * If our return value has VM_FAULT_RETRY set, it's because
 * lock_page_or_retry() returned 0.
 * The mmap_sem has usually been released in this case.
 * See __lock_page_or_retry() for the exception.
 *
 * If our return value does not have VM_FAULT_RETRY set, the mmap_sem
 * has not been released.
 *
 * We never return with VM_FAULT_RETRY and a bit from VM_FAULT_ERROR set.
 */
int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        int error;
        struct file *file = vma->vm_file;
        struct address_space *mapping = file->f_mapping;
        struct file_ra_state *ra = &file->f_ra;
        struct inode *inode = mapping->host;
        pgoff_t offset = vmf->pgoff;
        struct page *page;
        loff_t size;
        int ret = 0;

        size = round_up(i_size_read(inode), PAGE_SIZE);
        if (offset >= size >> PAGE_SHIFT)
                return VM_FAULT_SIGBUS;

        /*
         * Do we have something in the page cache already?
         */
        page = find_get_page(mapping, offset);
        if (likely(page) && !(vmf->flags & FAULT_FLAG_TRIED)) {
                /*
                 * We found the page, so try async readahead before
                 * waiting for the lock.
                 */
                do_async_mmap_readahead(vma, ra, file, page, offset);
        } else if (!page) {
                /* No page in the page cache at all */
                do_sync_mmap_readahead(vma, ra, file, offset);
                count_vm_event(PGMAJFAULT);
                mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
                ret = VM_FAULT_MAJOR;
retry_find:
                page = find_get_page(mapping, offset);
                if (!page)
                        goto no_cached_page;
        }

        if (!lock_page_or_retry(page, vma->vm_mm, vmf->flags)) {
                put_page(page);
                return ret | VM_FAULT_RETRY;
        }

        /* Did it get truncated? */
        if (unlikely(page->mapping != mapping)) {
                unlock_page(page);
                put_page(page);
                goto retry_find;
        }
        VM_BUG_ON_PAGE(page->index != offset, page);

        /*
         * We have a locked page in the page cache, now we need to check
         * that it's up-to-date. If not, it is going to be due to an error.
         */
        if (unlikely(!PageUptodate(page)))
                goto page_not_uptodate;

        /*
         * Found the page and have a reference on it.
         * We must recheck i_size under page lock.
         */
        size = round_up(i_size_read(inode), PAGE_SIZE);
        if (unlikely(offset >= size >> PAGE_SHIFT)) {
                unlock_page(page);
                put_page(page);
                return VM_FAULT_SIGBUS;
        }

        vmf->page = page;
        return ret | VM_FAULT_LOCKED;

no_cached_page:
        /*
         * We're only likely to ever get here if MADV_RANDOM is in
         * effect.
         */
        error = page_cache_read(file, offset, vmf->gfp_mask);

        /*
         * The page we want has now been added to the page cache.
         * In the unlikely event that someone removed it in the
         * meantime, we'll just come back here and read it again.
         */
        if (error >= 0)
                goto retry_find;

        /*
         * An error return from page_cache_read can result if the
         * system is low on memory, or a problem occurs while trying
         * to schedule I/O.
         */
        if (error == -ENOMEM)
                return VM_FAULT_OOM;
        return VM_FAULT_SIGBUS;

page_not_uptodate:
        /*
         * Umm, take care of errors if the page isn't up-to-date.
         * Try to re-read it _once_. We do this synchronously,
         * because there really aren't any performance issues here
         * and we need to check for errors.
         */
        ClearPageError(page);
        error = mapping->a_ops->readpage(file, page);
        if (!error) {
                wait_on_page_locked(page);
                if (!PageUptodate(page))
                        error = -EIO;
        }
        put_page(page);

        if (!error || error == AOP_TRUNCATED_PAGE)
                goto retry_find;

