filemap.c

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/*
 *  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/fs.h>
#include <linux/uaccess.h>
#include <linux/aio.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/memcontrol.h>
#include <linux/cleancache.h>
#include "internal.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_mutex      (truncate_pagecache)
 *    ->private_lock        (__free_pte->__set_page_dirty_buffers)
 *      ->swap_lock     (exclusive_swap_page, others)
 *        ->mapping->tree_lock
 *
 *  ->i_mutex
 *    ->i_mmap_mutex        (truncate->unmap_mapping_range)
 *
 *  ->mmap_sem
 *    ->i_mmap_mutex
 *      ->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_file_buffered_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_mutex
 *    ->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)
 *    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_mutex
 *   ->tasklist_lock            (memory_failure, collect_procs_ao)
 */

/*
 * 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)
{
    struct address_space *mapping = page->mapping;

    /*
     * 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);

    radix_tree_delete(&mapping->page_tree, page->index);
    page->mapping = NULL;
    /* Leave page->index set: truncation lookup relies upon it */
    mapping->nrpages--;
    __dec_zone_page_state(page, NR_FILE_PAGES);
    if (PageSwapBacked(page))
        __dec_zone_page_state(page, NR_SHMEM);
    BUG_ON(page_mapped(page));

    /*
     * Some filesystems seem to re-dirty the page even after
     * the VM has canceled the dirty bit (eg ext3 journaling).
     *
     * Fix it up by doing a final dirty accounting check after
     * having removed the page entirely.
     */
    if (PageDirty(page) && mapping_cap_account_dirty(mapping)) {
        dec_zone_page_state(page, NR_FILE_DIRTY);
        dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
    }
}

void delete_from_page_cache(struct page *page)
{
    struct address_space *mapping = page->mapping;
    void (*freepage)(struct page *);

    BUG_ON(!PageLocked(page));

    freepage = mapping->a_ops->freepage;
    spin_lock_irq(&mapping->tree_lock);
    __delete_from_page_cache(page);
    spin_unlock_irq(&mapping->tree_lock);
    mem_cgroup_uncharge_cache_page(page);

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

static int sleep_on_page(void *word)
{
    io_schedule();
    return 0;
}

static int sleep_on_page_killable(void *word)
{
    sleep_on_page(word);
    return fatal_signal_pending(current) ? -EINTR : 0;
}

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;

    ret = do_writepages(mapping, &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);

int filemap_flush(struct address_space *mapping)
{
    return __filemap_fdatawrite(mapping, WB_SYNC_NONE);
}
EXPORT_SYMBOL(filemap_flush);

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

    if (end_byte < start_byte)
        return 0;

    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();
    }

    /* Check for outstanding write errors */
    if (test_and_clear_bit(AS_ENOSPC, &mapping->flags))
        ret = -ENOSPC;
    if (test_and_clear_bit(AS_EIO, &mapping->flags))
        ret = -EIO;

    return ret;
}
EXPORT_SYMBOL(filemap_fdatawait_range);

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 (mapping->nrpages) {
        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;
        }
    }
    return err;
}
EXPORT_SYMBOL(filemap_write_and_wait);

int filemap_write_and_wait_range(struct address_space *mapping,
                 loff_t lstart, loff_t lend)
{
    int err = 0;

    if (mapping->nrpages) {
        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;
        }
    }
    return err;
}
EXPORT_SYMBOL(filemap_write_and_wait_range);

int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
{
    int error;

    VM_BUG_ON(!PageLocked(old));
    VM_BUG_ON(!PageLocked(new));
    VM_BUG_ON(new->mapping);

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

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

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

        spin_lock_irq(&mapping->tree_lock);
        __delete_from_page_cache(old);
        error = radix_tree_insert(&mapping->page_tree, offset, new);
        BUG_ON(error);
        mapping->nrpages++;
        __inc_zone_page_state(new, NR_FILE_PAGES);
        if (PageSwapBacked(new))
            __inc_zone_page_state(new, NR_SHMEM);
        spin_unlock_irq(&mapping->tree_lock);
        /* mem_cgroup codes must not be called under tree_lock */
        mem_cgroup_replace_page_cache(old, new);
        radix_tree_preload_end();
        if (freepage)
            freepage(old);
        page_cache_release(old);
    }

    return error;
}
EXPORT_SYMBOL_GPL(replace_page_cache_page);

int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
        pgoff_t offset, gfp_t gfp_mask)
{
    int error;

