writeback: implement bdi_wait_for_completion()

[deliverable/linux.git] / fs / fs-writeback.c

/*
 * 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/writeback.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/tracepoint.h>
#include <linux/device.h>
#include "internal.h"

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

struct wb_completion {
        atomic_t                cnt;
};

/*
 * 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;
        unsigned int for_sync:1;        /* sync(2) WB_SYNC_ALL writeback */
        unsigned int auto_free:1;       /* free on completion */
        enum wb_reason reason;          /* why was writeback initiated? */

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

/*
 * If one wants to wait for one or more wb_writeback_works, each work's
 * ->done should be set to a wb_completion defined using the following
 * macro.  Once all work items are issued with wb_queue_work(), the caller
 * can wait for the completion of all using wb_wait_for_completion().  Work
 * items which are waited upon aren't freed automatically on completion.
 */
#define DEFINE_WB_COMPLETION_ONSTACK(cmpl)                              \
        struct wb_completion cmpl = {                                   \
                .cnt            = ATOMIC_INIT(1),                       \
        }


/*
 * If an inode is constantly having its pages dirtied, but then the
 * updates stop dirtytime_expire_interval seconds in the past, it's
 * possible for the worst case time between when an inode has its
 * timestamps updated and when they finally get written out to be two
 * dirtytime_expire_intervals.  We set the default to 12 hours (in
 * seconds), which means most of the time inodes will have their
 * timestamps written to disk after 12 hours, but in the worst case a
 * few inodes might not their timestamps updated for 24 hours.
 */
unsigned int dirtytime_expire_interval = 12 * 60 * 60;

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>

EXPORT_TRACEPOINT_SYMBOL_GPL(wbc_writepage);

static bool wb_io_lists_populated(struct bdi_writeback *wb)
{
        if (wb_has_dirty_io(wb)) {
                return false;
        } else {
                set_bit(WB_has_dirty_io, &wb->state);
                WARN_ON_ONCE(!wb->avg_write_bandwidth);
                atomic_long_add(wb->avg_write_bandwidth,
                                &wb->bdi->tot_write_bandwidth);
                return true;
        }
}

static void wb_io_lists_depopulated(struct bdi_writeback *wb)
{
        if (wb_has_dirty_io(wb) && list_empty(&wb->b_dirty) &&
            list_empty(&wb->b_io) && list_empty(&wb->b_more_io)) {
                clear_bit(WB_has_dirty_io, &wb->state);
                WARN_ON_ONCE(atomic_long_sub_return(wb->avg_write_bandwidth,
                                        &wb->bdi->tot_write_bandwidth) < 0);
        }
}

/**
 * inode_wb_list_move_locked - move an inode onto a bdi_writeback IO list
 * @inode: inode to be moved
 * @wb: target bdi_writeback
 * @head: one of @wb->b_{dirty|io|more_io}
 *
 * Move @inode->i_wb_list to @list of @wb and set %WB_has_dirty_io.
 * Returns %true if @inode is the first occupant of the !dirty_time IO
 * lists; otherwise, %false.
 */
static bool inode_wb_list_move_locked(struct inode *inode,
                                      struct bdi_writeback *wb,
                                      struct list_head *head)
{
        assert_spin_locked(&wb->list_lock);

        list_move(&inode->i_wb_list, head);

        /* dirty_time doesn't count as dirty_io until expiration */
        if (head != &wb->b_dirty_time)
                return wb_io_lists_populated(wb);

        wb_io_lists_depopulated(wb);
        return false;
}

/**
 * inode_wb_list_del_locked - remove an inode from its bdi_writeback IO list
 * @inode: inode to be removed
 * @wb: bdi_writeback @inode is being removed from
 *
 * Remove @inode which may be on one of @wb->b_{dirty|io|more_io} lists and
 * clear %WB_has_dirty_io if all are empty afterwards.
 */
static void inode_wb_list_del_locked(struct inode *inode,
                                     struct bdi_writeback *wb)
{
        assert_spin_locked(&wb->list_lock);

        list_del_init(&inode->i_wb_list);
        wb_io_lists_depopulated(wb);
}

static void wb_wakeup(struct bdi_writeback *wb)
{
        spin_lock_bh(&wb->work_lock);
        if (test_bit(WB_registered, &wb->state))
                mod_delayed_work(bdi_wq, &wb->dwork, 0);
        spin_unlock_bh(&wb->work_lock);
}

static void wb_queue_work(struct bdi_writeback *wb,
                          struct wb_writeback_work *work)
{
        trace_writeback_queue(wb->bdi, work);

        spin_lock_bh(&wb->work_lock);
        if (!test_bit(WB_registered, &wb->state))
                goto out_unlock;
        if (work->done)
                atomic_inc(&work->done->cnt);
        list_add_tail(&work->list, &wb->work_list);
        mod_delayed_work(bdi_wq, &wb->dwork, 0);
out_unlock:
        spin_unlock_bh(&wb->work_lock);
}

