fs-writeback.c

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/*
 * fs/fs-writeback.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * Contains all the functions related to writing back and waiting
 * upon dirty inodes against superblocks, and writing back dirty
 * pages against inodes.  ie: data writeback.  Writeout of the
 * inode itself is not handled here.
 *
 * 10Apr2002    Andrew Morton
 *      Split out of fs/inode.c
 *      Additions for address_space-based writeback
 */

#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/spinlock.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/tracepoint.h>
#include "internal.h"

/*
 * 4MB minimal write chunk size
 */
#define MIN_WRITEBACK_PAGES (4096UL >> (PAGE_CACHE_SHIFT - 10))

/*
 * Passed into wb_writeback(), essentially a subset of writeback_control
 */
struct wb_writeback_work {
    long nr_pages;
    struct super_block *sb;
    unsigned long *older_than_this;
    enum writeback_sync_modes sync_mode;
    unsigned int tagged_writepages:1;
    unsigned int for_kupdate:1;
    unsigned int range_cyclic:1;
    unsigned int for_background:1;
    enum wb_reason reason;      /* why was writeback initiated? */

    struct list_head list;      /* pending work list */
    struct completion *done;    /* set if the caller waits */
};

int writeback_in_progress(struct backing_dev_info *bdi)
{
    return test_bit(BDI_writeback_running, &bdi->state);
}
EXPORT_SYMBOL(writeback_in_progress);

static inline struct backing_dev_info *inode_to_bdi(struct inode *inode)
{
    struct super_block *sb = inode->i_sb;

    if (strcmp(sb->s_type->name, "bdev") == 0)
        return inode->i_mapping->backing_dev_info;

    return sb->s_bdi;
}

static inline struct inode *wb_inode(struct list_head *head)
{
    return list_entry(head, struct inode, i_wb_list);
}

/*
 * Include the creation of the trace points after defining the
 * wb_writeback_work structure and inline functions so that the definition
 * remains local to this file.
 */
#define CREATE_TRACE_POINTS
#include <trace/events/writeback.h>

/* Wakeup flusher thread or forker thread to fork it. Requires bdi->wb_lock. */
static void bdi_wakeup_flusher(struct backing_dev_info *bdi)
{
    if (bdi->wb.task) {
        wake_up_process(bdi->wb.task);
    } else {
        /*
         * The bdi thread isn't there, wake up the forker thread which
         * will create and run it.
         */
        wake_up_process(default_backing_dev_info.wb.task);
    }
}

static void bdi_queue_work(struct backing_dev_info *bdi,
               struct wb_writeback_work *work)
{
    trace_writeback_queue(bdi, work);

    spin_lock_bh(&bdi->wb_lock);
    list_add_tail(&work->list, &bdi->work_list);
    if (!bdi->wb.task)
        trace_writeback_nothread(bdi, work);
    bdi_wakeup_flusher(bdi);
    spin_unlock_bh(&bdi->wb_lock);
}

static void
__bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
              bool range_cyclic, enum wb_reason reason)
{
    struct wb_writeback_work *work;

    /*
     * This is WB_SYNC_NONE writeback, so if allocation fails just
     * wakeup the thread for old dirty data writeback
     */
    work = kzalloc(sizeof(*work), GFP_ATOMIC);
    if (!work) {
        if (bdi->wb.task) {
            trace_writeback_nowork(bdi);
            wake_up_process(bdi->wb.task);
        }
        return;
    }

    work->sync_mode = WB_SYNC_NONE;
    work->nr_pages  = nr_pages;
    work->range_cyclic = range_cyclic;
    work->reason    = reason;

    bdi_queue_work(bdi, work);
}

void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
            enum wb_reason reason)
{
    __bdi_start_writeback(bdi, nr_pages, true, reason);
}

void bdi_start_background_writeback(struct backing_dev_info *bdi)
{
    /*
     * We just wake up the flusher thread. It will perform background
     * writeback as soon as there is no other work to do.
     */
    trace_writeback_wake_background(bdi);
    spin_lock_bh(&bdi->wb_lock);
    bdi_wakeup_flusher(bdi);
    spin_unlock_bh(&bdi->wb_lock);
}

/*
 * Remove the inode from the writeback list it is on.
 */
void inode_wb_list_del(struct inode *inode)
{
    struct backing_dev_info *bdi = inode_to_bdi(inode);

    spin_lock(&bdi->wb.list_lock);
    list_del_init(&inode->i_wb_list);
    spin_unlock(&bdi->wb.list_lock);
}

/*
 * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
 * furthest end of its superblock's dirty-inode list.
 *
 * Before stamping the inode's ->dirtied_when, we check to see whether it is
 * already the most-recently-dirtied inode on the b_dirty list.  If that is
 * the case then the inode must have been redirtied while it was being written
 * out and we don't reset its dirtied_when.
 */
static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
{
    assert_spin_locked(&wb->list_lock);
    if (!list_empty(&wb->b_dirty)) {
        struct inode *tail;

        tail = wb_inode(wb->b_dirty.next);
        if (time_before(inode->dirtied_when, tail->dirtied_when))
            inode->dirtied_when = jiffies;
    }
    list_move(&inode->i_wb_list, &wb->b_dirty);
}

