Merge tag 'mm-stable-2024-05-17-19-19' of git://git.kernel.org/pub/scm/linux/kernel...

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

// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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 <linux/memcontrol.h>
#include "internal.h"

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

/*
 * Passed into wb_writeback(), essentially a subset of writeback_control
 */
struct wb_writeback_work {
        long nr_pages;
        struct super_block *sb;
        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 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_io_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_io_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|dirty_time}
 *
 * Move @inode->i_io_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_io_list_move_locked(struct inode *inode,
                                      struct bdi_writeback *wb,
                                      struct list_head *head)
{
        assert_spin_locked(&wb->list_lock);
        assert_spin_locked(&inode->i_lock);
        WARN_ON_ONCE(inode->i_state & I_FREEING);

        list_move(&inode->i_io_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;
}

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

/*
 * This function is used when the first inode for this wb is marked dirty. It
 * wakes-up the corresponding bdi thread which should then take care of the
 * periodic background write-out of dirty inodes. Since the write-out would
 * starts only 'dirty_writeback_interval' centisecs from now anyway, we just
 * set up a timer which wakes the bdi thread up later.
 *
 * Note, we wouldn't bother setting up the timer, but this function is on the
 * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
 * by delaying the wake-up.
 *
 * We have to be careful not to postpone flush work if it is scheduled for
 * earlier. Thus we use queue_delayed_work().
 */
static void wb_wakeup_delayed(struct bdi_writeback *wb)
{
        unsigned long timeout;

        timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
        spin_lock_irq(&wb->work_lock);
        if (test_bit(WB_registered, &wb->state))
                queue_delayed_work(bdi_wq, &wb->dwork, timeout);
        spin_unlock_irq(&wb->work_lock);
}

static void finish_writeback_work(struct wb_writeback_work *work)
{
        struct wb_completion *done = work->done;

        if (work->auto_free)
                kfree(work);
        if (done) {
                wait_queue_head_t *waitq = done->waitq;

                /* @done can't be accessed after the following dec */
                if (atomic_dec_and_test(&done->cnt))
                        wake_up_all(waitq);
        }
}

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

        if (work->done)
                atomic_inc(&work->done->cnt);

        spin_lock_irq(&wb->work_lock);

        if (test_bit(WB_registered, &wb->state)) {
                list_add_tail(&work->list, &wb->work_list);
                mod_delayed_work(bdi_wq, &wb->dwork, 0);
        } else
                finish_writeback_work(work);

        spin_unlock_irq(&wb->work_lock);
}

/**
 * wb_wait_for_completion - wait for completion of bdi_writeback_works
 * @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 initialized with
 * DEFINE_WB_COMPLETION().  This function returns after all such work items
 * are completed.  Work items which are waited upon aren't freed
 * automatically on completion.
 */
void wb_wait_for_completion(struct wb_completion *done)
{
        atomic_dec(&done->cnt);         /* put down the initial count */
        wait_event(*done->waitq, !atomic_read(&done->cnt));
}

#ifdef CONFIG_CGROUP_WRITEBACK

/*
 * Parameters for foreign inode detection, see wbc_detach_inode() to see
 * how they're used.
 *
 * These paramters are inherently heuristical as the detection target
 * itself is fuzzy.  All we want to do is detaching an inode from the
 * current owner if it's being written to by some other cgroups too much.
 *
 * The current cgroup writeback is built on the assumption that multiple
 * cgroups writing to the same inode concurrently is very rare and a mode
 * of operation which isn't well supported.  As such, the goal is not
 * taking too long when a different cgroup takes over an inode while
 * avoiding too aggressive flip-flops from occasional foreign writes.
 *
 * We record, very roughly, 2s worth of IO time history and if more than
 * half of that is foreign, trigger the switch.  The recording is quantized
 * to 16 slots.  To avoid tiny writes from swinging the decision too much,
 * writes smaller than 1/8 of avg size are ignored.
 */
#define WB_FRN_TIME_SHIFT       13      /* 1s = 2^13, upto 8 secs w/ 16bit */
#define WB_FRN_TIME_AVG_SHIFT   3       /* avg = avg * 7/8 + new * 1/8 */
#define WB_FRN_TIME_CUT_DIV     8       /* ignore rounds < avg / 8 */
#define WB_FRN_TIME_PERIOD      (2 * (1 << WB_FRN_TIME_SHIFT))  /* 2s */

#define WB_FRN_HIST_SLOTS       16      /* inode->i_wb_frn_history is 16bit */
#define WB_FRN_HIST_UNIT        (WB_FRN_TIME_PERIOD / WB_FRN_HIST_SLOTS)
                                        /* each slot's duration is 2s / 16 */
#define WB_FRN_HIST_THR_SLOTS   (WB_FRN_HIST_SLOTS / 2)
                                        /* if foreign slots >= 8, switch */
#define WB_FRN_HIST_MAX_SLOTS   (WB_FRN_HIST_THR_SLOTS / 2 + 1)
                                        /* one round can affect upto 5 slots */
#define WB_FRN_MAX_IN_FLIGHT    1024    /* don't queue too many concurrently */

/*
 * Maximum inodes per isw.  A specific value has been chosen to make
 * struct inode_switch_wbs_context fit into 1024 bytes kmalloc.
 */
#define WB_MAX_INODES_PER_ISW  ((1024UL - sizeof(struct inode_switch_wbs_context)) \
                                / sizeof(struct inode *))

static atomic_t isw_nr_in_flight = ATOMIC_INIT(0);
static struct workqueue_struct *isw_wq;

void __inode_attach_wb(struct inode *inode, struct folio *folio)
{
        struct backing_dev_info *bdi = inode_to_bdi(inode);
        struct bdi_writeback *wb = NULL;

        if (inode_cgwb_enabled(inode)) {
                struct cgroup_subsys_state *memcg_css;

                if (folio) {
                        memcg_css = mem_cgroup_css_from_folio(folio);
                        wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
                } else {
                        /* must pin memcg_css, see wb_get_create() */
                        memcg_css = task_get_css(current, memory_cgrp_id);
                        wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
                        css_put(memcg_css);
                }
        }

        if (!wb)
                wb = &bdi->wb;

        /*
         * There may be multiple instances of this function racing to
         * update the same inode.  Use cmpxchg() to tell the winner.
         */
        if (unlikely(cmpxchg(&inode->i_wb, NULL, wb)))
                wb_put(wb);
}
EXPORT_SYMBOL_GPL(__inode_attach_wb);

/**
 * inode_cgwb_move_to_attached - put the inode onto wb->b_attached list
 * @inode: inode of interest with i_lock held
 * @wb: target bdi_writeback
 *
 * Remove the inode from wb's io lists and if necessarily put onto b_attached
 * list.  Only inodes attached to cgwb's are kept on this list.
 */
static void inode_cgwb_move_to_attached(struct inode *inode,
                                        struct bdi_writeback *wb)
{
        assert_spin_locked(&wb->list_lock);
        assert_spin_locked(&inode->i_lock);
        WARN_ON_ONCE(inode->i_state & I_FREEING);

        inode->i_state &= ~I_SYNC_QUEUED;
        if (wb != &wb->bdi->wb)
                list_move(&inode->i_io_list, &wb->b_attached);
        else
                list_del_init(&inode->i_io_list);
        wb_io_lists_depopulated(wb);
}

/**
 * locked_inode_to_wb_and_lock_list - determine a locked inode's wb and lock it
 * @inode: inode of interest with i_lock held
 *
 * Returns @inode's wb with its list_lock held.  @inode->i_lock must be
 * held on entry and is released on return.  The returned wb is guaranteed
 * to stay @inode's associated wb until its list_lock is released.
 */
static struct bdi_writeback *
locked_inode_to_wb_and_lock_list(struct inode *inode)
        __releases(&inode->i_lock)
        __acquires(&wb->list_lock)
{
        while (true) {
                struct bdi_writeback *wb = inode_to_wb(inode);