    VM_BUG_ON(!PageLocked(page));
    VM_BUG_ON(PageSwapBacked(page));

    error = mem_cgroup_cache_charge(page, current->mm,
                    gfp_mask & GFP_RECLAIM_MASK);
    if (error)
        goto out;

    error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
    if (error == 0) {
        page_cache_get(page);
        page->mapping = mapping;
        page->index = offset;

        spin_lock_irq(&mapping->tree_lock);
        error = radix_tree_insert(&mapping->page_tree, offset, page);
        if (likely(!error)) {
            mapping->nrpages++;
            __inc_zone_page_state(page, NR_FILE_PAGES);
            spin_unlock_irq(&mapping->tree_lock);
        } else {
            page->mapping = NULL;
            /* Leave page->index set: truncation relies upon it */
            spin_unlock_irq(&mapping->tree_lock);
            mem_cgroup_uncharge_cache_page(page);
            page_cache_release(page);
        }
        radix_tree_preload_end();
    } else
        mem_cgroup_uncharge_cache_page(page);
out:
    return error;
}
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)
{
    int ret;

    ret = add_to_page_cache(page, mapping, offset, gfp_mask);
    if (ret == 0)
        lru_cache_add_file(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 = get_mems_allowed();
            n = cpuset_mem_spread_node();
            page = alloc_pages_exact_node(n, gfp, 0);
        } while (!put_mems_allowed(cpuset_mems_cookie) && !page);

        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.
 */
static 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)];
}

static inline void wake_up_page(struct page *page, int bit)
{
    __wake_up_bit(page_waitqueue(page), &page->flags, bit);
}

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, sleep_on_page,
                            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,
                 sleep_on_page_killable, TASK_KILLABLE);
}

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);

void unlock_page(struct page *page)
{
    VM_BUG_ON(!PageLocked(page));
    clear_bit_unlock(PG_locked, &page->flags);
    smp_mb__after_clear_bit();
    wake_up_page(page, PG_locked);
}
EXPORT_SYMBOL(unlock_page);

void end_page_writeback(struct page *page)
{
    if (TestClearPageReclaim(page))
        rotate_reclaimable_page(page);

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

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

void __lock_page(struct page *page)
{
    DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);

    __wait_on_bit_lock(page_waitqueue(page), &wait, sleep_on_page,
                            TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(__lock_page);

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

    return __wait_on_bit_lock(page_waitqueue(page), &wait,
                    sleep_on_page_killable, TASK_KILLABLE);
}
EXPORT_SYMBOL_GPL(__lock_page_killable);

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;
    }
}

struct page *find_get_page(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;
            /*
             * Otherwise, shmem/tmpfs must be storing a swap entry
             * here as an exceptional entry: so 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)) {
            page_cache_release(page);
            goto repeat;
        }
    }
out:
    rcu_read_unlock();

    return page;
}
EXPORT_SYMBOL(find_get_page);

struct page *find_lock_page(struct address_space *mapping, pgoff_t offset)
{
    struct page *page;

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

struct page *find_or_create_page(struct address_space *mapping,
        pgoff_t index, gfp_t gfp_mask)
{
    struct page *page;
    int err;
repeat:
    page = find_lock_page(mapping, index);
    if (!page) {
        page = __page_cache_alloc(gfp_mask);
        if (!page)
            return NULL;
        /*
         * We want a regular kernel memory (not highmem or DMA etc)
         * allocation for the radix tree nodes, but we need to honour
         * the context-specific requirements the caller has asked for.
         * GFP_RECLAIM_MASK collects those requirements.
         */
        err = add_to_page_cache_lru(page, mapping, index,
            (gfp_mask & GFP_RECLAIM_MASK));
        if (unlikely(err)) {
            page_cache_release(page);
            page = NULL;
            if (err == -EEXIST)
                goto repeat;
        }
    }
    return page;
}
EXPORT_SYMBOL(find_or_create_page);