        /* Things didn't work out. Return zero to tell the mm layer so. */
        shrink_readahead_size_eio(file, ra);
        return VM_FAULT_SIGBUS;
}
EXPORT_SYMBOL(filemap_fault);

void filemap_map_pages(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        struct radix_tree_iter iter;
        void **slot;
        struct file *file = vma->vm_file;
        struct address_space *mapping = file->f_mapping;
        loff_t size;
        struct page *page;
        unsigned long address = (unsigned long) vmf->virtual_address;
        unsigned long addr;
        pte_t *pte;

        rcu_read_lock();
        radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, vmf->pgoff) {
                if (iter.index > vmf->max_pgoff)
                        break;
repeat:
                page = radix_tree_deref_slot(slot);
                if (unlikely(!page))
                        goto next;
                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page)) {
                                slot = radix_tree_iter_retry(&iter);
                                continue;
                        }
                        goto next;
                }

                if (!page_cache_get_speculative(page))
                        goto repeat;

                /* Has the page moved? */
                if (unlikely(page != *slot)) {
                        put_page(page);
                        goto repeat;
                }

                if (!PageUptodate(page) ||
                                PageReadahead(page) ||
                                PageHWPoison(page))
                        goto skip;
                if (!trylock_page(page))
                        goto skip;

                if (page->mapping != mapping || !PageUptodate(page))
                        goto unlock;

                size = round_up(i_size_read(mapping->host), PAGE_SIZE);
                if (page->index >= size >> PAGE_SHIFT)
                        goto unlock;

                pte = vmf->pte + page->index - vmf->pgoff;
                if (!pte_none(*pte))
                        goto unlock;

                if (file->f_ra.mmap_miss > 0)
                        file->f_ra.mmap_miss--;
                addr = address + (page->index - vmf->pgoff) * PAGE_SIZE;
                do_set_pte(vma, addr, page, pte, false, false);
                unlock_page(page);
                goto next;
unlock:
                unlock_page(page);
skip:
                put_page(page);
next:
                if (iter.index == vmf->max_pgoff)
                        break;
        }
        rcu_read_unlock();
}
EXPORT_SYMBOL(filemap_map_pages);

int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        struct page *page = vmf->page;
        struct inode *inode = file_inode(vma->vm_file);
        int ret = VM_FAULT_LOCKED;

        sb_start_pagefault(inode->i_sb);
        file_update_time(vma->vm_file);
        lock_page(page);
        if (page->mapping != inode->i_mapping) {
                unlock_page(page);
                ret = VM_FAULT_NOPAGE;
                goto out;
        }
        /*
         * We mark the page dirty already here so that when freeze is in
         * progress, we are guaranteed that writeback during freezing will
         * see the dirty page and writeprotect it again.
         */
        set_page_dirty(page);
        wait_for_stable_page(page);
out:
        sb_end_pagefault(inode->i_sb);
        return ret;
}
EXPORT_SYMBOL(filemap_page_mkwrite);

const struct vm_operations_struct generic_file_vm_ops = {
        .fault          = filemap_fault,
        .map_pages      = filemap_map_pages,
        .page_mkwrite   = filemap_page_mkwrite,
};

/* This is used for a general mmap of a disk file */

int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
{
        struct address_space *mapping = file->f_mapping;

        if (!mapping->a_ops->readpage)
                return -ENOEXEC;
        file_accessed(file);
        vma->vm_ops = &generic_file_vm_ops;
        return 0;
}

/*
 * This is for filesystems which do not implement ->writepage.
 */
int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
{
        if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
                return -EINVAL;
        return generic_file_mmap(file, vma);
}
#else
int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
{
        return -ENOSYS;
}
int generic_file_readonly_mmap(struct file * file, struct vm_area_struct * vma)
{
        return -ENOSYS;
}
#endif /* CONFIG_MMU */

EXPORT_SYMBOL(generic_file_mmap);
EXPORT_SYMBOL(generic_file_readonly_mmap);

static struct page *wait_on_page_read(struct page *page)
{
        if (!IS_ERR(page)) {
                wait_on_page_locked(page);
                if (!PageUptodate(page)) {
                        put_page(page);
                        page = ERR_PTR(-EIO);
                }
        }
        return page;
}

static struct page *do_read_cache_page(struct address_space *mapping,
                                pgoff_t index,
                                int (*filler)(void *, struct page *),
                                void *data,
                                gfp_t gfp)
{
        struct page *page;
        int err;
repeat:
        page = find_get_page(mapping, index);
        if (!page) {
                page = __page_cache_alloc(gfp | __GFP_COLD);
                if (!page)
                        return ERR_PTR(-ENOMEM);
                err = add_to_page_cache_lru(page, mapping, index, gfp);
                if (unlikely(err)) {
                        put_page(page);
                        if (err == -EEXIST)
                                goto repeat;
                        /* Presumably ENOMEM for radix tree node */
                        return ERR_PTR(err);
                }