/*
 * requeue inode for re-scanning after bdi->b_io list is exhausted.
 */
static void requeue_io(struct inode *inode, struct bdi_writeback *wb)
{
    assert_spin_locked(&wb->list_lock);
    list_move(&inode->i_wb_list, &wb->b_more_io);
}

static void inode_sync_complete(struct inode *inode)
{
    inode->i_state &= ~I_SYNC;
    /* If inode is clean an unused, put it into LRU now... */
    inode_add_lru(inode);
    /* Waiters must see I_SYNC cleared before being woken up */
    smp_mb();
    wake_up_bit(&inode->i_state, __I_SYNC);
}

static bool inode_dirtied_after(struct inode *inode, unsigned long t)
{
    bool ret = time_after(inode->dirtied_when, t);
#ifndef CONFIG_64BIT
    /*
     * For inodes being constantly redirtied, dirtied_when can get stuck.
     * It _appears_ to be in the future, but is actually in distant past.
     * This test is necessary to prevent such wrapped-around relative times
     * from permanently stopping the whole bdi writeback.
     */
    ret = ret && time_before_eq(inode->dirtied_when, jiffies);
#endif
    return ret;
}

/*
 * Move expired (dirtied before work->older_than_this) dirty inodes from
 * @delaying_queue to @dispatch_queue.
 */
static int move_expired_inodes(struct list_head *delaying_queue,
                   struct list_head *dispatch_queue,
                   struct wb_writeback_work *work)
{
    LIST_HEAD(tmp);
    struct list_head *pos, *node;
    struct super_block *sb = NULL;
    struct inode *inode;
    int do_sb_sort = 0;
    int moved = 0;

    while (!list_empty(delaying_queue)) {
        inode = wb_inode(delaying_queue->prev);
        if (work->older_than_this &&
            inode_dirtied_after(inode, *work->older_than_this))
            break;
        if (sb && sb != inode->i_sb)
            do_sb_sort = 1;
        sb = inode->i_sb;
        list_move(&inode->i_wb_list, &tmp);
        moved++;
    }

    /* just one sb in list, splice to dispatch_queue and we're done */
    if (!do_sb_sort) {
        list_splice(&tmp, dispatch_queue);
        goto out;
    }

    /* Move inodes from one superblock together */
    while (!list_empty(&tmp)) {
        sb = wb_inode(tmp.prev)->i_sb;
        list_for_each_prev_safe(pos, node, &tmp) {
            inode = wb_inode(pos);
            if (inode->i_sb == sb)
                list_move(&inode->i_wb_list, dispatch_queue);
        }
    }
out:
    return moved;
}

/*
 * Queue all expired dirty inodes for io, eldest first.
 * Before
 *         newly dirtied     b_dirty    b_io    b_more_io
 *         =============>    gf         edc     BA
 * After
 *         newly dirtied     b_dirty    b_io    b_more_io
 *         =============>    g          fBAedc
 *                                           |
 *                                           +--> dequeue for IO
 */
static void queue_io(struct bdi_writeback *wb, struct wb_writeback_work *work)
{
    int moved;
    assert_spin_locked(&wb->list_lock);
    list_splice_init(&wb->b_more_io, &wb->b_io);
    moved = move_expired_inodes(&wb->b_dirty, &wb->b_io, work);
    trace_writeback_queue_io(wb, work, moved);
}

static int write_inode(struct inode *inode, struct writeback_control *wbc)
{
    if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode))
        return inode->i_sb->s_op->write_inode(inode, wbc);
    return 0;
}

/*
 * Wait for writeback on an inode to complete. Called with i_lock held.
 * Caller must make sure inode cannot go away when we drop i_lock.
 */
static void __inode_wait_for_writeback(struct inode *inode)
    __releases(inode->i_lock)
    __acquires(inode->i_lock)
{
    DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
    wait_queue_head_t *wqh;

    wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
    while (inode->i_state & I_SYNC) {
        spin_unlock(&inode->i_lock);
        __wait_on_bit(wqh, &wq, inode_wait, TASK_UNINTERRUPTIBLE);
        spin_lock(&inode->i_lock);
    }
}

/*
 * Wait for writeback on an inode to complete. Caller must have inode pinned.
 */
void inode_wait_for_writeback(struct inode *inode)
{
    spin_lock(&inode->i_lock);
    __inode_wait_for_writeback(inode);
    spin_unlock(&inode->i_lock);
}

/*
 * Sleep until I_SYNC is cleared. This function must be called with i_lock
 * held and drops it. It is aimed for callers not holding any inode reference
 * so once i_lock is dropped, inode can go away.
 */
static void inode_sleep_on_writeback(struct inode *inode)
    __releases(inode->i_lock)
{
    DEFINE_WAIT(wait);
    wait_queue_head_t *wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
    int sleep;

    prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
    sleep = inode->i_state & I_SYNC;
    spin_unlock(&inode->i_lock);
    if (sleep)
        schedule();
    finish_wait(wqh, &wait);
}