                /*
                 * inode_to_wb() association is protected by both
                 * @inode->i_lock and @wb->list_lock but list_lock nests
                 * outside i_lock.  Drop i_lock and verify that the
                 * association hasn't changed after acquiring list_lock.
                 */
                wb_get(wb);
                spin_unlock(&inode->i_lock);
                spin_lock(&wb->list_lock);

                /* i_wb may have changed inbetween, can't use inode_to_wb() */
                if (likely(wb == inode->i_wb)) {
                        wb_put(wb);     /* @inode already has ref */
                        return wb;
                }

                spin_unlock(&wb->list_lock);
                wb_put(wb);
                cpu_relax();
                spin_lock(&inode->i_lock);
        }
}

/**
 * inode_to_wb_and_lock_list - determine an inode's wb and lock it
 * @inode: inode of interest
 *
 * Same as locked_inode_to_wb_and_lock_list() but @inode->i_lock isn't held
 * on entry.
 */
static struct bdi_writeback *inode_to_wb_and_lock_list(struct inode *inode)
        __acquires(&wb->list_lock)
{
        spin_lock(&inode->i_lock);
        return locked_inode_to_wb_and_lock_list(inode);
}

struct inode_switch_wbs_context {
        struct rcu_work         work;

        /*
         * Multiple inodes can be switched at once.  The switching procedure
         * consists of two parts, separated by a RCU grace period.  To make
         * sure that the second part is executed for each inode gone through
         * the first part, all inode pointers are placed into a NULL-terminated
         * array embedded into struct inode_switch_wbs_context.  Otherwise
         * an inode could be left in a non-consistent state.
         */
        struct bdi_writeback    *new_wb;
        struct inode            *inodes[];
};

static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi)
{
        down_write(&bdi->wb_switch_rwsem);
}

static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi)
{
        up_write(&bdi->wb_switch_rwsem);
}

static bool inode_do_switch_wbs(struct inode *inode,
                                struct bdi_writeback *old_wb,
                                struct bdi_writeback *new_wb)
{
        struct address_space *mapping = inode->i_mapping;
        XA_STATE(xas, &mapping->i_pages, 0);
        struct folio *folio;
        bool switched = false;

        spin_lock(&inode->i_lock);
        xa_lock_irq(&mapping->i_pages);

        /*
         * Once I_FREEING or I_WILL_FREE are visible under i_lock, the eviction
         * path owns the inode and we shouldn't modify ->i_io_list.
         */
        if (unlikely(inode->i_state & (I_FREEING | I_WILL_FREE)))
                goto skip_switch;

        trace_inode_switch_wbs(inode, old_wb, new_wb);

        /*
         * Count and transfer stats.  Note that PAGECACHE_TAG_DIRTY points
         * to possibly dirty folios while PAGECACHE_TAG_WRITEBACK points to
         * folios actually under writeback.
         */
        xas_for_each_marked(&xas, folio, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
                if (folio_test_dirty(folio)) {
                        long nr = folio_nr_pages(folio);
                        wb_stat_mod(old_wb, WB_RECLAIMABLE, -nr);
                        wb_stat_mod(new_wb, WB_RECLAIMABLE, nr);
                }
        }

        xas_set(&xas, 0);
        xas_for_each_marked(&xas, folio, ULONG_MAX, PAGECACHE_TAG_WRITEBACK) {
                long nr = folio_nr_pages(folio);
                WARN_ON_ONCE(!folio_test_writeback(folio));
                wb_stat_mod(old_wb, WB_WRITEBACK, -nr);
                wb_stat_mod(new_wb, WB_WRITEBACK, nr);
        }

        if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
                atomic_dec(&old_wb->writeback_inodes);
                atomic_inc(&new_wb->writeback_inodes);
        }

        wb_get(new_wb);

        /*
         * Transfer to @new_wb's IO list if necessary.  If the @inode is dirty,
         * the specific list @inode was on is ignored and the @inode is put on
         * ->b_dirty which is always correct including from ->b_dirty_time.
         * The transfer preserves @inode->dirtied_when ordering.  If the @inode
         * was clean, it means it was on the b_attached list, so move it onto
         * the b_attached list of @new_wb.
         */
        if (!list_empty(&inode->i_io_list)) {
                inode->i_wb = new_wb;

                if (inode->i_state & I_DIRTY_ALL) {
                        struct inode *pos;

                        list_for_each_entry(pos, &new_wb->b_dirty, i_io_list)
                                if (time_after_eq(inode->dirtied_when,
                                                  pos->dirtied_when))
                                        break;
                        inode_io_list_move_locked(inode, new_wb,
                                                  pos->i_io_list.prev);
                } else {
                        inode_cgwb_move_to_attached(inode, new_wb);
                }
        } else {
                inode->i_wb = new_wb;
        }

        /* ->i_wb_frn updates may race wbc_detach_inode() but doesn't matter */
        inode->i_wb_frn_winner = 0;
        inode->i_wb_frn_avg_time = 0;
        inode->i_wb_frn_history = 0;
        switched = true;
skip_switch:
        /*
         * Paired with load_acquire in unlocked_inode_to_wb_begin() and
         * ensures that the new wb is visible if they see !I_WB_SWITCH.
         */
        smp_store_release(&inode->i_state, inode->i_state & ~I_WB_SWITCH);

        xa_unlock_irq(&mapping->i_pages);
        spin_unlock(&inode->i_lock);

        return switched;
}

static void inode_switch_wbs_work_fn(struct work_struct *work)
{
        struct inode_switch_wbs_context *isw =
                container_of(to_rcu_work(work), struct inode_switch_wbs_context, work);
        struct backing_dev_info *bdi = inode_to_bdi(isw->inodes[0]);
        struct bdi_writeback *old_wb = isw->inodes[0]->i_wb;
        struct bdi_writeback *new_wb = isw->new_wb;
        unsigned long nr_switched = 0;
        struct inode **inodep;

        /*
         * If @inode switches cgwb membership while sync_inodes_sb() is
         * being issued, sync_inodes_sb() might miss it.  Synchronize.
         */
        down_read(&bdi->wb_switch_rwsem);

        /*
         * By the time control reaches here, RCU grace period has passed
         * since I_WB_SWITCH assertion and all wb stat update transactions
         * between unlocked_inode_to_wb_begin/end() are guaranteed to be
         * synchronizing against the i_pages lock.
         *
         * Grabbing old_wb->list_lock, inode->i_lock and the i_pages lock
         * gives us exclusion against all wb related operations on @inode
         * including IO list manipulations and stat updates.
         */
        if (old_wb < new_wb) {
                spin_lock(&old_wb->list_lock);
                spin_lock_nested(&new_wb->list_lock, SINGLE_DEPTH_NESTING);
        } else {
                spin_lock(&new_wb->list_lock);
                spin_lock_nested(&old_wb->list_lock, SINGLE_DEPTH_NESTING);
        }

        for (inodep = isw->inodes; *inodep; inodep++) {
                WARN_ON_ONCE((*inodep)->i_wb != old_wb);
                if (inode_do_switch_wbs(*inodep, old_wb, new_wb))
                        nr_switched++;
        }

        spin_unlock(&new_wb->list_lock);
        spin_unlock(&old_wb->list_lock);

        up_read(&bdi->wb_switch_rwsem);

        if (nr_switched) {
                wb_wakeup(new_wb);
                wb_put_many(old_wb, nr_switched);
        }

        for (inodep = isw->inodes; *inodep; inodep++)
                iput(*inodep);
        wb_put(new_wb);
        kfree(isw);
        atomic_dec(&isw_nr_in_flight);
}

static bool inode_prepare_wbs_switch(struct inode *inode,
                                     struct bdi_writeback *new_wb)
{
        /*
         * Paired with smp_mb() in cgroup_writeback_umount().
         * isw_nr_in_flight must be increased before checking SB_ACTIVE and
         * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0
         * in cgroup_writeback_umount() and the isw_wq will be not flushed.
         */
        smp_mb();

        if (IS_DAX(inode))
                return false;