/**
 * wb_wait_for_completion - wait for completion of bdi_writeback_works
 * @bdi: bdi work items were issued to
 * @done: target wb_completion
 *
 * Wait for one or more work items issued to @bdi with their ->done field
 * set to @done, which should have been defined with
 * DEFINE_WB_COMPLETION_ONSTACK().  This function returns after all such
 * work items are completed.  Work items which are waited upon aren't freed
 * automatically on completion.
 */
static void wb_wait_for_completion(struct backing_dev_info *bdi,
                                   struct wb_completion *done)
{
        atomic_dec(&done->cnt);         /* put down the initial count */
        wait_event(bdi->wb_waitq, !atomic_read(&done->cnt));
}

#ifdef CONFIG_CGROUP_WRITEBACK

/**
 * inode_congested - test whether an inode is congested
 * @inode: inode to test for congestion
 * @cong_bits: mask of WB_[a]sync_congested bits to test
 *
 * Tests whether @inode is congested.  @cong_bits is the mask of congestion
 * bits to test and the return value is the mask of set bits.
 *
 * If cgroup writeback is enabled for @inode, the congestion state is
 * determined by whether the cgwb (cgroup bdi_writeback) for the blkcg
 * associated with @inode is congested; otherwise, the root wb's congestion
 * state is used.
 */
int inode_congested(struct inode *inode, int cong_bits)
{
        if (inode) {
                struct bdi_writeback *wb = inode_to_wb(inode);
                if (wb)
                        return wb_congested(wb, cong_bits);
        }

        return wb_congested(&inode_to_bdi(inode)->wb, cong_bits);
}
EXPORT_SYMBOL_GPL(inode_congested);

/**
 * wb_split_bdi_pages - split nr_pages to write according to bandwidth
 * @wb: target bdi_writeback to split @nr_pages to
 * @nr_pages: number of pages to write for the whole bdi
 *
 * Split @wb's portion of @nr_pages according to @wb's write bandwidth in
 * relation to the total write bandwidth of all wb's w/ dirty inodes on
 * @wb->bdi.
 */
static long wb_split_bdi_pages(struct bdi_writeback *wb, long nr_pages)
{
        unsigned long this_bw = wb->avg_write_bandwidth;
        unsigned long tot_bw = atomic_long_read(&wb->bdi->tot_write_bandwidth);

        if (nr_pages == LONG_MAX)
                return LONG_MAX;

        /*
         * This may be called on clean wb's and proportional distribution
         * may not make sense, just use the original @nr_pages in those
         * cases.  In general, we wanna err on the side of writing more.
         */
        if (!tot_bw || this_bw >= tot_bw)
                return nr_pages;
        else
                return DIV_ROUND_UP_ULL((u64)nr_pages * this_bw, tot_bw);
}

#else   /* CONFIG_CGROUP_WRITEBACK */

static long wb_split_bdi_pages(struct bdi_writeback *wb, long nr_pages)
{
        return nr_pages;
}

#endif  /* CONFIG_CGROUP_WRITEBACK */

void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
                        bool range_cyclic, enum wb_reason reason)
{
        struct wb_writeback_work *work;

        if (!wb_has_dirty_io(wb))
                return;

        /*
         * 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) {
                trace_writeback_nowork(wb->bdi);
                wb_wakeup(wb);
                return;
        }

        work->sync_mode = WB_SYNC_NONE;
        work->nr_pages  = nr_pages;
        work->range_cyclic = range_cyclic;
        work->reason    = reason;
        work->auto_free = 1;

        wb_queue_work(wb, work);
}

/**
 * wb_start_background_writeback - start background writeback
 * @wb: bdi_writback to write from
 *
 * Description:
 *   This makes sure WB_SYNC_NONE background writeback happens. When
 *   this function returns, it is only guaranteed that for given wb
 *   some IO is happening if we are over background dirty threshold.
 *   Caller need not hold sb s_umount semaphore.
 */
void wb_start_background_writeback(struct bdi_writeback *wb)
{
        /*
         * 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(wb->bdi);
        wb_wakeup(wb);
}