/*
 * Find proper writeback list for the inode depending on its current state and
 * possibly also change of its state while we were doing writeback.  Here we
 * handle things such as livelock prevention or fairness of writeback among
 * inodes. This function can be called only by flusher thread - noone else
 * processes all inodes in writeback lists and requeueing inodes behind flusher
 * thread's back can have unexpected consequences.
 */
static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
              struct writeback_control *wbc)
{
    if (inode->i_state & I_FREEING)
        return;

    /*
     * Sync livelock prevention. Each inode is tagged and synced in one
     * shot. If still dirty, it will be redirty_tail()'ed below.  Update
     * the dirty time to prevent enqueue and sync it again.
     */
    if ((inode->i_state & I_DIRTY) &&
        (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages))
        inode->dirtied_when = jiffies;

    if (wbc->pages_skipped) {
        /*
         * writeback is not making progress due to locked
         * buffers. Skip this inode for now.
         */
        redirty_tail(inode, wb);
        return;
    }

    if (mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY)) {
        /*
         * We didn't write back all the pages.  nfs_writepages()
         * sometimes bales out without doing anything.
         */
        if (wbc->nr_to_write <= 0) {
            /* Slice used up. Queue for next turn. */
            requeue_io(inode, wb);
        } else {
            /*
             * Writeback blocked by something other than
             * congestion. Delay the inode for some time to
             * avoid spinning on the CPU (100% iowait)
             * retrying writeback of the dirty page/inode
             * that cannot be performed immediately.
             */
            redirty_tail(inode, wb);
        }
    } else if (inode->i_state & I_DIRTY) {
        /*
         * Filesystems can dirty the inode during writeback operations,
         * such as delayed allocation during submission or metadata
         * updates after data IO completion.
         */
        redirty_tail(inode, wb);
    } else {
        /* The inode is clean. Remove from writeback lists. */
        list_del_init(&inode->i_wb_list);
    }
}

/*
 * Write out an inode and its dirty pages. Do not update the writeback list
 * linkage. That is left to the caller. The caller is also responsible for
 * setting I_SYNC flag and calling inode_sync_complete() to clear it.
 */
static int
__writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
{
    struct address_space *mapping = inode->i_mapping;
    long nr_to_write = wbc->nr_to_write;
    unsigned dirty;
    int ret;

    WARN_ON(!(inode->i_state & I_SYNC));

    ret = do_writepages(mapping, wbc);

    /*
     * Make sure to wait on the data before writing out the metadata.
     * This is important for filesystems that modify metadata on data
     * I/O completion.
     */
    if (wbc->sync_mode == WB_SYNC_ALL) {
        int err = filemap_fdatawait(mapping);
        if (ret == 0)
            ret = err;
    }

    /*
     * Some filesystems may redirty the inode during the writeback
     * due to delalloc, clear dirty metadata flags right before
     * write_inode()
     */
    spin_lock(&inode->i_lock);
    /* Clear I_DIRTY_PAGES if we've written out all dirty pages */
    if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
        inode->i_state &= ~I_DIRTY_PAGES;
    dirty = inode->i_state & I_DIRTY;
    inode->i_state &= ~(I_DIRTY_SYNC | I_DIRTY_DATASYNC);
    spin_unlock(&inode->i_lock);
    /* Don't write the inode if only I_DIRTY_PAGES was set */
    if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
        int err = write_inode(inode, wbc);
        if (ret == 0)
            ret = err;
    }
    trace_writeback_single_inode(inode, wbc, nr_to_write);
    return ret;
}

/*
 * Write out an inode's dirty pages. Either the caller has an active reference
 * on the inode or the inode has I_WILL_FREE set.
 *
 * This function is designed to be called for writing back one inode which
 * we go e.g. from filesystem. Flusher thread uses __writeback_single_inode()
 * and does more profound writeback list handling in writeback_sb_inodes().
 */
static int
writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
               struct writeback_control *wbc)
{
    int ret = 0;

    spin_lock(&inode->i_lock);
    if (!atomic_read(&inode->i_count))
        WARN_ON(!(inode->i_state & (I_WILL_FREE|I_FREEING)));
    else
        WARN_ON(inode->i_state & I_WILL_FREE);

    if (inode->i_state & I_SYNC) {
        if (wbc->sync_mode != WB_SYNC_ALL)
            goto out;
        /*
         * It's a data-integrity sync. We must wait. Since callers hold
         * inode reference or inode has I_WILL_FREE set, it cannot go
         * away under us.
         */
        __inode_wait_for_writeback(inode);
    }
    WARN_ON(inode->i_state & I_SYNC);
    /*
     * Skip inode if it is clean. We don't want to mess with writeback
     * lists in this function since flusher thread may be doing for example
     * sync in parallel and if we move the inode, it could get skipped. So
     * here we make sure inode is on some writeback list and leave it there
     * unless we have completely cleaned the inode.
     */
    if (!(inode->i_state & I_DIRTY))
        goto out;
    inode->i_state |= I_SYNC;
    spin_unlock(&inode->i_lock);