filler:
                err = filler(data, page);
                if (err < 0) {
                        put_page(page);
                        return ERR_PTR(err);
                }

                page = wait_on_page_read(page);
                if (IS_ERR(page))
                        return page;
                goto out;
        }
        if (PageUptodate(page))
                goto out;

        /*
         * Page is not up to date and may be locked due one of the following
         * case a: Page is being filled and the page lock is held
         * case b: Read/write error clearing the page uptodate status
         * case c: Truncation in progress (page locked)
         * case d: Reclaim in progress
         *
         * Case a, the page will be up to date when the page is unlocked.
         *    There is no need to serialise on the page lock here as the page
         *    is pinned so the lock gives no additional protection. Even if the
         *    the page is truncated, the data is still valid if PageUptodate as
         *    it's a race vs truncate race.
         * Case b, the page will not be up to date
         * Case c, the page may be truncated but in itself, the data may still
         *    be valid after IO completes as it's a read vs truncate race. The
         *    operation must restart if the page is not uptodate on unlock but
         *    otherwise serialising on page lock to stabilise the mapping gives
         *    no additional guarantees to the caller as the page lock is
         *    released before return.
         * Case d, similar to truncation. If reclaim holds the page lock, it
         *    will be a race with remove_mapping that determines if the mapping
         *    is valid on unlock but otherwise the data is valid and there is
         *    no need to serialise with page lock.
         *
         * As the page lock gives no additional guarantee, we optimistically
         * wait on the page to be unlocked and check if it's up to date and
         * use the page if it is. Otherwise, the page lock is required to
         * distinguish between the different cases. The motivation is that we
         * avoid spurious serialisations and wakeups when multiple processes
         * wait on the same page for IO to complete.
         */
        wait_on_page_locked(page);
        if (PageUptodate(page))
                goto out;

        /* Distinguish between all the cases under the safety of the lock */
        lock_page(page);

        /* Case c or d, restart the operation */
        if (!page->mapping) {
                unlock_page(page);
                put_page(page);
                goto repeat;
        }

        /* Someone else locked and filled the page in a very small window */
        if (PageUptodate(page)) {
                unlock_page(page);
                goto out;
        }
        goto filler;

out:
        mark_page_accessed(page);
        return page;
}

/**
 * read_cache_page - read into page cache, fill it if needed
 * @mapping:    the page's address_space
 * @index:      the page index
 * @filler:     function to perform the read
 * @data:       first arg to filler(data, page) function, often left as NULL
 *
 * Read into the page cache. If a page already exists, and PageUptodate() is
 * not set, try to fill the page and wait for it to become unlocked.
 *
 * If the page does not get brought uptodate, return -EIO.
 */
struct page *read_cache_page(struct address_space *mapping,
                                pgoff_t index,
                                int (*filler)(void *, struct page *),
                                void *data)
{
        return do_read_cache_page(mapping, index, filler, data, mapping_gfp_mask(mapping));
}
EXPORT_SYMBOL(read_cache_page);

/**
 * read_cache_page_gfp - read into page cache, using specified page allocation flags.
 * @mapping:    the page's address_space
 * @index:      the page index
 * @gfp:        the page allocator flags to use if allocating
 *
 * This is the same as "read_mapping_page(mapping, index, NULL)", but with
 * any new page allocations done using the specified allocation flags.
 *
 * If the page does not get brought uptodate, return -EIO.
 */
struct page *read_cache_page_gfp(struct address_space *mapping,
                                pgoff_t index,
                                gfp_t gfp)
{
        filler_t *filler = (filler_t *)mapping->a_ops->readpage;

        return do_read_cache_page(mapping, index, filler, NULL, gfp);
}
EXPORT_SYMBOL(read_cache_page_gfp);