        /* while holding I_WB_SWITCH, no one else can update the association */
        spin_lock(&inode->i_lock);
        if (!(inode->i_sb->s_flags & SB_ACTIVE) ||
            inode->i_state & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) ||
            inode_to_wb(inode) == new_wb) {
                spin_unlock(&inode->i_lock);
                return false;
        }
        inode->i_state |= I_WB_SWITCH;
        __iget(inode);
        spin_unlock(&inode->i_lock);

        return true;
}

/**
 * inode_switch_wbs - change the wb association of an inode
 * @inode: target inode
 * @new_wb_id: ID of the new wb
 *
 * Switch @inode's wb association to the wb identified by @new_wb_id.  The
 * switching is performed asynchronously and may fail silently.
 */
static void inode_switch_wbs(struct inode *inode, int new_wb_id)
{
        struct backing_dev_info *bdi = inode_to_bdi(inode);
        struct cgroup_subsys_state *memcg_css;
        struct inode_switch_wbs_context *isw;

        /* noop if seems to be already in progress */
        if (inode->i_state & I_WB_SWITCH)
                return;

        /* avoid queueing a new switch if too many are already in flight */
        if (atomic_read(&isw_nr_in_flight) > WB_FRN_MAX_IN_FLIGHT)
                return;

        isw = kzalloc(struct_size(isw, inodes, 2), GFP_ATOMIC);
        if (!isw)
                return;

        atomic_inc(&isw_nr_in_flight);

        /* find and pin the new wb */
        rcu_read_lock();
        memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
        if (memcg_css && !css_tryget(memcg_css))
                memcg_css = NULL;
        rcu_read_unlock();
        if (!memcg_css)
                goto out_free;

        isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
        css_put(memcg_css);
        if (!isw->new_wb)
                goto out_free;

        if (!inode_prepare_wbs_switch(inode, isw->new_wb))
                goto out_free;

        isw->inodes[0] = inode;

        /*
         * In addition to synchronizing among switchers, I_WB_SWITCH tells
         * the RCU protected stat update paths to grab the i_page
         * lock so that stat transfer can synchronize against them.
         * Let's continue after I_WB_SWITCH is guaranteed to be visible.
         */
        INIT_RCU_WORK(&isw->work, inode_switch_wbs_work_fn);
        queue_rcu_work(isw_wq, &isw->work);
        return;

out_free:
        atomic_dec(&isw_nr_in_flight);
        if (isw->new_wb)
                wb_put(isw->new_wb);
        kfree(isw);
}

static bool isw_prepare_wbs_switch(struct inode_switch_wbs_context *isw,
                                   struct list_head *list, int *nr)
{
        struct inode *inode;

        list_for_each_entry(inode, list, i_io_list) {
                if (!inode_prepare_wbs_switch(inode, isw->new_wb))
                        continue;

                isw->inodes[*nr] = inode;
                (*nr)++;

                if (*nr >= WB_MAX_INODES_PER_ISW - 1)
                        return true;
        }
        return false;
}

/**
 * cleanup_offline_cgwb - detach associated inodes
 * @wb: target wb
 *
 * Switch all inodes attached to @wb to a nearest living ancestor's wb in order
 * to eventually release the dying @wb.  Returns %true if not all inodes were
 * switched and the function has to be restarted.
 */
bool cleanup_offline_cgwb(struct bdi_writeback *wb)
{
        struct cgroup_subsys_state *memcg_css;
        struct inode_switch_wbs_context *isw;
        int nr;
        bool restart = false;

        isw = kzalloc(struct_size(isw, inodes, WB_MAX_INODES_PER_ISW),
                      GFP_KERNEL);
        if (!isw)
                return restart;

        atomic_inc(&isw_nr_in_flight);

        for (memcg_css = wb->memcg_css->parent; memcg_css;
             memcg_css = memcg_css->parent) {
                isw->new_wb = wb_get_create(wb->bdi, memcg_css, GFP_KERNEL);
                if (isw->new_wb)
                        break;
        }
        if (unlikely(!isw->new_wb))
                isw->new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */

        nr = 0;
        spin_lock(&wb->list_lock);
        /*
         * In addition to the inodes that have completed writeback, also switch
         * cgwbs for those inodes only with dirty timestamps. Otherwise, those
         * inodes won't be written back for a long time when lazytime is
         * enabled, and thus pinning the dying cgwbs. It won't break the
         * bandwidth restrictions, as writeback of inode metadata is not
         * accounted for.
         */
        restart = isw_prepare_wbs_switch(isw, &wb->b_attached, &nr);
        if (!restart)
                restart = isw_prepare_wbs_switch(isw, &wb->b_dirty_time, &nr);
        spin_unlock(&wb->list_lock);

        /* no attached inodes? bail out */
        if (nr == 0) {
                atomic_dec(&isw_nr_in_flight);
                wb_put(isw->new_wb);
                kfree(isw);
                return restart;
        }

        /*
         * In addition to synchronizing among switchers, I_WB_SWITCH tells
         * the RCU protected stat update paths to grab the i_page
         * lock so that stat transfer can synchronize against them.
         * Let's continue after I_WB_SWITCH is guaranteed to be visible.
         */
        INIT_RCU_WORK(&isw->work, inode_switch_wbs_work_fn);
        queue_rcu_work(isw_wq, &isw->work);

        return restart;
}

/**
 * wbc_attach_and_unlock_inode - associate wbc with target inode and unlock it
 * @wbc: writeback_control of interest
 * @inode: target inode
 *
 * @inode is locked and about to be written back under the control of @wbc.
 * Record @inode's writeback context into @wbc and unlock the i_lock.  On
 * writeback completion, wbc_detach_inode() should be called.  This is used
 * to track the cgroup writeback context.
 */
void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
                                 struct inode *inode)
{
        if (!inode_cgwb_enabled(inode)) {
                spin_unlock(&inode->i_lock);
                return;
        }

        wbc->wb = inode_to_wb(inode);
        wbc->inode = inode;

        wbc->wb_id = wbc->wb->memcg_css->id;
        wbc->wb_lcand_id = inode->i_wb_frn_winner;
        wbc->wb_tcand_id = 0;
        wbc->wb_bytes = 0;
        wbc->wb_lcand_bytes = 0;
        wbc->wb_tcand_bytes = 0;

        wb_get(wbc->wb);
        spin_unlock(&inode->i_lock);

        /*
         * A dying wb indicates that either the blkcg associated with the
         * memcg changed or the associated memcg is dying.  In the first
         * case, a replacement wb should already be available and we should
         * refresh the wb immediately.  In the second case, trying to
         * refresh will keep failing.
         */
        if (unlikely(wb_dying(wbc->wb) && !css_is_dying(wbc->wb->memcg_css)))
                inode_switch_wbs(inode, wbc->wb_id);
}
EXPORT_SYMBOL_GPL(wbc_attach_and_unlock_inode);