    ret = __writeback_single_inode(inode, wbc);

    spin_lock(&wb->list_lock);
    spin_lock(&inode->i_lock);
    /*
     * If inode is clean, remove it from writeback lists. Otherwise don't
     * touch it. See comment above for explanation.
     */
    if (!(inode->i_state & I_DIRTY))
        list_del_init(&inode->i_wb_list);
    spin_unlock(&wb->list_lock);
    inode_sync_complete(inode);
out:
    spin_unlock(&inode->i_lock);
    return ret;
}

static long writeback_chunk_size(struct backing_dev_info *bdi,
                 struct wb_writeback_work *work)
{
    long pages;

    /*
     * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
     * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
     * here avoids calling into writeback_inodes_wb() more than once.
     *
     * The intended call sequence for WB_SYNC_ALL writeback is:
     *
     *      wb_writeback()
     *          writeback_sb_inodes()       <== called only once
     *              write_cache_pages()     <== called once for each inode
     *                   (quickly) tag currently dirty pages
     *                   (maybe slowly) sync all tagged pages
     */
    if (work->sync_mode == WB_SYNC_ALL || work->tagged_writepages)
        pages = LONG_MAX;
    else {
        pages = min(bdi->avg_write_bandwidth / 2,
                global_dirty_limit / DIRTY_SCOPE);
        pages = min(pages, work->nr_pages);
        pages = round_down(pages + MIN_WRITEBACK_PAGES,
                   MIN_WRITEBACK_PAGES);
    }

    return pages;
}

/*
 * Write a portion of b_io inodes which belong to @sb.
 *
 * Return the number of pages and/or inodes written.
 */
static long writeback_sb_inodes(struct super_block *sb,
                struct bdi_writeback *wb,
                struct wb_writeback_work *work)
{
    struct writeback_control wbc = {
        .sync_mode      = work->sync_mode,
        .tagged_writepages  = work->tagged_writepages,
        .for_kupdate        = work->for_kupdate,
        .for_background     = work->for_background,
        .range_cyclic       = work->range_cyclic,
        .range_start        = 0,
        .range_end      = LLONG_MAX,
    };
    unsigned long start_time = jiffies;
    long write_chunk;
    long wrote = 0;  /* count both pages and inodes */

    while (!list_empty(&wb->b_io)) {
        struct inode *inode = wb_inode(wb->b_io.prev);

        if (inode->i_sb != sb) {
            if (work->sb) {
                /*
                 * We only want to write back data for this
                 * superblock, move all inodes not belonging
                 * to it back onto the dirty list.
                 */
                redirty_tail(inode, wb);
                continue;
            }

            /*
             * The inode belongs to a different superblock.
             * Bounce back to the caller to unpin this and
             * pin the next superblock.
             */
            break;
        }

        /*
         * Don't bother with new inodes or inodes being freed, first
         * kind does not need periodic writeout yet, and for the latter
         * kind writeout is handled by the freer.
         */
        spin_lock(&inode->i_lock);
        if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
            spin_unlock(&inode->i_lock);
            redirty_tail(inode, wb);
            continue;
        }
        if ((inode->i_state & I_SYNC) && wbc.sync_mode != WB_SYNC_ALL) {
            /*
             * If this inode is locked for writeback and we are not
             * doing writeback-for-data-integrity, move it to
             * b_more_io so that writeback can proceed with the
             * other inodes on s_io.
             *
             * We'll have another go at writing back this inode
             * when we completed a full scan of b_io.
             */
            spin_unlock(&inode->i_lock);
            requeue_io(inode, wb);
            trace_writeback_sb_inodes_requeue(inode);
            continue;
        }
        spin_unlock(&wb->list_lock);

        /*
         * We already requeued the inode if it had I_SYNC set and we
         * are doing WB_SYNC_NONE writeback. So this catches only the
         * WB_SYNC_ALL case.
         */
        if (inode->i_state & I_SYNC) {
            /* Wait for I_SYNC. This function drops i_lock... */
            inode_sleep_on_writeback(inode);
            /* Inode may be gone, start again */
            spin_lock(&wb->list_lock);
            continue;
        }
        inode->i_state |= I_SYNC;
        spin_unlock(&inode->i_lock);

        write_chunk = writeback_chunk_size(wb->bdi, work);
        wbc.nr_to_write = write_chunk;
        wbc.pages_skipped = 0;

