// 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;

  	unsigned long *older_than_this;

  	enum writeback_sync_modes sync_mode;

  	unsigned int tagged_writepages:1;

  	unsigned int for_kupdate:1;
  	unsigned int range_cyclic:1;
  	unsigned int for_background:1;

  	unsigned int for_sync:1;	/* sync(2) WB_SYNC_ALL writeback */

  	unsigned int auto_free:1;	/* free on completion */

  	enum wb_reason reason;		/* why was writeback initiated? */

  	struct list_head list;		/* pending work list */

  	struct wb_completion *done;	/* set if the caller waits */

  };

  /*
   * If 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);

  	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;
  }
  
  /**

   * inode_io_list_del_locked - remove an inode from its bdi_writeback IO list

   * @inode: inode to be removed
   * @wb: bdi_writeback @inode is being removed from
   *
   * Remove @inode which may be on one of @wb->b_{dirty|io|more_io} lists and
   * clear %WB_has_dirty_io if all are empty afterwards.
   */

  static void inode_io_list_del_locked(struct inode *inode,

  				     struct bdi_writeback *wb)
  {
  	assert_spin_locked(&wb->list_lock);

  	list_del_init(&inode->i_io_list);

  	wb_io_lists_depopulated(wb);
  }

  static void wb_wakeup(struct bdi_writeback *wb)

  {

  	spin_lock_bh(&wb->work_lock);
  	if (test_bit(WB_registered, &wb->state))
  		mod_delayed_work(bdi_wq, &wb->dwork, 0);
  	spin_unlock_bh(&wb->work_lock);

  }

  static void 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_bh(&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_bh(&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 */

  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);

  /**

   * 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 inode		*inode;
  	struct bdi_writeback	*new_wb;
  
  	struct rcu_head		rcu_head;
  	struct work_struct	work;
  };

  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 void inode_switch_wbs_work_fn(struct work_struct *work)
  {
  	struct inode_switch_wbs_context *isw =
  		container_of(work, struct inode_switch_wbs_context, work);
  	struct inode *inode = isw->inode;

  	struct backing_dev_info *bdi = inode_to_bdi(inode);

  	struct address_space *mapping = inode->i_mapping;
  	struct bdi_writeback *old_wb = inode->i_wb;

  	struct bdi_writeback *new_wb = isw->new_wb;

  	XA_STATE(xas, &mapping->i_pages, 0);
  	struct page *page;

  	bool switched = false;

  
  	/*

  	 * 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);
  	}

  	spin_lock(&inode->i_lock);

  	xa_lock_irq(&mapping->i_pages);

  
  	/*
  	 * Once I_FREEING is 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))
  		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 pages while PAGECACHE_TAG_WRITEBACK points to

  	 * pages actually under writeback.

  	 */

  	xas_for_each_marked(&xas, page, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
  		if (PageDirty(page)) {

  			dec_wb_stat(old_wb, WB_RECLAIMABLE);
  			inc_wb_stat(new_wb, WB_RECLAIMABLE);

  		}
  	}

  	xas_set(&xas, 0);
  	xas_for_each_marked(&xas, page, ULONG_MAX, PAGECACHE_TAG_WRITEBACK) {
  		WARN_ON_ONCE(!PageWriteback(page));
  		dec_wb_stat(old_wb, WB_WRITEBACK);
  		inc_wb_stat(new_wb, WB_WRITEBACK);

  	}
  
  	wb_get(new_wb);
  
  	/*
  	 * Transfer to @new_wb's IO list if necessary.  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 (!list_empty(&inode->i_io_list)) {

  		struct inode *pos;

  		inode_io_list_del_locked(inode, old_wb);

  		inode->i_wb = new_wb;

  		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->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);

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

  	up_read(&bdi->wb_switch_rwsem);

  	if (switched) {
  		wb_wakeup(new_wb);
  		wb_put(old_wb);
  	}

  	wb_put(new_wb);

  
  	iput(inode);

  	kfree(isw);

  
  	atomic_dec(&isw_nr_in_flight);

  }
  
  static void inode_switch_wbs_rcu_fn(struct rcu_head *rcu_head)
  {
  	struct inode_switch_wbs_context *isw = container_of(rcu_head,
  				struct inode_switch_wbs_context, rcu_head);
  
  	/* needs to grab bh-unsafe locks, bounce to work item */
  	INIT_WORK(&isw->work, inode_switch_wbs_work_fn);

  	queue_work(isw_wq, &isw->work);

  }
  
  /**
   * 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(sizeof(*isw), GFP_ATOMIC);
  	if (!isw)

  		return;