/*
 * Performs necessary checks before doing a write
 *
 * Can adjust writing position or amount of bytes to write.
 * Returns appropriate error code that caller should return or
 * zero in case that write should be allowed.
 */
inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
{
        struct file *file = iocb->ki_filp;
        struct inode *inode = file->f_mapping->host;
        unsigned long limit = rlimit(RLIMIT_FSIZE);
        loff_t pos;

        if (!iov_iter_count(from))
                return 0;

        /* FIXME: this is for backwards compatibility with 2.4 */
        if (iocb->ki_flags & IOCB_APPEND)
                iocb->ki_pos = i_size_read(inode);

        pos = iocb->ki_pos;

        if (limit != RLIM_INFINITY) {
                if (iocb->ki_pos >= limit) {
                        send_sig(SIGXFSZ, current, 0);
                        return -EFBIG;
                }
                iov_iter_truncate(from, limit - (unsigned long)pos);
        }

        /*
         * LFS rule
         */
        if (unlikely(pos + iov_iter_count(from) > MAX_NON_LFS &&
                                !(file->f_flags & O_LARGEFILE))) {
                if (pos >= MAX_NON_LFS)
                        return -EFBIG;
                iov_iter_truncate(from, MAX_NON_LFS - (unsigned long)pos);
        }

        /*
         * Are we about to exceed the fs block limit ?
         *
         * If we have written data it becomes a short write.  If we have
         * exceeded without writing data we send a signal and return EFBIG.
         * Linus frestrict idea will clean these up nicely..
         */
        if (unlikely(pos >= inode->i_sb->s_maxbytes))
                return -EFBIG;

        iov_iter_truncate(from, inode->i_sb->s_maxbytes - pos);
        return iov_iter_count(from);
}
EXPORT_SYMBOL(generic_write_checks);

int pagecache_write_begin(struct file *file, struct address_space *mapping,
                                loff_t pos, unsigned len, unsigned flags,
                                struct page **pagep, void **fsdata)
{
        const struct address_space_operations *aops = mapping->a_ops;

        return aops->write_begin(file, mapping, pos, len, flags,
                                                        pagep, fsdata);
}
EXPORT_SYMBOL(pagecache_write_begin);

int pagecache_write_end(struct file *file, struct address_space *mapping,
                                loff_t pos, unsigned len, unsigned copied,
                                struct page *page, void *fsdata)
{
        const struct address_space_operations *aops = mapping->a_ops;

        return aops->write_end(file, mapping, pos, len, copied, page, fsdata);
}
EXPORT_SYMBOL(pagecache_write_end);

ssize_t
generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
{
        struct file     *file = iocb->ki_filp;
        struct address_space *mapping = file->f_mapping;
        struct inode    *inode = mapping->host;
        loff_t          pos = iocb->ki_pos;
        ssize_t         written;
        size_t          write_len;
        pgoff_t         end;
        struct iov_iter data;

        write_len = iov_iter_count(from);
        end = (pos + write_len - 1) >> PAGE_SHIFT;

        written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
        if (written)
                goto out;

        /*
         * After a write we want buffered reads to be sure to go to disk to get
         * the new data.  We invalidate clean cached page from the region we're
         * about to write.  We do this *before* the write so that we can return
         * without clobbering -EIOCBQUEUED from ->direct_IO().
         */
        if (mapping->nrpages) {
                written = invalidate_inode_pages2_range(mapping,
                                        pos >> PAGE_SHIFT, end);
                /*
                 * If a page can not be invalidated, return 0 to fall back
                 * to buffered write.
                 */
                if (written) {
                        if (written == -EBUSY)
                                return 0;
                        goto out;
                }
        }

        data = *from;
        written = mapping->a_ops->direct_IO(iocb, &data);

        /*
         * Finally, try again to invalidate clean pages which might have been
         * cached by non-direct readahead, or faulted in by get_user_pages()
         * if the source of the write was an mmap'ed region of the file
         * we're writing.  Either one is a pretty crazy thing to do,
         * so we don't support it 100%.  If this invalidation
         * fails, tough, the write still worked...
         */
        if (mapping->nrpages) {
                invalidate_inode_pages2_range(mapping,
                                              pos >> PAGE_SHIFT, end);
        }

        if (written > 0) {
                pos += written;
                iov_iter_advance(from, written);
                if (pos > i_size_read(inode) && !S_ISBLK(inode->i_mode)) {
                        i_size_write(inode, pos);
                        mark_inode_dirty(inode);
                }
                iocb->ki_pos = pos;
        }
out:
        return written;
}
EXPORT_SYMBOL(generic_file_direct_write);