        /*
         * We use I_SYNC to pin the inode in memory. While it is set
         * evict_inode() will wait so the inode cannot be freed.
         */
        __writeback_single_inode(inode, &wbc);

        work->nr_pages -= write_chunk - wbc.nr_to_write;
        wrote += write_chunk - wbc.nr_to_write;
        spin_lock(&wb->list_lock);
        spin_lock(&inode->i_lock);
        if (!(inode->i_state & I_DIRTY))
            wrote++;
        requeue_inode(inode, wb, &wbc);
        inode_sync_complete(inode);
        spin_unlock(&inode->i_lock);
        cond_resched_lock(&wb->list_lock);
        /*
         * bail out to wb_writeback() often enough to check
         * background threshold and other termination conditions.
         */
        if (wrote) {
            if (time_is_before_jiffies(start_time + HZ / 10UL))
                break;
            if (work->nr_pages <= 0)
                break;
        }
    }
    return wrote;
}

static long __writeback_inodes_wb(struct bdi_writeback *wb,
                  struct wb_writeback_work *work)
{
    unsigned long start_time = jiffies;
    long wrote = 0;

    while (!list_empty(&wb->b_io)) {
        struct inode *inode = wb_inode(wb->b_io.prev);
        struct super_block *sb = inode->i_sb;

        if (!grab_super_passive(sb)) {
            /*
             * grab_super_passive() may fail consistently due to
             * s_umount being grabbed by someone else. Don't use
             * requeue_io() to avoid busy retrying the inode/sb.
             */
            redirty_tail(inode, wb);
            continue;
        }
        wrote += writeback_sb_inodes(sb, wb, work);
        drop_super(sb);

        /* refer to the same tests at the end of writeback_sb_inodes */
        if (wrote) {
            if (time_is_before_jiffies(start_time + HZ / 10UL))
                break;
            if (work->nr_pages <= 0)
                break;
        }
    }
    /* Leave any unwritten inodes on b_io */
    return wrote;
}

long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages,
                enum wb_reason reason)
{
    struct wb_writeback_work work = {
        .nr_pages   = nr_pages,
        .sync_mode  = WB_SYNC_NONE,
        .range_cyclic   = 1,
        .reason     = reason,
    };

    spin_lock(&wb->list_lock);
    if (list_empty(&wb->b_io))
        queue_io(wb, &work);
    __writeback_inodes_wb(wb, &work);
    spin_unlock(&wb->list_lock);

    return nr_pages - work.nr_pages;
}

static bool over_bground_thresh(struct backing_dev_info *bdi)
{
    unsigned long background_thresh, dirty_thresh;

    global_dirty_limits(&background_thresh, &dirty_thresh);

    if (global_page_state(NR_FILE_DIRTY) +
        global_page_state(NR_UNSTABLE_NFS) > background_thresh)
        return true;

    if (bdi_stat(bdi, BDI_RECLAIMABLE) >
                bdi_dirty_limit(bdi, background_thresh))
        return true;

    return false;
}

/*
 * Called under wb->list_lock. If there are multiple wb per bdi,
 * only the flusher working on the first wb should do it.
 */
static void wb_update_bandwidth(struct bdi_writeback *wb,
                unsigned long start_time)
{
    __bdi_update_bandwidth(wb->bdi, 0, 0, 0, 0, 0, start_time);
}

/*
 * Explicit flushing or periodic writeback of "old" data.
 *
 * Define "old": the first time one of an inode's pages is dirtied, we mark the
 * dirtying-time in the inode's address_space.  So this periodic writeback code
 * just walks the superblock inode list, writing back any inodes which are
 * older than a specific point in time.
 *
 * Try to run once per dirty_writeback_interval.  But if a writeback event
 * takes longer than a dirty_writeback_interval interval, then leave a
 * one-second gap.
 *
 * older_than_this takes precedence over nr_to_write.  So we'll only write back
 * all dirty pages if they are all attached to "old" mappings.
 */
static long wb_writeback(struct bdi_writeback *wb,
             struct wb_writeback_work *work)
{
    unsigned long wb_start = jiffies;
    long nr_pages = work->nr_pages;
    unsigned long oldest_jif;
    struct inode *inode;
    long progress;

    oldest_jif = jiffies;
    work->older_than_this = &oldest_jif;

    spin_lock(&wb->list_lock);
    for (;;) {
        /*
         * Stop writeback when nr_pages has been consumed
         */
        if (work->nr_pages <= 0)
            break;

        /*
         * Background writeout and kupdate-style writeback may
         * run forever. Stop them if there is other work to do
         * so that e.g. sync can proceed. They'll be restarted
         * after the other works are all done.
         */
        if ((work->for_background || work->for_kupdate) &&
            !list_empty(&wb->bdi->work_list))
            break;

        /*
         * For background writeout, stop when we are below the
         * background dirty threshold
         */
        if (work->for_background && !over_bground_thresh(wb->bdi))
            break;

        /*
         * Kupdate and background works are special and we want to
         * include all inodes that need writing. Livelock avoidance is
         * handled by these works yielding to any other work so we are
         * safe.
         */
        if (work->for_kupdate) {
            oldest_jif = jiffies -
                msecs_to_jiffies(dirty_expire_interval * 10);
        } else if (work->for_background)
            oldest_jif = jiffies;

        trace_writeback_start(wb->bdi, work);
        if (list_empty(&wb->b_io))
            queue_io(wb, work);
        if (work->sb)
            progress = writeback_sb_inodes(work->sb, wb, work);
        else
            progress = __writeback_inodes_wb(wb, work);
        trace_writeback_written(wb->bdi, work);

        wb_update_bandwidth(wb, wb_start);