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();
restart:
    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)) {
                /*
                 * Transient condition which can only trigger
                 * when entry at index 0 moves out of or back
                 * to root: none yet gotten, safe to restart.
                 */
                WARN_ON(iter.index);
                goto restart;
            }
            /*
             * Otherwise, shmem/tmpfs must be storing a swap entry
             * here as an exceptional entry: so skip over it -
             * we only reach this from invalidate_mapping_pages().
             */
            continue;
        }

        if (!page_cache_get_speculative(page))
            goto repeat;

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

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

    rcu_read_unlock();
    return ret;
}

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();
restart:
    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)) {
                /*
                 * Transient condition which can only trigger
                 * when entry at index 0 moves out of or back
                 * to root: none yet gotten, safe to restart.
                 */
                goto restart;
            }
            /*
             * Otherwise, shmem/tmpfs must be storing a swap entry
             * here as an exceptional entry: so stop looking for
             * contiguous pages.
             */
            break;
        }

        if (!page_cache_get_speculative(page))
            goto repeat;

        /* Has the page moved? */
        if (unlikely(page != *slot)) {
            page_cache_release(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) {
            page_cache_release(page);
            break;
        }

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

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();
restart:
    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)) {
                /*
                 * Transient condition which can only trigger
                 * when entry at index 0 moves out of or back
                 * to root: none yet gotten, safe to restart.
                 */
                goto restart;
            }
            /*
             * This function is never used on a shmem/tmpfs
             * mapping, so a swap entry won't be found here.
             */
            BUG();
        }

        if (!page_cache_get_speculative(page))
            goto repeat;

        /* Has the page moved? */
        if (unlikely(page != *slot)) {
            page_cache_release(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);

struct page *
grab_cache_page_nowait(struct address_space *mapping, pgoff_t index)
{
    struct page *page = find_get_page(mapping, index);

    if (page) {
        if (trylock_page(page))
            return page;
        page_cache_release(page);
        return NULL;
    }
    page = __page_cache_alloc(mapping_gfp_mask(mapping) & ~__GFP_FS);
    if (page && add_to_page_cache_lru(page, mapping, index, GFP_NOFS)) {
        page_cache_release(page);
        page = NULL;
    }
    return page;
}
EXPORT_SYMBOL(grab_cache_page_nowait);

/*
 * 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;
}

static void do_generic_file_read(struct file *filp, loff_t *ppos,
        read_descriptor_t *desc, read_actor_t actor)
{
    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;

    index = *ppos >> PAGE_CACHE_SHIFT;
    prev_index = ra->prev_pos >> PAGE_CACHE_SHIFT;
    prev_offset = ra->prev_pos & (PAGE_CACHE_SIZE-1);
    last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
    offset = *ppos & ~PAGE_CACHE_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)) {
            if (inode->i_blkbits == PAGE_CACHE_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,
                                desc, offset))
                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_CACHE_SHIFT;
        if (unlikely(!isize || index > end_index)) {
            page_cache_release(page);
            goto out;
        }

        /* nr is the maximum number of bytes to copy from this page */
        nr = PAGE_CACHE_SIZE;
        if (index == end_index) {
            nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
            if (nr <= offset) {
                page_cache_release(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...
         *
         * The actor routine returns how many bytes were actually used..
         * NOTE! This may not be the same as how much of a user buffer
         * we filled up (we may be padding etc), so we can only update
         * "pos" here (the actor routine has to update the user buffer
         * pointers and the remaining count).
         */
        ret = actor(desc, page, offset, nr);
        offset += ret;
        index += offset >> PAGE_CACHE_SHIFT;
        offset &= ~PAGE_CACHE_MASK;
        prev_offset = offset;

        page_cache_release(page);
        if (ret == nr && desc->count)
            continue;
        goto out;