/*
 * Remove the inode from the writeback list it is on.
 */
void inode_wb_list_del(struct inode *inode)
{
        struct bdi_writeback *wb = inode_to_wb(inode);

        spin_lock(&wb->list_lock);
        inode_wb_list_del_locked(inode, wb);
        spin_unlock(&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)
{
        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;
        }
        inode_wb_list_move_locked(inode, wb, &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)
{
        inode_wb_list_move_locked(inode, wb, &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;
}

#define EXPIRE_DIRTY_ATIME 0x0001

/*
 * 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,
                               int flags,
                               struct wb_writeback_work *work)
{
        unsigned long *older_than_this = NULL;
        unsigned long expire_time;
        LIST_HEAD(tmp);
        struct list_head *pos, *node;
        struct super_block *sb = NULL;
        struct inode *inode;
        int do_sb_sort = 0;
        int moved = 0;

        if ((flags & EXPIRE_DIRTY_ATIME) == 0)
                older_than_this = work->older_than_this;
        else if (!work->for_sync) {
                expire_time = jiffies - (dirtytime_expire_interval * HZ);
                older_than_this = &expire_time;
        }
        while (!list_empty(delaying_queue)) {
                inode = wb_inode(delaying_queue->prev);
                if (older_than_this &&
                    inode_dirtied_after(inode, *older_than_this))
                        break;
                list_move(&inode->i_wb_list, &tmp);
                moved++;
                if (flags & EXPIRE_DIRTY_ATIME)
                        set_bit(__I_DIRTY_TIME_EXPIRED, &inode->i_state);
                if (sb_is_blkdev_sb(inode->i_sb))
                        continue;
                if (sb && sb != inode->i_sb)
                        do_sb_sort = 1;
                sb = inode->i_sb;
        }

        /* 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, 0, work);
        moved += move_expired_inodes(&wb->b_dirty_time, &wb->b_io,
                                     EXPIRE_DIRTY_ATIME, work);
        if (moved)
                wb_io_lists_populated(wb);
        trace_writeback_queue_io(wb, work, moved);
}

static int write_inode(struct inode *inode, struct writeback_control *wbc)
{
        int ret;

        if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode)) {
                trace_writeback_write_inode_start(inode, wbc);
                ret = inode->i_sb->s_op->write_inode(inode, wbc);
                trace_writeback_write_inode(inode, wbc);
                return ret;
        }
        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, bit_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 if (inode->i_state & I_DIRTY_TIME) {
                inode->dirtied_when = jiffies;
                inode_wb_list_move_locked(inode, wb, &wb->b_dirty_time);
        } else {
                /* The inode is clean. Remove from writeback lists. */
                inode_wb_list_del_locked(inode, wb);
        }
}

/*
 * 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));

        trace_writeback_single_inode_start(inode, wbc, nr_to_write);

        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. We don't do it for sync(2) writeback because it has a
         * separate, external IO completion path and ->sync_fs for guaranteeing
         * inode metadata is written back correctly.
         */
        if (wbc->sync_mode == WB_SYNC_ALL && !wbc->for_sync) {
                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);

        dirty = inode->i_state & I_DIRTY;
        if (inode->i_state & I_DIRTY_TIME) {
                if ((dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) ||
                    unlikely(inode->i_state & I_DIRTY_TIME_EXPIRED) ||
                    unlikely(time_after(jiffies,
                                        (inode->dirtied_time_when +
                                         dirtytime_expire_interval * HZ)))) {
                        dirty |= I_DIRTY_TIME | I_DIRTY_TIME_EXPIRED;
                        trace_writeback_lazytime(inode);
                }
        } else
                inode->i_state &= ~I_DIRTY_TIME_EXPIRED;
        inode->i_state &= ~dirty;

        /*
         * Paired with smp_mb() in __mark_inode_dirty().  This allows
         * __mark_inode_dirty() to test i_state without grabbing i_lock -
         * either they see the I_DIRTY bits cleared or we see the dirtied
         * inode.
         *
         * I_DIRTY_PAGES is always cleared together above even if @mapping
         * still has dirty pages.  The flag is reinstated after smp_mb() if
         * necessary.  This guarantees that either __mark_inode_dirty()
         * sees clear I_DIRTY_PAGES or we see PAGECACHE_TAG_DIRTY.
         */
        smp_mb();

        if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
                inode->i_state |= I_DIRTY_PAGES;

        spin_unlock(&inode->i_lock);