        /*
         * Did we write something? Try for more
         *
         * Dirty inodes are moved to b_io for writeback in batches.
         * The completion of the current batch does not necessarily
         * mean the overall work is done. So we keep looping as long
         * as made some progress on cleaning pages or inodes.
         */
        if (progress)
            continue;
        /*
         * No more inodes for IO, bail
         */
        if (list_empty(&wb->b_more_io))
            break;
        /*
         * Nothing written. Wait for some inode to
         * become available for writeback. Otherwise
         * we'll just busyloop.
         */
        if (!list_empty(&wb->b_more_io))  {
            trace_writeback_wait(wb->bdi, work);
            inode = wb_inode(wb->b_more_io.prev);
            spin_lock(&inode->i_lock);
            spin_unlock(&wb->list_lock);
            /* This function drops i_lock... */
            inode_sleep_on_writeback(inode);
            spin_lock(&wb->list_lock);
        }
    }
    spin_unlock(&wb->list_lock);

    return nr_pages - work->nr_pages;
}

/*
 * Return the next wb_writeback_work struct that hasn't been processed yet.
 */
static struct wb_writeback_work *
get_next_work_item(struct backing_dev_info *bdi)
{
    struct wb_writeback_work *work = NULL;

    spin_lock_bh(&bdi->wb_lock);
    if (!list_empty(&bdi->work_list)) {
        work = list_entry(bdi->work_list.next,
                  struct wb_writeback_work, list);
        list_del_init(&work->list);
    }
    spin_unlock_bh(&bdi->wb_lock);
    return work;
}

/*
 * Add in the number of potentially dirty inodes, because each inode
 * write can dirty pagecache in the underlying blockdev.
 */
static unsigned long get_nr_dirty_pages(void)
{
    return global_page_state(NR_FILE_DIRTY) +
        global_page_state(NR_UNSTABLE_NFS) +
        get_nr_dirty_inodes();
}

static long wb_check_background_flush(struct bdi_writeback *wb)
{
    if (over_bground_thresh(wb->bdi)) {

        struct wb_writeback_work work = {
            .nr_pages   = LONG_MAX,
            .sync_mode  = WB_SYNC_NONE,
            .for_background = 1,
            .range_cyclic   = 1,
            .reason     = WB_REASON_BACKGROUND,
        };

        return wb_writeback(wb, &work);
    }

    return 0;
}

static long wb_check_old_data_flush(struct bdi_writeback *wb)
{
    unsigned long expired;
    long nr_pages;

    /*
     * When set to zero, disable periodic writeback
     */
    if (!dirty_writeback_interval)
        return 0;

    expired = wb->last_old_flush +
            msecs_to_jiffies(dirty_writeback_interval * 10);
    if (time_before(jiffies, expired))
        return 0;

    wb->last_old_flush = jiffies;
    nr_pages = get_nr_dirty_pages();

    if (nr_pages) {
        struct wb_writeback_work work = {
            .nr_pages   = nr_pages,
            .sync_mode  = WB_SYNC_NONE,
            .for_kupdate    = 1,
            .range_cyclic   = 1,
            .reason     = WB_REASON_PERIODIC,
        };

        return wb_writeback(wb, &work);
    }

    return 0;
}

/*
 * Retrieve work items and do the writeback they describe
 */
long wb_do_writeback(struct bdi_writeback *wb, int force_wait)
{
    struct backing_dev_info *bdi = wb->bdi;
    struct wb_writeback_work *work;
    long wrote = 0;

    set_bit(BDI_writeback_running, &wb->bdi->state);
    while ((work = get_next_work_item(bdi)) != NULL) {
        /*
         * Override sync mode, in case we must wait for completion
         * because this thread is exiting now.
         */
        if (force_wait)
            work->sync_mode = WB_SYNC_ALL;

        trace_writeback_exec(bdi, work);

        wrote += wb_writeback(wb, work);

        /*
         * Notify the caller of completion if this is a synchronous
         * work item, otherwise just free it.
         */
        if (work->done)
            complete(work->done);
        else
            kfree(work);
    }

    /*
     * Check for periodic writeback, kupdated() style
     */
    wrote += wb_check_old_data_flush(wb);
    wrote += wb_check_background_flush(wb);
    clear_bit(BDI_writeback_running, &wb->bdi->state);

    return wrote;
}

/*
 * Handle writeback of dirty data for the device backed by this bdi. Also
 * wakes up periodically and does kupdated style flushing.
 */
int bdi_writeback_thread(void *data)
{
    struct bdi_writeback *wb = data;
    struct backing_dev_info *bdi = wb->bdi;
    long pages_written;

    current->flags |= PF_SWAPWRITE;
    set_freezable();
    wb->last_active = jiffies;