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);
            page_cache_release(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) {
                page_cache_release(page);
                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);
                    page_cache_release(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 */
        desc->error = error;
        page_cache_release(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) {
            desc->error = -ENOMEM;
            goto out;
        }
        error = add_to_page_cache_lru(page, mapping,
                        index, GFP_KERNEL);
        if (error) {
            page_cache_release(page);
            if (error == -EEXIST)
                goto find_page;
            desc->error = error;
            goto out;
        }
        goto readpage;
    }

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

    *ppos = ((loff_t)index << PAGE_CACHE_SHIFT) + offset;
    file_accessed(filp);
}

int file_read_actor(read_descriptor_t *desc, struct page *page,
            unsigned long offset, unsigned long size)
{
    char *kaddr;
    unsigned long left, count = desc->count;

    if (size > count)
        size = count;

    /*
     * Faults on the destination of a read are common, so do it before
     * taking the kmap.
     */
    if (!fault_in_pages_writeable(desc->arg.buf, size)) {
        kaddr = kmap_atomic(page);
        left = __copy_to_user_inatomic(desc->arg.buf,
                        kaddr + offset, size);
        kunmap_atomic(kaddr);
        if (left == 0)
            goto success;
    }

    /* Do it the slow way */
    kaddr = kmap(page);
    left = __copy_to_user(desc->arg.buf, kaddr + offset, size);
    kunmap(page);

    if (left) {
        size -= left;
        desc->error = -EFAULT;
    }
success:
    desc->count = count - size;
    desc->written += size;
    desc->arg.buf += size;
    return size;
}

/*
 * Performs necessary checks before doing a write
 * @iov:    io vector request
 * @nr_segs:    number of segments in the iovec
 * @count:  number of bytes to write
 * @access_flags: type of access: %VERIFY_READ or %VERIFY_WRITE
 *
 * Adjust number of segments and amount of bytes to write (nr_segs should be
 * properly initialized first). Returns appropriate error code that caller
 * should return or zero in case that write should be allowed.
 */
int generic_segment_checks(const struct iovec *iov,
            unsigned long *nr_segs, size_t *count, int access_flags)
{
    unsigned long   seg;
    size_t cnt = 0;
    for (seg = 0; seg < *nr_segs; seg++) {
        const struct iovec *iv = &iov[seg];

        /*
         * If any segment has a negative length, or the cumulative
         * length ever wraps negative then return -EINVAL.
         */
        cnt += iv->iov_len;
        if (unlikely((ssize_t)(cnt|iv->iov_len) < 0))
            return -EINVAL;
        if (access_ok(access_flags, iv->iov_base, iv->iov_len))
            continue;
        if (seg == 0)
            return -EFAULT;
        *nr_segs = seg;
        cnt -= iv->iov_len; /* This segment is no good */
        break;
    }
    *count = cnt;
    return 0;
}
EXPORT_SYMBOL(generic_segment_checks);

ssize_t
generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
        unsigned long nr_segs, loff_t pos)
{
    struct file *filp = iocb->ki_filp;
    ssize_t retval;
    unsigned long seg = 0;
    size_t count;
    loff_t *ppos = &iocb->ki_pos;

    count = 0;
    retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
    if (retval)
        return retval;

    /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
    if (filp->f_flags & O_DIRECT) {
        loff_t size;
        struct address_space *mapping;
        struct inode *inode;

        mapping = filp->f_mapping;
        inode = mapping->host;
        if (!count)
            goto out; /* skip atime */
        size = i_size_read(inode);
        if (pos < size) {
            retval = filemap_write_and_wait_range(mapping, pos,
                    pos + iov_length(iov, nr_segs) - 1);
            if (!retval) {
                retval = mapping->a_ops->direct_IO(READ, iocb,
                            iov, pos, nr_segs);
            }
            if (retval > 0) {
                *ppos = pos + retval;
                count -= 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.
             */
            if (retval < 0 || !count || *ppos >= size) {
                file_accessed(filp);
                goto out;
            }
        }
    }

    count = retval;
    for (seg = 0; seg < nr_segs; seg++) {
        read_descriptor_t desc;
        loff_t offset = 0;