/**
 * wbc_detach_inode - disassociate wbc from inode and perform foreign detection
 * @wbc: writeback_control of the just finished writeback
 *
 * To be called after a writeback attempt of an inode finishes and undoes
 * wbc_attach_and_unlock_inode().  Can be called under any context.
 *
 * As concurrent write sharing of an inode is expected to be very rare and
 * memcg only tracks page ownership on first-use basis severely confining
 * the usefulness of such sharing, cgroup writeback tracks ownership
 * per-inode.  While the support for concurrent write sharing of an inode
 * is deemed unnecessary, an inode being written to by different cgroups at
 * different points in time is a lot more common, and, more importantly,
 * charging only by first-use can too readily lead to grossly incorrect
 * behaviors (single foreign page can lead to gigabytes of writeback to be
 * incorrectly attributed).
 *
 * To resolve this issue, cgroup writeback detects the majority dirtier of
 * an inode and transfers the ownership to it.  To avoid unnecessary
 * oscillation, the detection mechanism keeps track of history and gives
 * out the switch verdict only if the foreign usage pattern is stable over
 * a certain amount of time and/or writeback attempts.
 *
 * On each writeback attempt, @wbc tries to detect the majority writer
 * using Boyer-Moore majority vote algorithm.  In addition to the byte
 * count from the majority voting, it also counts the bytes written for the
 * current wb and the last round's winner wb (max of last round's current
 * wb, the winner from two rounds ago, and the last round's majority
 * candidate).  Keeping track of the historical winner helps the algorithm
 * to semi-reliably detect the most active writer even when it's not the
 * absolute majority.
 *
 * Once the winner of the round is determined, whether the winner is
 * foreign or not and how much IO time the round consumed is recorded in
 * inode->i_wb_frn_history.  If the amount of recorded foreign IO time is
 * over a certain threshold, the switch verdict is given.
 */
void wbc_detach_inode(struct writeback_control *wbc)
{
        struct bdi_writeback *wb = wbc->wb;
        struct inode *inode = wbc->inode;
        unsigned long avg_time, max_bytes, max_time;
        u16 history;
        int max_id;

        if (!wb)
                return;

        history = inode->i_wb_frn_history;
        avg_time = inode->i_wb_frn_avg_time;

        /* pick the winner of this round */
        if (wbc->wb_bytes >= wbc->wb_lcand_bytes &&
            wbc->wb_bytes >= wbc->wb_tcand_bytes) {
                max_id = wbc->wb_id;
                max_bytes = wbc->wb_bytes;
        } else if (wbc->wb_lcand_bytes >= wbc->wb_tcand_bytes) {
                max_id = wbc->wb_lcand_id;
                max_bytes = wbc->wb_lcand_bytes;
        } else {
                max_id = wbc->wb_tcand_id;
                max_bytes = wbc->wb_tcand_bytes;
        }

        /*
         * Calculate the amount of IO time the winner consumed and fold it
         * into the running average kept per inode.  If the consumed IO
         * time is lower than avag / WB_FRN_TIME_CUT_DIV, ignore it for
         * deciding whether to switch or not.  This is to prevent one-off
         * small dirtiers from skewing the verdict.
         */
        max_time = DIV_ROUND_UP((max_bytes >> PAGE_SHIFT) << WB_FRN_TIME_SHIFT,
                                wb->avg_write_bandwidth);
        if (avg_time)
                avg_time += (max_time >> WB_FRN_TIME_AVG_SHIFT) -
                            (avg_time >> WB_FRN_TIME_AVG_SHIFT);
        else
                avg_time = max_time;    /* immediate catch up on first run */

        if (max_time >= avg_time / WB_FRN_TIME_CUT_DIV) {
                int slots;

                /*
                 * The switch verdict is reached if foreign wb's consume
                 * more than a certain proportion of IO time in a
                 * WB_FRN_TIME_PERIOD.  This is loosely tracked by 16 slot
                 * history mask where each bit represents one sixteenth of
                 * the period.  Determine the number of slots to shift into
                 * history from @max_time.
                 */
                slots = min(DIV_ROUND_UP(max_time, WB_FRN_HIST_UNIT),
                            (unsigned long)WB_FRN_HIST_MAX_SLOTS);
                history <<= slots;
                if (wbc->wb_id != max_id)
                        history |= (1U << slots) - 1;

                if (history)
                        trace_inode_foreign_history(inode, wbc, history);

                /*
                 * Switch if the current wb isn't the consistent winner.
                 * If there are multiple closely competing dirtiers, the
                 * inode may switch across them repeatedly over time, which
                 * is okay.  The main goal is avoiding keeping an inode on
                 * the wrong wb for an extended period of time.
                 */
                if (hweight16(history) > WB_FRN_HIST_THR_SLOTS)
                        inode_switch_wbs(inode, max_id);
        }

        /*
         * Multiple instances of this function may race to update the
         * following fields but we don't mind occassional inaccuracies.
         */
        inode->i_wb_frn_winner = max_id;
        inode->i_wb_frn_avg_time = min(avg_time, (unsigned long)U16_MAX);
        inode->i_wb_frn_history = history;

        wb_put(wbc->wb);
        wbc->wb = NULL;
}
EXPORT_SYMBOL_GPL(wbc_detach_inode);

/**
 * wbc_account_cgroup_owner - account writeback to update inode cgroup ownership
 * @wbc: writeback_control of the writeback in progress
 * @page: page being written out
 * @bytes: number of bytes being written out
 *
 * @bytes from @page are about to written out during the writeback
 * controlled by @wbc.  Keep the book for foreign inode detection.  See
 * wbc_detach_inode().
 */
void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
                              size_t bytes)
{
        struct folio *folio;
        struct cgroup_subsys_state *css;
        int id;

        /*
         * pageout() path doesn't attach @wbc to the inode being written
         * out.  This is intentional as we don't want the function to block
         * behind a slow cgroup.  Ultimately, we want pageout() to kick off
         * regular writeback instead of writing things out itself.
         */
        if (!wbc->wb || wbc->no_cgroup_owner)
                return;

        folio = page_folio(page);
        css = mem_cgroup_css_from_folio(folio);
        /* dead cgroups shouldn't contribute to inode ownership arbitration */
        if (!(css->flags & CSS_ONLINE))
                return;

        id = css->id;

        if (id == wbc->wb_id) {
                wbc->wb_bytes += bytes;
                return;
        }

        if (id == wbc->wb_lcand_id)
                wbc->wb_lcand_bytes += bytes;

        /* Boyer-Moore majority vote algorithm */
        if (!wbc->wb_tcand_bytes)
                wbc->wb_tcand_id = id;
        if (id == wbc->wb_tcand_id)
                wbc->wb_tcand_bytes += bytes;
        else
                wbc->wb_tcand_bytes -= min(bytes, wbc->wb_tcand_bytes);
}
EXPORT_SYMBOL_GPL(wbc_account_cgroup_owner);

/**
 * 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);
}

/**
 * bdi_split_work_to_wbs - split a wb_writeback_work to all wb's of a bdi
 * @bdi: target backing_dev_info
 * @base_work: wb_writeback_work to issue
 * @skip_if_busy: skip wb's which already have writeback in progress
 *
 * Split and issue @base_work to all wb's (bdi_writeback's) of @bdi which
 * have dirty inodes.  If @base_work->nr_page isn't %LONG_MAX, it's
 * distributed to the busy wbs according to each wb's proportion in the
 * total active write bandwidth of @bdi.
 */
static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
                                  struct wb_writeback_work *base_work,
                                  bool skip_if_busy)
{
        struct bdi_writeback *last_wb = NULL;
        struct bdi_writeback *wb = list_entry(&bdi->wb_list,
                                              struct bdi_writeback, bdi_node);

        might_sleep();
restart:
        rcu_read_lock();
        list_for_each_entry_continue_rcu(wb, &bdi->wb_list, bdi_node) {
                DEFINE_WB_COMPLETION(fallback_work_done, bdi);
                struct wb_writeback_work fallback_work;
                struct wb_writeback_work *work;
                long nr_pages;

                if (last_wb) {
                        wb_put(last_wb);
                        last_wb = NULL;
                }

                /* SYNC_ALL writes out I_DIRTY_TIME too */
                if (!wb_has_dirty_io(wb) &&
                    (base_work->sync_mode == WB_SYNC_NONE ||
                     list_empty(&wb->b_dirty_time)))
                        continue;
                if (skip_if_busy && writeback_in_progress(wb))
                        continue;

                nr_pages = wb_split_bdi_pages(wb, base_work->nr_pages);

                work = kmalloc(sizeof(*work), GFP_ATOMIC);
                if (work) {
                        *work = *base_work;
                        work->nr_pages = nr_pages;
                        work->auto_free = 1;
                        wb_queue_work(wb, work);
                        continue;
                }

                /*
                 * If wb_tryget fails, the wb has been shutdown, skip it.
                 *
                 * Pin @wb so that it stays on @bdi->wb_list.  This allows
                 * continuing iteration from @wb after dropping and
                 * regrabbing rcu read lock.
                 */
                if (!wb_tryget(wb))
                        continue;