    /*
     * Our parent may run at a different priority, just set us to normal
     */
    set_user_nice(current, 0);

    trace_writeback_thread_start(bdi);

    while (!kthread_freezable_should_stop(NULL)) {
        /*
         * Remove own delayed wake-up timer, since we are already awake
         * and we'll take care of the preriodic write-back.
         */
        del_timer(&wb->wakeup_timer);

        pages_written = wb_do_writeback(wb, 0);

        trace_writeback_pages_written(pages_written);

        if (pages_written)
            wb->last_active = jiffies;

        set_current_state(TASK_INTERRUPTIBLE);
        if (!list_empty(&bdi->work_list) || kthread_should_stop()) {
            __set_current_state(TASK_RUNNING);
            continue;
        }

        if (wb_has_dirty_io(wb) && dirty_writeback_interval)
            schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
        else {
            /*
             * We have nothing to do, so can go sleep without any
             * timeout and save power. When a work is queued or
             * something is made dirty - we will be woken up.
             */
            schedule();
        }
    }

    /* Flush any work that raced with us exiting */
    if (!list_empty(&bdi->work_list))
        wb_do_writeback(wb, 1);

    trace_writeback_thread_stop(bdi);
    return 0;
}


/*
 * Start writeback of `nr_pages' pages.  If `nr_pages' is zero, write back
 * the whole world.
 */
void wakeup_flusher_threads(long nr_pages, enum wb_reason reason)
{
    struct backing_dev_info *bdi;

    if (!nr_pages) {
        nr_pages = global_page_state(NR_FILE_DIRTY) +
                global_page_state(NR_UNSTABLE_NFS);
    }

    rcu_read_lock();
    list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
        if (!bdi_has_dirty_io(bdi))
            continue;
        __bdi_start_writeback(bdi, nr_pages, false, reason);
    }
    rcu_read_unlock();
}

static noinline void block_dump___mark_inode_dirty(struct inode *inode)
{
    if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
        struct dentry *dentry;
        const char *name = "?";

        dentry = d_find_alias(inode);
        if (dentry) {
            spin_lock(&dentry->d_lock);
            name = (const char *) dentry->d_name.name;
        }
        printk(KERN_DEBUG
               "%s(%d): dirtied inode %lu (%s) on %s\n",
               current->comm, task_pid_nr(current), inode->i_ino,
               name, inode->i_sb->s_id);
        if (dentry) {
            spin_unlock(&dentry->d_lock);
            dput(dentry);
        }
    }
}

void __mark_inode_dirty(struct inode *inode, int flags)
{
    struct super_block *sb = inode->i_sb;
    struct backing_dev_info *bdi = NULL;

    /*
     * Don't do this for I_DIRTY_PAGES - that doesn't actually
     * dirty the inode itself
     */
    if (flags & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
        if (sb->s_op->dirty_inode)
            sb->s_op->dirty_inode(inode, flags);
    }

    /*
     * make sure that changes are seen by all cpus before we test i_state
     * -- mikulas
     */
    smp_mb();

    /* avoid the locking if we can */
    if ((inode->i_state & flags) == flags)
        return;

    if (unlikely(block_dump))
        block_dump___mark_inode_dirty(inode);

    spin_lock(&inode->i_lock);
    if ((inode->i_state & flags) != flags) {
        const int was_dirty = inode->i_state & I_DIRTY;

        inode->i_state |= flags;

        /*
         * If the inode is being synced, just update its dirty state.
         * The unlocker will place the inode on the appropriate
         * superblock list, based upon its state.
         */
        if (inode->i_state & I_SYNC)
            goto out_unlock_inode;

        /*
         * Only add valid (hashed) inodes to the superblock's
         * dirty list.  Add blockdev inodes as well.
         */
        if (!S_ISBLK(inode->i_mode)) {
            if (inode_unhashed(inode))
                goto out_unlock_inode;
        }
        if (inode->i_state & I_FREEING)
            goto out_unlock_inode;

        /*
         * If the inode was already on b_dirty/b_io/b_more_io, don't
         * reposition it (that would break b_dirty time-ordering).
         */
        if (!was_dirty) {
            bool wakeup_bdi = false;
            bdi = inode_to_bdi(inode);

            if (bdi_cap_writeback_dirty(bdi)) {
                WARN(!test_bit(BDI_registered, &bdi->state),
                     "bdi-%s not registered\n", bdi->name);

                /*
                 * If this is the first dirty inode for this
                 * bdi, we have to wake-up the corresponding
                 * bdi thread to make sure background
                 * write-back happens later.
                 */
                if (!wb_has_dirty_io(&bdi->wb))
                    wakeup_bdi = true;
            }

            spin_unlock(&inode->i_lock);
            spin_lock(&bdi->wb.list_lock);
            inode->dirtied_when = jiffies;
            list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
            spin_unlock(&bdi->wb.list_lock);

            if (wakeup_bdi)
                bdi_wakeup_thread_delayed(bdi);
            return;
        }
    }
out_unlock_inode:
    spin_unlock(&inode->i_lock);

}
EXPORT_SYMBOL(__mark_inode_dirty);

static void wait_sb_inodes(struct super_block *sb)
{
    struct inode *inode, *old_inode = NULL;