        /*
         * If we did a short DIO read we need to skip the section of the
         * iov that we've already read data into.
         */
        if (count) {
            if (count > iov[seg].iov_len) {
                count -= iov[seg].iov_len;
                continue;
            }
            offset = count;
            count = 0;
        }

        desc.written = 0;
        desc.arg.buf = iov[seg].iov_base + offset;
        desc.count = iov[seg].iov_len - offset;
        if (desc.count == 0)
            continue;
        desc.error = 0;
        do_generic_file_read(filp, ppos, &desc, file_read_actor);
        retval += desc.written;
        if (desc.error) {
            retval = retval ?: desc.error;
            break;
        }
        if (desc.count > 0)
            break;
    }
out:
    return retval;
}
EXPORT_SYMBOL(generic_file_aio_read);

#ifdef CONFIG_MMU

static int page_cache_read(struct file *file, pgoff_t offset)
{
    struct address_space *mapping = file->f_mapping;
    struct page *page; 
    int ret;

    do {
        page = page_cache_alloc_cold(mapping);
        if (!page)
            return -ENOMEM;

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

        page_cache_release(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)
{
    unsigned long ra_pages;
    struct address_space *mapping = file->f_mapping;

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

    if (VM_SequentialReadHint(vma)) {
        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_pages = max_sane_readahead(ra->ra_pages);
    ra->start = max_t(long, 0, offset - ra_pages / 2);
    ra->size = ra_pages;
    ra->async_size = 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 (VM_RandomReadHint(vma))
        return;
    if (ra->mmap_miss > 0)
        ra->mmap_miss--;
    if (PageReadahead(page))
        page_cache_async_readahead(mapping, ra, file,
                       page, offset, ra->ra_pages);
}

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;
    pgoff_t size;
    int ret = 0;

    size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
    if (offset >= size)
        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)) {
        page_cache_release(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->index != offset);

    /*
     * 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 = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
    if (unlikely(offset >= size)) {
        unlock_page(page);
        page_cache_release(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);

    /*
     * 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;
    }
    page_cache_release(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);

int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
{
    struct page *page = vmf->page;
    struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
    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);
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,
    .page_mkwrite   = filemap_page_mkwrite,
    .remap_pages    = generic_file_remap_pages,
};

/* 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 *__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)) {
            page_cache_release(page);
            if (err == -EEXIST)
                goto repeat;
            /* Presumably ENOMEM for radix tree node */
            return ERR_PTR(err);
        }
        err = filler(data, page);
        if (err < 0) {
            page_cache_release(page);
            page = ERR_PTR(err);
        }
    }
    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;

retry:
    page = __read_cache_page(mapping, index, filler, data, gfp);
    if (IS_ERR(page))
        return page;
    if (PageUptodate(page))
        goto out;

    lock_page(page);
    if (!page->mapping) {
        unlock_page(page);
        page_cache_release(page);
        goto retry;
    }
    if (PageUptodate(page)) {
        unlock_page(page);
        goto out;
    }
    err = filler(data, page);
    if (err < 0) {
        page_cache_release(page);
        return ERR_PTR(err);
    }
out:
    mark_page_accessed(page);
    return page;
}

struct page *read_cache_page_async(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_async);

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

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

struct page *read_cache_page(struct address_space *mapping,
                pgoff_t index,
                int (*filler)(void *, struct page *),
                void *data)
{
    return wait_on_page_read(read_cache_page_async(mapping, index, filler, data));
}
EXPORT_SYMBOL(read_cache_page);

static size_t __iovec_copy_from_user_inatomic(char *vaddr,
            const struct iovec *iov, size_t base, size_t bytes)
{
    size_t copied = 0, left = 0;

    while (bytes) {
        char __user *buf = iov->iov_base + base;
        int copy = min(bytes, iov->iov_len - base);

        base = 0;
        left = __copy_from_user_inatomic(vaddr, buf, copy);
        copied += copy;
        bytes -= copy;
        vaddr += copy;
        iov++;

        if (unlikely(left))
            break;
    }
    return copied - left;
}

/*
 * Copy as much as we can into the page and return the number of bytes which
 * were successfully copied.  If a fault is encountered then return the number of
 * bytes which were copied.
 */
size_t iov_iter_copy_from_user_atomic(struct page *page,
        struct iov_iter *i, unsigned long offset, size_t bytes)
{
    char *kaddr;
    size_t copied;