/*
 * Find or create a page at the given pagecache position. Return the locked
 * page. This function is specifically for buffered writes.
 */
struct page *grab_cache_page_write_begin(struct address_space *mapping,
                                        pgoff_t index, unsigned flags)
{
        struct page *page;
        int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT;

        if (flags & AOP_FLAG_NOFS)
                fgp_flags |= FGP_NOFS;

        page = pagecache_get_page(mapping, index, fgp_flags,
                        mapping_gfp_mask(mapping));
        if (page)
                wait_for_stable_page(page);

        return page;
}
EXPORT_SYMBOL(grab_cache_page_write_begin);

ssize_t generic_perform_write(struct file *file,
                                struct iov_iter *i, loff_t pos)
{
        struct address_space *mapping = file->f_mapping;
        const struct address_space_operations *a_ops = mapping->a_ops;
        long status = 0;
        ssize_t written = 0;
        unsigned int flags = 0;

        /*
         * Copies from kernel address space cannot fail (NFSD is a big user).
         */
        if (!iter_is_iovec(i))
                flags |= AOP_FLAG_UNINTERRUPTIBLE;

        do {
                struct page *page;
                unsigned long offset;   /* Offset into pagecache page */
                unsigned long bytes;    /* Bytes to write to page */
                size_t copied;          /* Bytes copied from user */
                void *fsdata;

                offset = (pos & (PAGE_SIZE - 1));
                bytes = min_t(unsigned long, PAGE_SIZE - offset,
                                                iov_iter_count(i));

again:
                /*
                 * Bring in the user page that we will copy from _first_.
                 * Otherwise there's a nasty deadlock on copying from the
                 * same page as we're writing to, without it being marked
                 * up-to-date.
                 *
                 * Not only is this an optimisation, but it is also required
                 * to check that the address is actually valid, when atomic
                 * usercopies are used, below.
                 */
                if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
                        status = -EFAULT;
                        break;
                }

                if (fatal_signal_pending(current)) {
                        status = -EINTR;
                        break;
                }

                status = a_ops->write_begin(file, mapping, pos, bytes, flags,
                                                &page, &fsdata);
                if (unlikely(status < 0))
                        break;

                if (mapping_writably_mapped(mapping))
                        flush_dcache_page(page);

                copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
                flush_dcache_page(page);

                status = a_ops->write_end(file, mapping, pos, bytes, copied,
                                                page, fsdata);
                if (unlikely(status < 0))
                        break;
                copied = status;

                cond_resched();

                iov_iter_advance(i, copied);
                if (unlikely(copied == 0)) {
                        /*
                         * If we were unable to copy any data at all, we must
                         * fall back to a single segment length write.
                         *
                         * If we didn't fallback here, we could livelock
                         * because not all segments in the iov can be copied at
                         * once without a pagefault.
                         */
                        bytes = min_t(unsigned long, PAGE_SIZE - offset,
                                                iov_iter_single_seg_count(i));
                        goto again;
                }
                pos += copied;
                written += copied;

                balance_dirty_pages_ratelimited(mapping);
        } while (iov_iter_count(i));

        return written ? written : status;
}
EXPORT_SYMBOL(generic_perform_write);

/**
 * __generic_file_write_iter - write data to a file
 * @iocb:       IO state structure (file, offset, etc.)
 * @from:       iov_iter with data to write
 *
 * This function does all the work needed for actually writing data to a
 * file. It does all basic checks, removes SUID from the file, updates
 * modification times and calls proper subroutines depending on whether we
 * do direct IO or a standard buffered write.
 *
 * It expects i_mutex to be grabbed unless we work on a block device or similar
 * object which does not need locking at all.
 *
 * This function does *not* take care of syncing data in case of O_SYNC write.
 * A caller has to handle it. This is mainly due to the fact that we want to
 * avoid syncing under i_mutex.
 */
ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
        struct file *file = iocb->ki_filp;
        struct address_space * mapping = file->f_mapping;
        struct inode    *inode = mapping->host;
        ssize_t         written = 0;
        ssize_t         err;
        ssize_t         status;