        if (dirty & I_DIRTY_TIME)
                mark_inode_dirty_sync(inode);
        /* Don't write the inode if only I_DIRTY_PAGES was set */
        if (dirty & ~I_DIRTY_PAGES) {
                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 and we have no outstanding writeback in
         * WB_SYNC_ALL mode. 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_ALL) &&
            (wbc->sync_mode != WB_SYNC_ALL ||
             !mapping_tagged(inode->i_mapping, PAGECACHE_TAG_WRITEBACK)))
                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_ALL))
                inode_wb_list_del_locked(inode, wb);
        spin_unlock(&wb->list_lock);
        inode_sync_complete(inode);
out:
        spin_unlock(&inode->i_lock);
        return ret;
}

static long writeback_chunk_size(struct bdi_writeback *wb,
                                 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(wb->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,
                .for_sync               = work->for_sync,
                .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, 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_ALL))
                        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 (!trylock_super(sb)) {
                        /*
                         * trylock_super() 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);
                up_read(&sb->s_umount);

                /* 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;
}

static 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 bdi_writeback *wb)
{
        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 (wb_stat(wb, WB_RECLAIMABLE) > wb_dirty_limit(wb, 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)
{
        __wb_update_bandwidth(wb, 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->work_list))
                        break;

                /*
                 * For background writeout, stop when we are below the
                 * background dirty threshold
                 */
                if (work->for_background && !over_bground_thresh(wb))
                        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 bdi_writeback *wb)
{
        struct wb_writeback_work *work = NULL;

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

                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
 */
static long wb_do_writeback(struct bdi_writeback *wb)
{
        struct wb_writeback_work *work;
        long wrote = 0;

        set_bit(WB_writeback_running, &wb->state);
        while ((work = get_next_work_item(wb)) != NULL) {
                struct wb_completion *done = work->done;

                trace_writeback_exec(wb->bdi, work);

                wrote += wb_writeback(wb, work);

                if (work->auto_free)
                        kfree(work);
                if (done && atomic_dec_and_test(&done->cnt))
                        wake_up_all(&wb->bdi->wb_waitq);
        }

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

        return wrote;
}

/*
 * Handle writeback of dirty data for the device backed by this bdi. Also
 * reschedules periodically and does kupdated style flushing.
 */
void wb_workfn(struct work_struct *work)
{
        struct bdi_writeback *wb = container_of(to_delayed_work(work),
                                                struct bdi_writeback, dwork);
        long pages_written;

        set_worker_desc("flush-%s", dev_name(wb->bdi->dev));
        current->flags |= PF_SWAPWRITE;

        if (likely(!current_is_workqueue_rescuer() ||
                   !test_bit(WB_registered, &wb->state))) {
                /*
                 * The normal path.  Keep writing back @wb until its
                 * work_list is empty.  Note that this path is also taken
                 * if @wb is shutting down even when we're running off the
                 * rescuer as work_list needs to be drained.
                 */
                do {
                        pages_written = wb_do_writeback(wb);
                        trace_writeback_pages_written(pages_written);
                } while (!list_empty(&wb->work_list));
        } else {
                /*
                 * bdi_wq can't get enough workers and we're running off
                 * the emergency worker.  Don't hog it.  Hopefully, 1024 is
                 * enough for efficient IO.
                 */
                pages_written = writeback_inodes_wb(wb, 1024,
                                                    WB_REASON_FORKER_THREAD);
                trace_writeback_pages_written(pages_written);
        }

        if (!list_empty(&wb->work_list))
                mod_delayed_work(bdi_wq, &wb->dwork, 0);
        else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
                wb_wakeup_delayed(wb);

        current->flags &= ~PF_SWAPWRITE;
}

/*
 * 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 = get_nr_dirty_pages();

        rcu_read_lock();
        list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
                struct bdi_writeback *wb;
                struct wb_iter iter;

                if (!bdi_has_dirty_io(bdi))
                        continue;

                bdi_for_each_wb(wb, bdi, &iter, 0)
                        wb_start_writeback(wb, wb_split_bdi_pages(wb, nr_pages),
                                           false, reason);
        }
        rcu_read_unlock();
}

