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 // 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);
 
     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);
 }
 
 static void finish_writeback_work(struct bdi_writeback *wb,
                   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(wb, 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 page *page)
 {
     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 (page) {
             memcg_css = mem_cgroup_css_from_page(page);
             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);
 
     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);
 }
 
 /**
  * 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;
     struct inode *inode;
     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);
     list_for_each_entry(inode, &wb->b_attached, i_io_list) {
         if (!inode_prepare_wbs_switch(inode, isw->new_wb))
             continue;
 
         isw->inodes[nr++] = inode;
 
         if (nr >= WB_MAX_INODES_PER_ISW - 1) {
             restart = true;
             break;
         }
     }
     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 (hweight32(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 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;
 
     css = mem_cgroup_css_from_page(page);
     /* 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;
         }
 
         /* 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);
 
         /*
          * Pin @wb so that it stays on @bdi->wb_list.  This allows
          * continuing iteration from @wb after dropping and
          * regrabbing rcu read lock.
          */
         wb_get(wb);
         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);
 
     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);
 
     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);
     inode->i_state &= ~I_SYNC_QUEUED;
 }
 
 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;
 }
 
 #define EXPIRE_DIRTY_ATIME 0x0001
 
 /*
  * 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)
 {
     if (inode->i_state & I_FREEING)
         return;
 
     /*
      * Sync livelock prevention. Each inode is tagged and synced in one
      * shot. If still dirty, it will be redirty_tail()'ed below.  Update
      * the dirty time to prevent enqueue and sync it again.
      */
     if ((inode->i_state & I_DIRTY) &&
         (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages))
         inode->dirtied_when = jiffies;
 
     if (wbc->pages_skipped) {
         /*
          * writeback is not making progress due to locked
          * buffers. Skip this inode for now.
          */
         redirty_tail_locked(inode, wb);
         return;
     }
 
     if (mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY)) {
         /*
          * We didn't write back all the pages.  nfs_writepages()
          * sometimes bales out without doing anything.
          */
         if (wbc->nr_to_write <= 0) {
             /* Slice used up. Queue for next turn. */
             requeue_io(inode, wb);
         } else {
             /*
              * Writeback blocked by something other than
              * congestion. Delay the inode for some time to
              * avoid spinning on the CPU (100% iowait)
              * retrying writeback of the dirty page/inode
              * that cannot be performed immediately.
              */
             redirty_tail_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_FSCACHE_WB)) {
         if (!(inode->i_state & I_DIRTY_PAGES)) {
             inode->i_state &= ~I_PINNING_FSCACHE_WB;
             wbc->unpinned_fscache_wb = true;
             dirty |= I_PINNING_FSCACHE_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_fscache_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 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) &&
          (inode->i_state & I_DIRTY))
         redirty_tail_locked(inode, wb);
 
     spin_unlock(&wb->list_lock);
     inode_sync_complete(inode);
 out:
     spin_unlock(&inode->i_lock);
     return ret;
 }
 
 static long writeback_chunk_size(struct bdi_writeback *wb,
                  struct wb_writeback_work *work)
 {
     long pages;
 
     /*
      * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
      * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
      * here avoids calling into writeback_inodes_wb() more than once.
      *
      * The intended call sequence for WB_SYNC_ALL writeback is:
      *
      *      wb_writeback()
      *          writeback_sb_inodes()       <== called only once
      *              write_cache_pages()     <== called once for each inode
      *                   (quickly) tag currently dirty pages
      *                   (maybe slowly) sync all tagged pages
      */
     if (work->sync_mode == WB_SYNC_ALL || work->tagged_writepages)
         pages = LONG_MAX;
     else {
         pages = min(wb->avg_write_bandwidth / 2,
                 global_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 */
 
     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);
         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 (!trylock_super(sb)) {
             /*
              * trylock_super() may fail consistently due to
              * s_umount being grabbed by someone else. Don't use
              * requeue_io() to avoid busy retrying the inode/sb.
              */
             redirty_tail(inode, wb);
             continue;
         }
         wrote += writeback_sb_inodes(sb, wb, work);
         up_read(&sb->s_umount);
 
         /* refer to the same tests at the end of writeback_sb_inodes */
         if (wrote) {
             if (time_is_before_jiffies(start_time + HZ / 10UL))
                 break;
             if (work->nr_pages <= 0)
                 break;
         }
     }
     /* Leave any unwritten inodes on b_io */
     return wrote;
 }
 
 static long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages,
                 enum wb_reason reason)
 {
     struct wb_writeback_work work = {
         .nr_pages   = nr_pages,
         .sync_mode  = WB_SYNC_NONE,
         .range_cyclic   = 1,
         .reason     = reason,
     };
     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;
 
     blk_start_plug(&plug);
     spin_lock(&wb->list_lock);
     for (;;) {
         /*
          * Stop writeback when nr_pages has been consumed
          */
         if (work->nr_pages <= 0)
             break;
 
         /*
          * Background writeout and kupdate-style writeback may
          * run forever. Stop them if there is other work to do
          * so that e.g. sync can proceed. They'll be restarted
          * after the other works are all done.
          */
         if ((work->for_background || work->for_kupdate) &&
             !list_empty(&wb->work_list))
             break;
 
         /*
          * For background writeout, stop when we are below the
          * background dirty threshold
          */
         if (work->for_background && !wb_over_bg_thresh(wb))
             break;
 
         /*
          * Kupdate and background works are special and we want to
          * include all inodes that need writing. Livelock avoidance is
          * handled by these works yielding to any other work so we are
          * safe.
          */
         if (work->for_kupdate) {
             dirtied_before = jiffies -
                 msecs_to_jiffies(dirty_expire_interval * 10);
         } else if (work->for_background)
             dirtied_before = jiffies;
 
         trace_writeback_start(wb, work);
         if (list_empty(&wb->b_io))
             queue_io(wb, work, dirtied_before);
         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)
             continue;
         /*
          * No more inodes for IO, bail
          */
         if (list_empty(&wb->b_more_io))
             break;
         /*
          * Nothing written. Wait for some inode to
          * become available for writeback. Otherwise
          * we'll just busyloop.
          */
         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);
         spin_lock(&wb->list_lock);
     }
     spin_unlock(&wb->list_lock);
     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(wb, 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 `nr_pages' pages on this bdi. If `nr_pages' is zero,
  * write back the whole world.
  */
 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) {
         /*
          * 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);
         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) ||
         (dirtytime && (inode->i_state & I_DIRTY_INODE)))
         return;
 
     spin_lock(&inode->i_lock);
     if (dirtytime && (inode->i_state & I_DIRTY_INODE))
         goto out_unlock_inode;
     if ((inode->i_state & flags) != flags) {
         const int was_dirty = inode->i_state & I_DIRTY;
 
         inode_attach_wb(inode, NULL);
 
         /* I_DIRTY_INODE supersedes I_DIRTY_TIME. */
         if (flags & I_DIRTY_INODE)
             inode->i_state &= ~I_DIRTY_TIME;
         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);
 out_unlock_inode:
     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)
 {
     return 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);

This page was automatically generated by the 2.1.0 LXR engine. The LXR team