    /*
     * We need to be protected against the filesystem going from
     * r/o to r/w or vice versa.
     */
    WARN_ON(!rwsem_is_locked(&sb->s_umount));

    spin_lock(&inode_sb_list_lock);

    /*
     * Data integrity sync. Must wait for all pages under writeback,
     * because there may have been pages dirtied before our sync
     * call, but which had writeout started before we write it out.
     * In which case, the inode may not be on the dirty list, but
     * we still have to wait for that writeout.
     */
    list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
        struct address_space *mapping = inode->i_mapping;

        spin_lock(&inode->i_lock);
        if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
            (mapping->nrpages == 0)) {
            spin_unlock(&inode->i_lock);
            continue;
        }
        __iget(inode);
        spin_unlock(&inode->i_lock);
        spin_unlock(&inode_sb_list_lock);

        /*
         * We hold a reference to 'inode' so it couldn't have been
         * removed from s_inodes list while we dropped the
         * inode_sb_list_lock.  We cannot iput the inode now as we can
         * be holding the last reference and we cannot iput it under
         * inode_sb_list_lock. So we keep the reference and iput it
         * later.
         */
        iput(old_inode);
        old_inode = inode;

        filemap_fdatawait(mapping);

        cond_resched();

        spin_lock(&inode_sb_list_lock);
    }
    spin_unlock(&inode_sb_list_lock);
    iput(old_inode);
}

void writeback_inodes_sb_nr(struct super_block *sb,
                unsigned long nr,
                enum wb_reason reason)
{
    DECLARE_COMPLETION_ONSTACK(done);
    struct wb_writeback_work work = {
        .sb         = sb,
        .sync_mode      = WB_SYNC_NONE,
        .tagged_writepages  = 1,
        .done           = &done,
        .nr_pages       = nr,
        .reason         = reason,
    };

    if (sb->s_bdi == &noop_backing_dev_info)
        return;
    WARN_ON(!rwsem_is_locked(&sb->s_umount));
    bdi_queue_work(sb->s_bdi, &work);
    wait_for_completion(&done);
}
EXPORT_SYMBOL(writeback_inodes_sb_nr);

void writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
{
    return writeback_inodes_sb_nr(sb, get_nr_dirty_pages(), reason);
}
EXPORT_SYMBOL(writeback_inodes_sb);

int writeback_inodes_sb_if_idle(struct super_block *sb, enum wb_reason reason)
{
    if (!writeback_in_progress(sb->s_bdi)) {
        down_read(&sb->s_umount);
        writeback_inodes_sb(sb, reason);
        up_read(&sb->s_umount);
        return 1;
    } else
        return 0;
}
EXPORT_SYMBOL(writeback_inodes_sb_if_idle);

int writeback_inodes_sb_nr_if_idle(struct super_block *sb,
                   unsigned long nr,
                   enum wb_reason reason)
{
    if (!writeback_in_progress(sb->s_bdi)) {
        down_read(&sb->s_umount);
        writeback_inodes_sb_nr(sb, nr, reason);
        up_read(&sb->s_umount);
        return 1;
    } else
        return 0;
}
EXPORT_SYMBOL(writeback_inodes_sb_nr_if_idle);

void sync_inodes_sb(struct super_block *sb)
{
    DECLARE_COMPLETION_ONSTACK(done);
    struct wb_writeback_work work = {
        .sb     = sb,
        .sync_mode  = WB_SYNC_ALL,
        .nr_pages   = LONG_MAX,
        .range_cyclic   = 0,
        .done       = &done,
        .reason     = WB_REASON_SYNC,
    };

    /* Nothing to do? */
    if (sb->s_bdi == &noop_backing_dev_info)
        return;
    WARN_ON(!rwsem_is_locked(&sb->s_umount));

    bdi_queue_work(sb->s_bdi, &work);
    wait_for_completion(&done);

    wait_sb_inodes(sb);
}
EXPORT_SYMBOL(sync_inodes_sb);

int write_inode_now(struct inode *inode, int sync)
{
    struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
    struct writeback_control wbc = {
        .nr_to_write = LONG_MAX,
        .sync_mode = sync ? WB_SYNC_ALL : WB_SYNC_NONE,
        .range_start = 0,
        .range_end = LLONG_MAX,
    };

    if (!mapping_cap_writeback_dirty(inode->i_mapping))
        wbc.nr_to_write = 0;

    might_sleep();
    return writeback_single_inode(inode, wb, &wbc);
}
EXPORT_SYMBOL(write_inode_now);

int sync_inode(struct inode *inode, struct writeback_control *wbc)
{
    return writeback_single_inode(inode, &inode_to_bdi(inode)->wb, wbc);
}
EXPORT_SYMBOL(sync_inode);

int sync_inode_metadata(struct inode *inode, int wait)
{
    struct writeback_control wbc = {
        .sync_mode = wait ? WB_SYNC_ALL : WB_SYNC_NONE,
        .nr_to_write = 0, /* metadata-only */
    };

    return sync_inode(inode, &wbc);
}
EXPORT_SYMBOL(sync_inode_metadata);