/*
 * Wake up bdi's periodically to make sure dirtytime inodes gets
 * written back periodically.  We deliberately do *not* check the
 * b_dirtytime list in wb_has_dirty_io(), since this would cause the
 * kernel to be constantly waking up once there are any dirtytime
 * inodes on the system.  So instead we define a separate delayed work
 * function which gets called much more rarely.  (By default, only
 * once every 12 hours.)
 *
 * If there is any other write activity going on in the file system,
 * this function won't be necessary.  But if the only thing that has
 * happened on the file system is a dirtytime inode caused by an atime
 * update, we need this infrastructure below to make sure that inode
 * eventually gets pushed out to disk.
 */
static void wakeup_dirtytime_writeback(struct work_struct *w);
static DECLARE_DELAYED_WORK(dirtytime_work, wakeup_dirtytime_writeback);

static void wakeup_dirtytime_writeback(struct work_struct *w)
{
        struct backing_dev_info *bdi;

        rcu_read_lock();
        list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
                struct bdi_writeback *wb;
                struct wb_iter iter;

                bdi_for_each_wb(wb, bdi, &iter, 0)
                        if (!list_empty(&bdi->wb.b_dirty_time))
                                wb_wakeup(&bdi->wb);
        }
        rcu_read_unlock();
        schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
}

static int __init start_dirtytime_writeback(void)
{
        schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);
        return 0;
}
__initcall(start_dirtytime_writeback);

int dirtytime_interval_handler(struct ctl_table *table, int write,
                               void __user *buffer, size_t *lenp, loff_t *ppos)
{
        int ret;

        ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
        if (ret == 0 && write)
                mod_delayed_work(system_wq, &dirtytime_work, 0);
        return ret;
}

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);
                }
        }
}

/**
 *      __mark_inode_dirty -    internal function
 *      @inode: inode to mark
 *      @flags: what kind of dirty (i.e. I_DIRTY_SYNC)
 *      Mark an inode as dirty. Callers should use mark_inode_dirty or
 *      mark_inode_dirty_sync.
 *
 * Put the inode on the super block's dirty list.
 *
 * CAREFUL! We mark it dirty unconditionally, but move it onto the
 * dirty list only if it is hashed or if it refers to a blockdev.
 * If it was not hashed, it will never be added to the dirty list
 * even if it is later hashed, as it will have been marked dirty already.
 *
 * In short, make sure you hash any inodes _before_ you start marking
 * them dirty.
 *
 * Note that for blockdevs, inode->dirtied_when represents the dirtying time of
 * the block-special inode (/dev/hda1) itself.  And the ->dirtied_when field of
 * the kernel-internal blockdev inode represents the dirtying time of the
 * blockdev's pages.  This is why for I_DIRTY_PAGES we always use
 * page->mapping->host, so the page-dirtying time is recorded in the internal
 * blockdev inode.
 */
#define I_DIRTY_INODE (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
void __mark_inode_dirty(struct inode *inode, int flags)
{
        struct super_block *sb = inode->i_sb;
        struct backing_dev_info *bdi = NULL;
        int dirtytime;

        trace_writeback_mark_inode_dirty(inode, flags);

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

                if (sb->s_op->dirty_inode)
                        sb->s_op->dirty_inode(inode, flags);

                trace_writeback_dirty_inode(inode, flags);
        }
        if (flags & I_DIRTY_INODE)
                flags &= ~I_DIRTY_TIME;
        dirtytime = flags & I_DIRTY_TIME;

        /*
         * Paired with smp_mb() in __writeback_single_inode() for the
         * following lockless i_state test.  See there for details.
         */
        smp_mb();

        if (((inode->i_state & flags) == flags) ||
            (dirtytime && (inode->i_state & I_DIRTY_INODE)))
                return;

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

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

                inode_attach_wb(inode, NULL);

                if (flags & I_DIRTY_INODE)
                        inode->i_state &= ~I_DIRTY_TIME;
                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) {
                        struct list_head *dirty_list;
                        bool wakeup_bdi = false;
                        bdi = inode_to_bdi(inode);

                        spin_unlock(&inode->i_lock);
                        spin_lock(&bdi->wb.list_lock);

                        WARN(bdi_cap_writeback_dirty(bdi) &&
                             !test_bit(WB_registered, &bdi->wb.state),
                             "bdi-%s not registered\n", bdi->name);

                        inode->dirtied_when = jiffies;
                        if (dirtytime)
                                inode->dirtied_time_when = jiffies;

                        if (inode->i_state & (I_DIRTY_INODE | I_DIRTY_PAGES))
                                dirty_list = &bdi->wb.b_dirty;
                        else
                                dirty_list = &bdi->wb.b_dirty_time;

                        wakeup_bdi = inode_wb_list_move_locked(inode, &bdi->wb,
                                                               dirty_list);