    BUG_ON(!in_atomic());
    kaddr = kmap_atomic(page);
    if (likely(i->nr_segs == 1)) {
        int left;
        char __user *buf = i->iov->iov_base + i->iov_offset;
        left = __copy_from_user_inatomic(kaddr + offset, buf, bytes);
        copied = bytes - left;
    } else {
        copied = __iovec_copy_from_user_inatomic(kaddr + offset,
                        i->iov, i->iov_offset, bytes);
    }
    kunmap_atomic(kaddr);

    return copied;
}
EXPORT_SYMBOL(iov_iter_copy_from_user_atomic);

/*
 * This has the same sideeffects and return value as
 * iov_iter_copy_from_user_atomic().
 * The difference is that it attempts to resolve faults.
 * Page must not be locked.
 */
size_t iov_iter_copy_from_user(struct page *page,
        struct iov_iter *i, unsigned long offset, size_t bytes)
{
    char *kaddr;
    size_t copied;

    kaddr = kmap(page);
    if (likely(i->nr_segs == 1)) {
        int left;
        char __user *buf = i->iov->iov_base + i->iov_offset;
        left = __copy_from_user(kaddr + offset, buf, bytes);
        copied = bytes - left;
    } else {
        copied = __iovec_copy_from_user_inatomic(kaddr + offset,
                        i->iov, i->iov_offset, bytes);
    }
    kunmap(page);
    return copied;
}
EXPORT_SYMBOL(iov_iter_copy_from_user);

void iov_iter_advance(struct iov_iter *i, size_t bytes)
{
    BUG_ON(i->count < bytes);

    if (likely(i->nr_segs == 1)) {
        i->iov_offset += bytes;
        i->count -= bytes;
    } else {
        const struct iovec *iov = i->iov;
        size_t base = i->iov_offset;
        unsigned long nr_segs = i->nr_segs;

        /*
         * The !iov->iov_len check ensures we skip over unlikely
         * zero-length segments (without overruning the iovec).
         */
        while (bytes || unlikely(i->count && !iov->iov_len)) {
            int copy;

            copy = min(bytes, iov->iov_len - base);
            BUG_ON(!i->count || i->count < copy);
            i->count -= copy;
            bytes -= copy;
            base += copy;
            if (iov->iov_len == base) {
                iov++;
                nr_segs--;
                base = 0;
            }
        }
        i->iov = iov;
        i->iov_offset = base;
        i->nr_segs = nr_segs;
    }
}
EXPORT_SYMBOL(iov_iter_advance);

/*
 * Fault in the first iovec of the given iov_iter, to a maximum length
 * of bytes. Returns 0 on success, or non-zero if the memory could not be
 * accessed (ie. because it is an invalid address).
 *
 * writev-intensive code may want this to prefault several iovecs -- that
 * would be possible (callers must not rely on the fact that _only_ the
 * first iovec will be faulted with the current implementation).
 */
int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes)
{
    char __user *buf = i->iov->iov_base + i->iov_offset;
    bytes = min(bytes, i->iov->iov_len - i->iov_offset);
    return fault_in_pages_readable(buf, bytes);
}
EXPORT_SYMBOL(iov_iter_fault_in_readable);

/*
 * Return the count of just the current iov_iter segment.
 */
size_t iov_iter_single_seg_count(struct iov_iter *i)
{
    const struct iovec *iov = i->iov;
    if (i->nr_segs == 1)
        return i->count;
    else
        return min(i->count, iov->iov_len - i->iov_offset);
}
EXPORT_SYMBOL(iov_iter_single_seg_count);