        /* We can write back this queue in page reclaim */
        current->backing_dev_info = inode_to_bdi(inode);
        err = file_remove_privs(file);
        if (err)
                goto out;

        err = file_update_time(file);
        if (err)
                goto out;

        if (iocb->ki_flags & IOCB_DIRECT) {
                loff_t pos, endbyte;

                written = generic_file_direct_write(iocb, from);
                /*
                 * If the write stopped short of completing, fall back to
                 * buffered writes.  Some filesystems do this for writes to
                 * holes, for example.  For DAX files, a buffered write will
                 * not succeed (even if it did, DAX does not handle dirty
                 * page-cache pages correctly).
                 */
                if (written < 0 || !iov_iter_count(from) || IS_DAX(inode))
                        goto out;

                status = generic_perform_write(file, from, pos = iocb->ki_pos);
                /*
                 * If generic_perform_write() returned a synchronous error
                 * then we want to return the number of bytes which were
                 * direct-written, or the error code if that was zero.  Note
                 * that this differs from normal direct-io semantics, which
                 * will return -EFOO even if some bytes were written.
                 */
                if (unlikely(status < 0)) {
                        err = status;
                        goto out;
                }
                /*
                 * We need to ensure that the page cache pages are written to
                 * disk and invalidated to preserve the expected O_DIRECT
                 * semantics.
                 */
                endbyte = pos + status - 1;
                err = filemap_write_and_wait_range(mapping, pos, endbyte);
                if (err == 0) {
                        iocb->ki_pos = endbyte + 1;
                        written += status;
                        invalidate_mapping_pages(mapping,
                                                 pos >> PAGE_SHIFT,
                                                 endbyte >> PAGE_SHIFT);
                } else {
                        /*
                         * We don't know how much we wrote, so just return
                         * the number of bytes which were direct-written
                         */
                }
        } else {
                written = generic_perform_write(file, from, iocb->ki_pos);
                if (likely(written > 0))
                        iocb->ki_pos += written;
        }
out:
        current->backing_dev_info = NULL;
        return written ? written : err;
}
EXPORT_SYMBOL(__generic_file_write_iter);

/**
 * generic_file_write_iter - write data to a file
 * @iocb:       IO state structure
 * @from:       iov_iter with data to write
 *
 * This is a wrapper around __generic_file_write_iter() to be used by most
 * filesystems. It takes care of syncing the file in case of O_SYNC file
 * and acquires i_mutex as needed.
 */
ssize_t generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
        struct file *file = iocb->ki_filp;
        struct inode *inode = file->f_mapping->host;
        ssize_t ret;

        inode_lock(inode);
        ret = generic_write_checks(iocb, from);
        if (ret > 0)
                ret = __generic_file_write_iter(iocb, from);
        inode_unlock(inode);

        if (ret > 0)
                ret = generic_write_sync(iocb, ret);
        return ret;
}
EXPORT_SYMBOL(generic_file_write_iter);

/**
 * try_to_release_page() - release old fs-specific metadata on a page
 *
 * @page: the page which the kernel is trying to free
 * @gfp_mask: memory allocation flags (and I/O mode)
 *
 * The address_space is to try to release any data against the page
 * (presumably at page->private).  If the release was successful, return `1'.
 * Otherwise return zero.
 *
 * This may also be called if PG_fscache is set on a page, indicating that the
 * page is known to the local caching routines.
 *
 * The @gfp_mask argument specifies whether I/O may be performed to release
 * this page (__GFP_IO), and whether the call may block (__GFP_RECLAIM & __GFP_FS).
 *
 */
int try_to_release_page(struct page *page, gfp_t gfp_mask)
{
        struct address_space * const mapping = page->mapping;

        BUG_ON(!PageLocked(page));
        if (PageWriteback(page))
                return 0;

        if (mapping && mapping->a_ops->releasepage)
                return mapping->a_ops->releasepage(page, gfp_mask);
        return try_to_free_buffers(page);
}

EXPORT_SYMBOL(try_to_release_page);