                        spin_unlock(&bdi->wb.list_lock);
                        trace_writeback_dirty_inode_enqueue(inode);

                        /*
                         * 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 (bdi_cap_writeback_dirty(bdi) && wakeup_bdi)
                                wb_wakeup_delayed(&bdi->wb);
                        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);
}

/**
 * writeback_inodes_sb_nr -     writeback dirty inodes from given super_block
 * @sb: the superblock
 * @nr: the number of pages to write
 * @reason: reason why some writeback work initiated
 *
 * Start writeback on some inodes on this super_block. No guarantees are made
 * on how many (if any) will be written, and this function does not wait
 * for IO completion of submitted IO.
 */
void writeback_inodes_sb_nr(struct super_block *sb,
                            unsigned long nr,
                            enum wb_reason reason)
{
        DEFINE_WB_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,
        };
        struct backing_dev_info *bdi = sb->s_bdi;

        if (!bdi_has_dirty_io(bdi) || bdi == &noop_backing_dev_info)
                return;
        WARN_ON(!rwsem_is_locked(&sb->s_umount));
        wb_queue_work(&bdi->wb, &work);
        wb_wait_for_completion(bdi, &done);
}
EXPORT_SYMBOL(writeback_inodes_sb_nr);

/**
 * writeback_inodes_sb  -       writeback dirty inodes from given super_block
 * @sb: the superblock
 * @reason: reason why some writeback work was initiated
 *
 * Start writeback on some inodes on this super_block. No guarantees are made
 * on how many (if any) will be written, and this function does not wait
 * for IO completion of submitted IO.
 */
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);

/**
 * try_to_writeback_inodes_sb_nr - try to start writeback if none underway
 * @sb: the superblock
 * @nr: the number of pages to write
 * @reason: the reason of writeback
 *
 * Invoke writeback_inodes_sb_nr if no writeback is currently underway.
 * Returns 1 if writeback was started, 0 if not.
 */
int try_to_writeback_inodes_sb_nr(struct super_block *sb,
                                  unsigned long nr,
                                  enum wb_reason reason)
{
        if (writeback_in_progress(&sb->s_bdi->wb))
                return 1;

        if (!down_read_trylock(&sb->s_umount))
                return 0;

        writeback_inodes_sb_nr(sb, nr, reason);
        up_read(&sb->s_umount);
        return 1;
}
EXPORT_SYMBOL(try_to_writeback_inodes_sb_nr);

/**
 * try_to_writeback_inodes_sb - try to start writeback if none underway
 * @sb: the superblock
 * @reason: reason why some writeback work was initiated
 *
 * Implement by try_to_writeback_inodes_sb_nr()
 * Returns 1 if writeback was started, 0 if not.
 */
int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
{
        return try_to_writeback_inodes_sb_nr(sb, get_nr_dirty_pages(), reason);
}
EXPORT_SYMBOL(try_to_writeback_inodes_sb);

/**
 * sync_inodes_sb       -       sync sb inode pages
 * @sb: the superblock
 *
 * This function writes and waits on any dirty inode belonging to this
 * super_block.
 */
void sync_inodes_sb(struct super_block *sb)
{
        DEFINE_WB_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,
                .for_sync       = 1,
        };
        struct backing_dev_info *bdi = sb->s_bdi;

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

        wb_queue_work(&bdi->wb, &work);
        wb_wait_for_completion(bdi, &done);

        wait_sb_inodes(sb);
}
EXPORT_SYMBOL(sync_inodes_sb);

/**
 * write_inode_now      -       write an inode to disk
 * @inode: inode to write to disk
 * @sync: whether the write should be synchronous or not
 *
 * This function commits an inode to disk immediately if it is dirty. This is
 * primarily needed by knfsd.
 *
 * The caller must either have a ref on the inode or must have set I_WILL_FREE.
 */
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);

/**
 * sync_inode - write an inode and its pages to disk.
 * @inode: the inode to sync
 * @wbc: controls the writeback mode
 *
 * sync_inode() will write an inode and its pages to disk.  It will also
 * correctly update the inode on its superblock's dirty inode lists and will
 * update inode->i_state.
 *
 * The caller must have a ref on the inode.
 */
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);

/**
 * sync_inode_metadata - write an inode to disk
 * @inode: the inode to sync
 * @wait: wait for I/O to complete.
 *
 * Write an inode to disk and adjust its dirty state after completion.
 *
 * Note: only writes the actual inode, no associated data or other metadata.
 */
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);