/*
 * 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 int generic_write_checks(struct file *file, loff_t *pos, size_t *count, int isblk)
{
    struct inode *inode = file->f_mapping->host;
    unsigned long limit = rlimit(RLIMIT_FSIZE);

        if (unlikely(*pos < 0))
                return -EINVAL;

    if (!isblk) {
        /* FIXME: this is for backwards compatibility with 2.4 */
        if (file->f_flags & O_APPEND)
                        *pos = i_size_read(inode);

        if (limit != RLIM_INFINITY) {
            if (*pos >= limit) {
                send_sig(SIGXFSZ, current, 0);
                return -EFBIG;
            }
            if (*count > limit - (typeof(limit))*pos) {
                *count = limit - (typeof(limit))*pos;
            }
        }
    }

    /*
     * LFS rule
     */
    if (unlikely(*pos + *count > MAX_NON_LFS &&
                !(file->f_flags & O_LARGEFILE))) {
        if (*pos >= MAX_NON_LFS) {
            return -EFBIG;
        }
        if (*count > MAX_NON_LFS - (unsigned long)*pos) {
            *count = 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 (likely(!isblk)) {
        if (unlikely(*pos >= inode->i_sb->s_maxbytes)) {
            if (*count || *pos > inode->i_sb->s_maxbytes) {
                return -EFBIG;
            }
            /* zero-length writes at ->s_maxbytes are OK */
        }

        if (unlikely(*pos + *count > inode->i_sb->s_maxbytes))
            *count = inode->i_sb->s_maxbytes - *pos;
    } else {
#ifdef CONFIG_BLOCK
        loff_t isize;
        if (bdev_read_only(I_BDEV(inode)))
            return -EPERM;
        isize = i_size_read(inode);
        if (*pos >= isize) {
            if (*count || *pos > isize)
                return -ENOSPC;
        }

        if (*pos + *count > isize)
            *count = isize - *pos;
#else
        return -EPERM;
#endif
    }
    return 0;
}
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;

    mark_page_accessed(page);
    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, const struct iovec *iov,
        unsigned long *nr_segs, loff_t pos, loff_t *ppos,
        size_t count, size_t ocount)
{
    struct file *file = iocb->ki_filp;
    struct address_space *mapping = file->f_mapping;
    struct inode    *inode = mapping->host;
    ssize_t     written;
    size_t      write_len;
    pgoff_t     end;

    if (count != ocount)
        *nr_segs = iov_shorten((struct iovec *)iov, *nr_segs, count);

    write_len = iov_length(iov, *nr_segs);
    end = (pos + write_len - 1) >> PAGE_CACHE_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_CACHE_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;
        }
    }

    written = mapping->a_ops->direct_IO(WRITE, iocb, iov, pos, *nr_segs);

    /*
     * 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_CACHE_SHIFT, end);
    }

    if (written > 0) {
        pos += written;
        if (pos > i_size_read(inode) && !S_ISBLK(inode->i_mode)) {
            i_size_write(inode, pos);
            mark_inode_dirty(inode);
        }
        *ppos = 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)
{
    int status;
    gfp_t gfp_mask;
    struct page *page;
    gfp_t gfp_notmask = 0;

    gfp_mask = mapping_gfp_mask(mapping);
    if (mapping_cap_account_dirty(mapping))
        gfp_mask |= __GFP_WRITE;
    if (flags & AOP_FLAG_NOFS)
        gfp_notmask = __GFP_FS;
repeat:
    page = find_lock_page(mapping, index);
    if (page)
        goto found;

    page = __page_cache_alloc(gfp_mask & ~gfp_notmask);
    if (!page)
        return NULL;
    status = add_to_page_cache_lru(page, mapping, index,
                        GFP_KERNEL & ~gfp_notmask);
    if (unlikely(status)) {
        page_cache_release(page);
        if (status == -EEXIST)
            goto repeat;
        return NULL;
    }
found:
    wait_on_page_writeback(page);
    return page;
}
EXPORT_SYMBOL(grab_cache_page_write_begin);

static 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 (segment_eq(get_fs(), KERNEL_DS))
        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_CACHE_SIZE - 1));
        bytes = min_t(unsigned long, PAGE_CACHE_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;
        }