                /* alloc failed, execute synchronously using on-stack fallback */
                work = &fallback_work;
                *work = *base_work;
                work->nr_pages = nr_pages;
                work->auto_free = 0;
                work->done = &fallback_work_done;

                wb_queue_work(wb, work);
                last_wb = wb;

                rcu_read_unlock();
                wb_wait_for_completion(&fallback_work_done);
                goto restart;
        }
        rcu_read_unlock();

        if (last_wb)
                wb_put(last_wb);
}

/**
 * cgroup_writeback_by_id - initiate cgroup writeback from bdi and memcg IDs
 * @bdi_id: target bdi id
 * @memcg_id: target memcg css id
 * @reason: reason why some writeback work initiated
 * @done: target wb_completion
 *
 * Initiate flush of the bdi_writeback identified by @bdi_id and @memcg_id
 * with the specified parameters.
 */
int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
                           enum wb_reason reason, struct wb_completion *done)
{
        struct backing_dev_info *bdi;
        struct cgroup_subsys_state *memcg_css;
        struct bdi_writeback *wb;
        struct wb_writeback_work *work;
        unsigned long dirty;
        int ret;

        /* lookup bdi and memcg */
        bdi = bdi_get_by_id(bdi_id);
        if (!bdi)
                return -ENOENT;

        rcu_read_lock();
        memcg_css = css_from_id(memcg_id, &memory_cgrp_subsys);
        if (memcg_css && !css_tryget(memcg_css))
                memcg_css = NULL;
        rcu_read_unlock();
        if (!memcg_css) {
                ret = -ENOENT;
                goto out_bdi_put;
        }

        /*
         * And find the associated wb.  If the wb isn't there already
         * there's nothing to flush, don't create one.
         */
        wb = wb_get_lookup(bdi, memcg_css);
        if (!wb) {
                ret = -ENOENT;
                goto out_css_put;
        }

        /*
         * The caller is attempting to write out most of
         * the currently dirty pages.  Let's take the current dirty page
         * count and inflate it by 25% which should be large enough to
         * flush out most dirty pages while avoiding getting livelocked by
         * concurrent dirtiers.
         *
         * BTW the memcg stats are flushed periodically and this is best-effort
         * estimation, so some potential error is ok.
         */
        dirty = memcg_page_state(mem_cgroup_from_css(memcg_css), NR_FILE_DIRTY);
        dirty = dirty * 10 / 8;

        /* issue the writeback work */
        work = kzalloc(sizeof(*work), GFP_NOWAIT | __GFP_NOWARN);
        if (work) {
                work->nr_pages = dirty;
                work->sync_mode = WB_SYNC_NONE;
                work->range_cyclic = 1;
                work->reason = reason;
                work->done = done;
                work->auto_free = 1;
                wb_queue_work(wb, work);
                ret = 0;
        } else {
                ret = -ENOMEM;
        }

        wb_put(wb);
out_css_put:
        css_put(memcg_css);
out_bdi_put:
        bdi_put(bdi);
        return ret;
}

/**
 * cgroup_writeback_umount - flush inode wb switches for umount
 *
 * This function is called when a super_block is about to be destroyed and
 * flushes in-flight inode wb switches.  An inode wb switch goes through
 * RCU and then workqueue, so the two need to be flushed in order to ensure
 * that all previously scheduled switches are finished.  As wb switches are
 * rare occurrences and synchronize_rcu() can take a while, perform
 * flushing iff wb switches are in flight.
 */
void cgroup_writeback_umount(void)
{
        /*
         * SB_ACTIVE should be reliably cleared before checking
         * isw_nr_in_flight, see generic_shutdown_super().
         */
        smp_mb();

        if (atomic_read(&isw_nr_in_flight)) {
                /*
                 * Use rcu_barrier() to wait for all pending callbacks to
                 * ensure that all in-flight wb switches are in the workqueue.
                 */
                rcu_barrier();
                flush_workqueue(isw_wq);
        }
}

static int __init cgroup_writeback_init(void)
{
        isw_wq = alloc_workqueue("inode_switch_wbs", 0, 0);
        if (!isw_wq)
                return -ENOMEM;
        return 0;
}
fs_initcall(cgroup_writeback_init);

#else   /* CONFIG_CGROUP_WRITEBACK */

static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }
static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }

static void inode_cgwb_move_to_attached(struct inode *inode,
                                        struct bdi_writeback *wb)
{
        assert_spin_locked(&wb->list_lock);
        assert_spin_locked(&inode->i_lock);
        WARN_ON_ONCE(inode->i_state & I_FREEING);

        inode->i_state &= ~I_SYNC_QUEUED;
        list_del_init(&inode->i_io_list);
        wb_io_lists_depopulated(wb);
}

static struct bdi_writeback *
locked_inode_to_wb_and_lock_list(struct inode *inode)
        __releases(&inode->i_lock)
        __acquires(&wb->list_lock)
{
        struct bdi_writeback *wb = inode_to_wb(inode);

        spin_unlock(&inode->i_lock);
        spin_lock(&wb->list_lock);
        return wb;
}

static struct bdi_writeback *inode_to_wb_and_lock_list(struct inode *inode)
        __acquires(&wb->list_lock)
{
        struct bdi_writeback *wb = inode_to_wb(inode);

        spin_lock(&wb->list_lock);
        return wb;
}

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

static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
                                  struct wb_writeback_work *base_work,
                                  bool skip_if_busy)
{
        might_sleep();

        if (!skip_if_busy || !writeback_in_progress(&bdi->wb)) {
                base_work->auto_free = 0;
                wb_queue_work(&bdi->wb, base_work);
        }
}

#endif  /* CONFIG_CGROUP_WRITEBACK */

/*
 * 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_node_page_state(NR_FILE_DIRTY) +
                get_nr_dirty_inodes();
}

static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)
{
        if (!wb_has_dirty_io(wb))
                return;

        /*
         * All callers of this function want to start writeback of all
         * dirty pages. Places like vmscan can call this at a very
         * high frequency, causing pointless allocations of tons of
         * work items and keeping the flusher threads busy retrieving
         * that work. Ensure that we only allow one of them pending and
         * inflight at the time.
         */
        if (test_bit(WB_start_all, &wb->state) ||
            test_and_set_bit(WB_start_all, &wb->state))
                return;

        wb->start_all_reason = reason;
        wb_wakeup(wb);
}

/**
 * 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);
        wb_wakeup(wb);
}

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

        wb = inode_to_wb_and_lock_list(inode);
        spin_lock(&inode->i_lock);

        inode->i_state &= ~I_SYNC_QUEUED;
        list_del_init(&inode->i_io_list);
        wb_io_lists_depopulated(wb);

        spin_unlock(&inode->i_lock);
        spin_unlock(&wb->list_lock);
}
EXPORT_SYMBOL(inode_io_list_del);

/*
 * mark an inode as under writeback on the sb
 */
void sb_mark_inode_writeback(struct inode *inode)
{
        struct super_block *sb = inode->i_sb;
        unsigned long flags;

        if (list_empty(&inode->i_wb_list)) {
                spin_lock_irqsave(&sb->s_inode_wblist_lock, flags);
                if (list_empty(&inode->i_wb_list)) {
                        list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
                        trace_sb_mark_inode_writeback(inode);
                }
                spin_unlock_irqrestore(&sb->s_inode_wblist_lock, flags);
        }
}

/*
 * clear an inode as under writeback on the sb
 */
void sb_clear_inode_writeback(struct inode *inode)
{
        struct super_block *sb = inode->i_sb;
        unsigned long flags;

        if (!list_empty(&inode->i_wb_list)) {
                spin_lock_irqsave(&sb->s_inode_wblist_lock, flags);
                if (!list_empty(&inode->i_wb_list)) {
                        list_del_init(&inode->i_wb_list);
                        trace_sb_clear_inode_writeback(inode);
                }
                spin_unlock_irqrestore(&sb->s_inode_wblist_lock, flags);
        }
}