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

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

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

        mark_page_accessed(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_CACHE_SIZE - offset,
                        iov_iter_single_seg_count(i));
            goto again;
        }
        pos += copied;
        written += copied;

        balance_dirty_pages_ratelimited(mapping);
        if (fatal_signal_pending(current)) {
            status = -EINTR;
            break;
        }
    } while (iov_iter_count(i));

    return written ? written : status;
}

ssize_t
generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
        unsigned long nr_segs, loff_t pos, loff_t *ppos,
        size_t count, ssize_t written)
{
    struct file *file = iocb->ki_filp;
    ssize_t status;
    struct iov_iter i;

    iov_iter_init(&i, iov, nr_segs, count, written);
    status = generic_perform_write(file, &i, pos);

    if (likely(status >= 0)) {
        written += status;
        *ppos = pos + status;
    }
    
    return written ? written : status;
}
EXPORT_SYMBOL(generic_file_buffered_write);

ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
                 unsigned long nr_segs, loff_t *ppos)
{
    struct file *file = iocb->ki_filp;
    struct address_space * mapping = file->f_mapping;
    size_t ocount;      /* original count */
    size_t count;       /* after file limit checks */
    struct inode    *inode = mapping->host;
    loff_t      pos;
    ssize_t     written;
    ssize_t     err;

    ocount = 0;
    err = generic_segment_checks(iov, &nr_segs, &ocount, VERIFY_READ);
    if (err)
        return err;

    count = ocount;
    pos = *ppos;

    /* We can write back this queue in page reclaim */
    current->backing_dev_info = mapping->backing_dev_info;
    written = 0;

    err = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
    if (err)
        goto out;

    if (count == 0)
        goto out;

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

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

    /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
    if (unlikely(file->f_flags & O_DIRECT)) {
        loff_t endbyte;
        ssize_t written_buffered;

        written = generic_file_direct_write(iocb, iov, &nr_segs, pos,
                            ppos, count, ocount);
        if (written < 0 || written == count)
            goto out;
        /*
         * direct-io write to a hole: fall through to buffered I/O
         * for completing the rest of the request.
         */
        pos += written;
        count -= written;
        written_buffered = generic_file_buffered_write(iocb, iov,
                        nr_segs, pos, ppos, count,
                        written);
        /*
         * If generic_file_buffered_write() retuned 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 (written_buffered < 0) {
            err = written_buffered;
            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 + written_buffered - written - 1;
        err = filemap_write_and_wait_range(file->f_mapping, pos, endbyte);
        if (err == 0) {
            written = written_buffered;
            invalidate_mapping_pages(mapping,
                         pos >> PAGE_CACHE_SHIFT,
                         endbyte >> PAGE_CACHE_SHIFT);
        } else {
            /*
             * We don't know how much we wrote, so just return
             * the number of bytes which were direct-written
             */
        }
    } else {
        written = generic_file_buffered_write(iocb, iov, nr_segs,
                pos, ppos, count, written);
    }
out:
    current->backing_dev_info = NULL;
    return written ? written : err;
}
EXPORT_SYMBOL(__generic_file_aio_write);

ssize_t generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
        unsigned long nr_segs, loff_t pos)
{
    struct file *file = iocb->ki_filp;
    struct inode *inode = file->f_mapping->host;
    ssize_t ret;

    BUG_ON(iocb->ki_pos != pos);

    sb_start_write(inode->i_sb);
    mutex_lock(&inode->i_mutex);
    ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
    mutex_unlock(&inode->i_mutex);

    if (ret > 0 || ret == -EIOCBQUEUED) {
        ssize_t err;

        err = generic_write_sync(file, pos, ret);
        if (err < 0 && ret > 0)
            ret = err;
    }
    sb_end_write(inode->i_sb);
    return ret;
}
EXPORT_SYMBOL(generic_file_aio_write);

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);