/*
 * 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_locked(struct inode *inode, struct bdi_writeback *wb)
{
        assert_spin_locked(&inode->i_lock);

        inode->i_state &= ~I_SYNC_QUEUED;
        /*
         * When the inode is being freed just don't bother with dirty list
         * tracking. Flush worker will ignore this inode anyway and it will
         * trigger assertions in inode_io_list_move_locked().
         */
        if (inode->i_state & I_FREEING) {
                list_del_init(&inode->i_io_list);
                wb_io_lists_depopulated(wb);
                return;
        }
        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_io_list_move_locked(inode, wb, &wb->b_dirty);
}

static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
{
        spin_lock(&inode->i_lock);
        redirty_tail_locked(inode, wb);
        spin_unlock(&inode->i_lock);
}

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

/*
 * Move expired (dirtied before dirtied_before) dirty inodes from
 * @delaying_queue to @dispatch_queue.
 */
static int move_expired_inodes(struct list_head *delaying_queue,
                               struct list_head *dispatch_queue,
                               unsigned long dirtied_before)
{
        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 (inode_dirtied_after(inode, dirtied_before))
                        break;
                spin_lock(&inode->i_lock);
                list_move(&inode->i_io_list, &tmp);
                moved++;
                inode->i_state |= I_SYNC_QUEUED;
                spin_unlock(&inode->i_lock);
                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;
        }

        /*
         * Although inode's i_io_list is moved from 'tmp' to 'dispatch_queue',
         * we don't take inode->i_lock here because it is just a pointless overhead.
         * Inode is already marked as I_SYNC_QUEUED so writeback list handling is
         * fully under our control.
         */
        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_io_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,
                     unsigned long dirtied_before)
{
        int moved;
        unsigned long time_expire_jif = dirtied_before;

        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, dirtied_before);
        if (!work->for_sync)
                time_expire_jif = jiffies - dirtytime_expire_interval * HZ;
        moved += move_expired_inodes(&wb->b_dirty_time, &wb->b_io,
                                     time_expire_jif);
        if (moved)
                wb_io_lists_populated(wb);
        trace_writeback_queue_io(wb, work, dirtied_before, 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,
                          unsigned long dirtied_before)
{
        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. Although having skipped pages
                 * is odd for clean inodes, it can happen for some
                 * filesystems so handle that gracefully.
                 */
                if (inode->i_state & I_DIRTY_ALL)
                        redirty_tail_locked(inode, wb);
                else
                        inode_cgwb_move_to_attached(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 &&
                    !inode_dirtied_after(inode, dirtied_before)) {
                        /* 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_locked(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_locked(inode, wb);
        } else if (inode->i_state & I_DIRTY_TIME) {
                inode->dirtied_when = jiffies;
                inode_io_list_move_locked(inode, wb, &wb->b_dirty_time);
                inode->i_state &= ~I_SYNC_QUEUED;
        } else {
                /* The inode is clean. Remove from writeback lists. */
                inode_cgwb_move_to_attached(inode, wb);
        }
}

/*
 * Write out an inode and its dirty pages (or some of its dirty pages, depending
 * on @wbc->nr_to_write), and clear the relevant dirty flags from i_state.
 *
 * This doesn't remove the inode from the writeback list it is on, except
 * potentially to move it from b_dirty_time to b_dirty due to timestamp
 * expiration.  The caller is otherwise responsible for writeback list handling.
 *
 * The caller is also responsible for setting the I_SYNC flag beforehand and
 * calling inode_sync_complete() to clear it afterwards.
 */
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;
        }

        /*
         * If the inode has dirty timestamps and we need to write them, call
         * mark_inode_dirty_sync() to notify the filesystem about it and to
         * change I_DIRTY_TIME into I_DIRTY_SYNC.
         */
        if ((inode->i_state & I_DIRTY_TIME) &&
            (wbc->sync_mode == WB_SYNC_ALL ||
             time_after(jiffies, inode->dirtied_time_when +
                        dirtytime_expire_interval * HZ))) {
                trace_writeback_lazytime(inode);
                mark_inode_dirty_sync(inode);
        }

        /*
         * Get and clear the dirty flags from i_state.  This needs to be done
         * after calling writepages because some filesystems may redirty the
         * inode during writepages due to delalloc.  It also needs to be done
         * after handling timestamp expiration, as that may dirty the inode too.
         */
        spin_lock(&inode->i_lock);
        dirty = inode->i_state & I_DIRTY;
        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;
        else if (unlikely(inode->i_state & I_PINNING_NETFS_WB)) {
                if (!(inode->i_state & I_DIRTY_PAGES)) {
                        inode->i_state &= ~I_PINNING_NETFS_WB;
                        wbc->unpinned_netfs_wb = true;
                        dirty |= I_PINNING_NETFS_WB; /* Cause write_inode */
                }
        }

        spin_unlock(&inode->i_lock);

        /* 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;
        }
        wbc->unpinned_netfs_wb = false;
        trace_writeback_single_inode(inode, wbc, nr_to_write);
        return ret;
}

/*
 * Write out an inode's dirty data and metadata on-demand, i.e. separately from
 * the regular batched writeback done by the flusher threads in
 * writeback_sb_inodes().  @wbc controls various aspects of the write, such as
 * whether it is a data-integrity sync (%WB_SYNC_ALL) or not (%WB_SYNC_NONE).
 *
 * To prevent the inode from going away, either the caller must have a reference
 * to the inode, or the inode must have I_WILL_FREE or I_FREEING set.
 */
static int writeback_single_inode(struct inode *inode,
                                  struct writeback_control *wbc)
{
        struct bdi_writeback *wb;
        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) {
                /*
                 * Writeback is already running on the inode.  For WB_SYNC_NONE,
                 * that's enough and we can just return.  For WB_SYNC_ALL, we
                 * must wait for the existing writeback to complete, then do
                 * writeback again if there's anything left.
                 */
                if (wbc->sync_mode != WB_SYNC_ALL)
                        goto out;
                __inode_wait_for_writeback(inode);
        }
        WARN_ON(inode->i_state & I_SYNC);
        /*
         * If the inode is already fully clean, then there's nothing to do.
         *
         * For data-integrity syncs we also need to check whether any pages are
         * still under writeback, e.g. due to prior WB_SYNC_NONE writeback.  If
         * there are any such pages, we'll need to wait for them.
         */
        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;
        wbc_attach_and_unlock_inode(wbc, inode);

        ret = __writeback_single_inode(inode, wbc);

        wbc_detach_inode(wbc);

        wb = inode_to_wb_and_lock_list(inode);
        spin_lock(&inode->i_lock);
        /*
         * If the inode is freeing, its i_io_list shoudn't be updated
         * as it can be finally deleted at this moment.
         */
        if (!(inode->i_state & I_FREEING)) {
                /*
                 * If the inode is now fully clean, then it can be safely
                 * removed from its writeback list (if any). Otherwise the
                 * flusher threads are responsible for the writeback lists.
                 */
                if (!(inode->i_state & I_DIRTY_ALL))
                        inode_cgwb_move_to_attached(inode, wb);
                else if (!(inode->i_state & I_SYNC_QUEUED)) {
                        if ((inode->i_state & I_DIRTY))
                                redirty_tail_locked(inode, wb);
                        else if (inode->i_state & I_DIRTY_TIME) {
                                inode->dirtied_when = jiffies;
                                inode_io_list_move_locked(inode,
                                                          wb,
                                                          &wb->b_dirty_time);
                        }
                }
        }

        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_wb_domain.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.
 *
 * NOTE! This is called with wb->list_lock held, and will
 * unlock and relock that for each inode it ends up doing
 * IO for.
 */
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 total_wrote = 0;  /* count both pages and inodes */
        unsigned long dirtied_before = jiffies;

        if (work->for_kupdate)
                dirtied_before = jiffies -
                        msecs_to_jiffies(dirty_expire_interval * 10);

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

                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)) {
                        redirty_tail_locked(inode, wb);
                        spin_unlock(&inode->i_lock);
                        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.
                         */
                        requeue_io(inode, wb);
                        spin_unlock(&inode->i_lock);
                        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;
                wbc_attach_and_unlock_inode(&wbc, inode);

                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);

                wbc_detach_inode(&wbc);
                work->nr_pages -= write_chunk - wbc.nr_to_write;
                wrote = write_chunk - wbc.nr_to_write - wbc.pages_skipped;
                wrote = wrote < 0 ? 0 : wrote;
                total_wrote += wrote;

                if (need_resched()) {
                        /*
                         * We're trying to balance between building up a nice
                         * long list of IOs to improve our merge rate, and
                         * getting those IOs out quickly for anyone throttling
                         * in balance_dirty_pages().  cond_resched() doesn't
                         * unplug, so get our IOs out the door before we
                         * give up the CPU.
                         */
                        blk_flush_plug(current->plug, false);
                        cond_resched();
                }

                /*
                 * Requeue @inode if still dirty.  Be careful as @inode may
                 * have been switched to another wb in the meantime.
                 */
                tmp_wb = inode_to_wb_and_lock_list(inode);
                spin_lock(&inode->i_lock);
                if (!(inode->i_state & I_DIRTY_ALL))
                        total_wrote++;
                requeue_inode(inode, tmp_wb, &wbc, dirtied_before);
                inode_sync_complete(inode);
                spin_unlock(&inode->i_lock);

                if (unlikely(tmp_wb != wb)) {
                        spin_unlock(&tmp_wb->list_lock);
                        spin_lock(&wb->list_lock);
                }

                /*
                 * bail out to wb_writeback() often enough to check
                 * background threshold and other termination conditions.
                 */
                if (total_wrote) {
                        if (time_is_before_jiffies(start_time + HZ / 10UL))
                                break;
                        if (work->nr_pages <= 0)
                                break;
                }
        }
        return total_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 (!super_trylock_shared(sb)) {
                        /*
                         * super_trylock_shared() 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,
        };
        struct blk_plug plug;

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

        return nr_pages - work.nr_pages;
}

/*
 * 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.
 *
 * dirtied_before 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)
{
        long nr_pages = work->nr_pages;
        unsigned long dirtied_before = jiffies;
        struct inode *inode;
        long progress;
        struct blk_plug plug;
        bool queued = false;

        blk_start_plug(&plug);
        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 && !wb_over_bg_thresh(wb))
                        break;


                spin_lock(&wb->list_lock);

                trace_writeback_start(wb, work);
                if (list_empty(&wb->b_io)) {
                        /*
                         * 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) {
                                dirtied_before = jiffies -
                                        msecs_to_jiffies(dirty_expire_interval *
                                                         10);
                        } else if (work->for_background)
                                dirtied_before = jiffies;

                        queue_io(wb, work, dirtied_before);
                        queued = true;
                }
                if (work->sb)
                        progress = writeback_sb_inodes(work->sb, wb, work);
                else
                        progress = __writeback_inodes_wb(wb, work);
                trace_writeback_written(wb, work);

                /*
                 * 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 || !queued) {
                        spin_unlock(&wb->list_lock);
                        continue;
                }

                /*
                 * No more inodes for IO, bail
                 */
                if (list_empty(&wb->b_more_io)) {
                        spin_unlock(&wb->list_lock);
                        break;
                }

                /*
                 * Nothing written. Wait for some inode to
                 * become available for writeback. Otherwise
                 * we'll just busyloop.
                 */
                trace_writeback_wait(wb, 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);
        }
        blk_finish_plug(&plug);

        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_irq(&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_irq(&wb->work_lock);
        return work;
}

static long wb_check_background_flush(struct bdi_writeback *wb)
{
        if (wb_over_bg_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;
}

static long wb_check_start_all(struct bdi_writeback *wb)
{
        long nr_pages;

        if (!test_bit(WB_start_all, &wb->state))
                return 0;

        nr_pages = get_nr_dirty_pages();
        if (nr_pages) {
                struct wb_writeback_work work = {
                        .nr_pages       = wb_split_bdi_pages(wb, nr_pages),
                        .sync_mode      = WB_SYNC_NONE,
                        .range_cyclic   = 1,
                        .reason         = wb->start_all_reason,
                };

                nr_pages = wb_writeback(wb, &work);
        }

        clear_bit(WB_start_all, &wb->state);
        return nr_pages;
}


/*
 * 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) {
                trace_writeback_exec(wb, work);
                wrote += wb_writeback(wb, work);
                finish_writeback_work(work);
        }

        /*
         * Check for a flush-everything request
         */
        wrote += wb_check_start_all(wb);

        /*
         * 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", bdi_dev_name(wb->bdi));

        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))
                wb_wakeup(wb);
        else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
                wb_wakeup_delayed(wb);
}

/*
 * Start writeback of all dirty pages on this bdi.
 */
static void __wakeup_flusher_threads_bdi(struct backing_dev_info *bdi,
                                         enum wb_reason reason)
{
        struct bdi_writeback *wb;

        if (!bdi_has_dirty_io(bdi))
                return;

        list_for_each_entry_rcu(wb, &bdi->wb_list, bdi_node)
                wb_start_writeback(wb, reason);
}

void wakeup_flusher_threads_bdi(struct backing_dev_info *bdi,
                                enum wb_reason reason)
{
        rcu_read_lock();
        __wakeup_flusher_threads_bdi(bdi, reason);
        rcu_read_unlock();
}

/*
 * Wakeup the flusher threads to start writeback of all currently dirty pages
 */
void wakeup_flusher_threads(enum wb_reason reason)
{
        struct backing_dev_info *bdi;

        /*
         * If we are expecting writeback progress we must submit plugged IO.
         */
        blk_flush_plug(current->plug, true);

        rcu_read_lock();
        list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
                __wakeup_flusher_threads_bdi(bdi, 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;

                list_for_each_entry_rcu(wb, &bdi->wb_list, bdi_node)
                        if (!list_empty(&wb->b_dirty_time))
                                wb_wakeup(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 *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;
}

/**
 * __mark_inode_dirty - internal function to mark an inode dirty
 *
 * @inode: inode to mark
 * @flags: what kind of dirty, e.g. I_DIRTY_SYNC.  This can be a combination of
 *         multiple I_DIRTY_* flags, except that I_DIRTY_TIME can't be combined
 *         with I_DIRTY_PAGES.
 *
 * Mark an inode as dirty.  We notify the filesystem, then update the inode's
 * dirty flags.  Then, if needed we add the inode to the appropriate dirty list.
 *
 * Most callers should use mark_inode_dirty() or mark_inode_dirty_sync()
 * instead of calling this directly.
 *
 * CAREFUL!  We only add the inode to the dirty list if it is hashed or if it
 * refers to a blockdev.  Unhashed inodes will never be added to the dirty list
 * even if they are later hashed, as they will have been marked dirty already.
 *
 * In short, ensure 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.
 */
void __mark_inode_dirty(struct inode *inode, int flags)
{
        struct super_block *sb = inode->i_sb;
        int dirtytime = 0;
        struct bdi_writeback *wb = NULL;

        trace_writeback_mark_inode_dirty(inode, flags);

        if (flags & I_DIRTY_INODE) {
                /*
                 * Inode timestamp update will piggback on this dirtying.
                 * We tell ->dirty_inode callback that timestamps need to
                 * be updated by setting I_DIRTY_TIME in flags.
                 */
                if (inode->i_state & I_DIRTY_TIME) {
                        spin_lock(&inode->i_lock);
                        if (inode->i_state & I_DIRTY_TIME) {
                                inode->i_state &= ~I_DIRTY_TIME;
                                flags |= I_DIRTY_TIME;
                        }
                        spin_unlock(&inode->i_lock);
                }

                /*
                 * Notify the filesystem about the inode being dirtied, so that
                 * (if needed) it can update on-disk fields and journal the
                 * inode.  This is only needed when the inode itself is being
                 * dirtied now.  I.e. it's only needed for I_DIRTY_INODE, not
                 * for just I_DIRTY_PAGES or I_DIRTY_TIME.
                 */
                trace_writeback_dirty_inode_start(inode, flags);
                if (sb->s_op->dirty_inode)
                        sb->s_op->dirty_inode(inode,
                                flags & (I_DIRTY_INODE | I_DIRTY_TIME));
                trace_writeback_dirty_inode(inode, flags);

                /* I_DIRTY_INODE supersedes I_DIRTY_TIME. */
                flags &= ~I_DIRTY_TIME;
        } else {
                /*
                 * Else it's either I_DIRTY_PAGES, I_DIRTY_TIME, or nothing.
                 * (We don't support setting both I_DIRTY_PAGES and I_DIRTY_TIME
                 * in one call to __mark_inode_dirty().)
                 */
                dirtytime = flags & I_DIRTY_TIME;
                WARN_ON_ONCE(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)
                return;

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

                inode_attach_wb(inode, NULL);

                inode->i_state |= flags;

                /*
                 * Grab inode's wb early because it requires dropping i_lock and we
                 * need to make sure following checks happen atomically with dirty
                 * list handling so that we don't move inodes under flush worker's
                 * hands.
                 */
                if (!was_dirty) {
                        wb = locked_inode_to_wb_and_lock_list(inode);
                        spin_lock(&inode->i_lock);
                }

                /*
                 * If the inode is queued for writeback by flush worker, just
                 * update its dirty state. Once the flush worker is done with
                 * the inode it will place it on the appropriate superblock
                 * list, based upon its state.
                 */
                if (inode->i_state & I_SYNC_QUEUED)
                        goto out_unlock;

                /*
                 * 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;
                }
                if (inode->i_state & I_FREEING)
                        goto out_unlock;

                /*
                 * 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;

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

                        if (inode->i_state & I_DIRTY)
                                dirty_list = &wb->b_dirty;
                        else
                                dirty_list = &wb->b_dirty_time;

                        wakeup_bdi = inode_io_list_move_locked(inode, wb,
                                                               dirty_list);

                        spin_unlock(&wb->list_lock);
                        spin_unlock(&inode->i_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 (wakeup_bdi &&
                            (wb->bdi->capabilities & BDI_CAP_WRITEBACK))
                                wb_wakeup_delayed(wb);
                        return;
                }
        }
out_unlock:
        if (wb)
                spin_unlock(&wb->list_lock);
        spin_unlock(&inode->i_lock);
}
EXPORT_SYMBOL(__mark_inode_dirty);

/*
 * The @s_sync_lock is used to serialise concurrent sync operations
 * to avoid lock contention problems with concurrent wait_sb_inodes() calls.
 * Concurrent callers will block on the s_sync_lock rather than doing contending
 * walks. The queueing maintains sync(2) required behaviour as all the IO that
 * has been issued up to the time this function is enter is guaranteed to be
 * completed by the time we have gained the lock and waited for all IO that is
 * in progress regardless of the order callers are granted the lock.
 */
static void wait_sb_inodes(struct super_block *sb)
{
        LIST_HEAD(sync_list);

        /*
         * 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));

        mutex_lock(&sb->s_sync_lock);

        /*
         * Splice the writeback list onto a temporary list to avoid waiting on
         * inodes that have started writeback after this point.
         *
         * Use rcu_read_lock() to keep the inodes around until we have a
         * reference. s_inode_wblist_lock protects sb->s_inodes_wb as well as
         * the local list because inodes can be dropped from either by writeback
         * completion.
         */
        rcu_read_lock();
        spin_lock_irq(&sb->s_inode_wblist_lock);
        list_splice_init(&sb->s_inodes_wb, &sync_list);

        /*
         * 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.
         */
        while (!list_empty(&sync_list)) {
                struct inode *inode = list_first_entry(&sync_list, struct inode,
                                                       i_wb_list);
                struct address_space *mapping = inode->i_mapping;

                /*
                 * Move each inode back to the wb list before we drop the lock
                 * to preserve consistency between i_wb_list and the mapping
                 * writeback tag. Writeback completion is responsible to remove
                 * the inode from either list once the writeback tag is cleared.
                 */
                list_move_tail(&inode->i_wb_list, &sb->s_inodes_wb);

                /*
                 * The mapping can appear untagged while still on-list since we
                 * do not have the mapping lock. Skip it here, wb completion
                 * will remove it.
                 */
                if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
                        continue;

                spin_unlock_irq(&sb->s_inode_wblist_lock);

                spin_lock(&inode->i_lock);
                if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) {
                        spin_unlock(&inode->i_lock);

                        spin_lock_irq(&sb->s_inode_wblist_lock);
                        continue;
                }
                __iget(inode);
                spin_unlock(&inode->i_lock);
                rcu_read_unlock();

                /*
                 * We keep the error status of individual mapping so that
                 * applications can catch the writeback error using fsync(2).
                 * See filemap_fdatawait_keep_errors() for details.
                 */
                filemap_fdatawait_keep_errors(mapping);

                cond_resched();

                iput(inode);

                rcu_read_lock();
                spin_lock_irq(&sb->s_inode_wblist_lock);
        }
        spin_unlock_irq(&sb->s_inode_wblist_lock);
        rcu_read_unlock();
        mutex_unlock(&sb->s_sync_lock);
}

static void __writeback_inodes_sb_nr(struct super_block *sb, unsigned long nr,
                                     enum wb_reason reason, bool skip_if_busy)
{
        struct backing_dev_info *bdi = sb->s_bdi;
        DEFINE_WB_COMPLETION(done, bdi);
        struct wb_writeback_work work = {
                .sb                     = sb,
                .sync_mode              = WB_SYNC_NONE,
                .tagged_writepages      = 1,
                .done                   = &done,
                .nr_pages               = nr,
                .reason                 = reason,
        };

        if (!bdi_has_dirty_io(bdi) || bdi == &noop_backing_dev_info)
                return;
        WARN_ON(!rwsem_is_locked(&sb->s_umount));

        bdi_split_work_to_wbs(sb->s_bdi, &work, skip_if_busy);
        wb_wait_for_completion(&done);
}

/**
 * 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)
{
        __writeback_inodes_sb_nr(sb, nr, reason, false);
}
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)
{
        writeback_inodes_sb_nr(sb, get_nr_dirty_pages(), reason);
}
EXPORT_SYMBOL(writeback_inodes_sb);

/**
 * try_to_writeback_inodes_sb - try to start writeback if none underway
 * @sb: the superblock
 * @reason: reason why some writeback work was initiated
 *
 * Invoke __writeback_inodes_sb_nr if no writeback is currently underway.
 */
void try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
{
        if (!down_read_trylock(&sb->s_umount))
                return;

        __writeback_inodes_sb_nr(sb, get_nr_dirty_pages(), reason, true);
        up_read(&sb->s_umount);
}
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)
{
        struct backing_dev_info *bdi = sb->s_bdi;
        DEFINE_WB_COMPLETION(done, bdi);
        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,
        };

        /*
         * Can't skip on !bdi_has_dirty() because we should wait for !dirty
         * inodes under writeback and I_DIRTY_TIME inodes ignored by
         * bdi_has_dirty() need to be written out too.
         */
        if (bdi == &noop_backing_dev_info)
                return;
        WARN_ON(!rwsem_is_locked(&sb->s_umount));

        /* protect against inode wb switch, see inode_switch_wbs_work_fn() */
        bdi_down_write_wb_switch_rwsem(bdi);
        bdi_split_work_to_wbs(bdi, &work, false);
        wb_wait_for_completion(&done);
        bdi_up_write_wb_switch_rwsem(bdi);

        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 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_can_writeback(inode->i_mapping))
                wbc.nr_to_write = 0;

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

/**
 * 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 writeback_single_inode(inode, &wbc);
}
EXPORT_SYMBOL(sync_inode_metadata);