  
  	/* find and pin the new wb */
  	rcu_read_lock();
  	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
  	if (memcg_css)
  		isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
  	rcu_read_unlock();
  	if (!isw->new_wb)
  		goto out_free;
  
  	/* 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) ||
  	    inode_to_wb(inode) == isw->new_wb) {
  		spin_unlock(&inode->i_lock);
  		goto out_free;
  	}

  	inode->i_state |= I_WB_SWITCH;

  	__iget(inode);

  	spin_unlock(&inode->i_lock);

  	isw->inode = 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.
  	 */
  	call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);

  
  	atomic_inc(&isw_nr_in_flight);

  	return;

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

  /**

   * 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 unnnecessary
   * 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);

  
  /**

   * inode_congested - test whether an inode is congested

   * @inode: inode to test for congestion (may be NULL)

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

   *
   * @inode is allowed to be NULL as this function is often called on
   * mapping->host which is NULL for the swapper space.

   */
  int inode_congested(struct inode *inode, int cong_bits)
  {

  	/*
  	 * Once set, ->i_wb never becomes NULL while the inode is alive.
  	 * Start transaction iff ->i_wb is visible.
  	 */

  	if (inode && inode_to_wb_is_valid(inode)) {

  		struct bdi_writeback *wb;

  		struct wb_lock_cookie lock_cookie = {};
  		bool congested;

  		wb = unlocked_inode_to_wb_begin(inode, &lock_cookie);

  		congested = wb_congested(wb, cong_bits);

  		unlocked_inode_to_wb_end(inode, &lock_cookie);

  		return congested;

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

  /**
   * wb_split_bdi_pages - split nr_pages to write according to bandwidth
   * @wb: target bdi_writeback to split @nr_pages to
   * @nr_pages: number of pages to write for the whole bdi
   *
   * Split @wb's portion of @nr_pages according to @wb's write bandwidth in
   * relation to the total write bandwidth of all wb's w/ dirty inodes on
   * @wb->bdi.
   */
  static long wb_split_bdi_pages(struct bdi_writeback *wb, long nr_pages)
  {
  	unsigned long this_bw = wb->avg_write_bandwidth;
  	unsigned long tot_bw = atomic_long_read(&wb->bdi->tot_write_bandwidth);
  
  	if (nr_pages == LONG_MAX)
  		return LONG_MAX;
  
  	/*
  	 * This may be called on clean wb's and proportional distribution
  	 * may not make sense, just use the original @nr_pages in those
  	 * cases.  In general, we wanna err on the side of writing more.
  	 */
  	if (!tot_bw || this_bw >= tot_bw)
  		return nr_pages;
  	else
  		return DIV_ROUND_UP_ULL((u64)nr_pages * this_bw, tot_bw);
  }

  /**

   * 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

   * @nr: number of pages to write, 0 for best-effort dirty flushing

   * @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, unsigned long nr,
  			   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;
  	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;
  	}
  
  	/*
  	 * If @nr is zero, 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.
  	 */
  	if (!nr) {
  		unsigned long filepages, headroom, dirty, writeback;
  
  		mem_cgroup_wb_stats(wb, &filepages, &headroom, &dirty,
  				      &writeback);
  		nr = dirty * 10 / 8;
  	}
  
  	/* issue the writeback work */
  	work = kzalloc(sizeof(*work), GFP_NOWAIT | __GFP_NOWARN);
  	if (work) {
  		work->nr_pages = nr;
  		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)
  {
  	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 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) +
  		global_node_page_state(NR_UNSTABLE_NFS) +
  		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);

  	inode_io_list_del_locked(inode, wb);

  	spin_unlock(&wb->list_lock);

  }

  /*

   * 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(struct inode *inode, struct bdi_writeback *wb)

  {

  	if (!list_empty(&wb->b_dirty)) {

  		struct inode *tail;

  		tail = wb_inode(wb->b_dirty.next);

  		if (time_before(inode->dirtied_when, tail->dirtied_when))

  			inode->dirtied_when = jiffies;
  	}

  	inode_io_list_move_locked(inode, wb, &wb->b_dirty);

  }
  
  /*

   * requeue inode for re-scanning after bdi->b_io list is exhausted.

   */

  static void requeue_io(struct inode *inode, struct bdi_writeback *wb)

  {

  	inode_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 work->older_than_this) dirty inodes from

   * @delaying_queue to @dispatch_queue.

   */

  static int move_expired_inodes(struct list_head *delaying_queue,

  			       struct list_head *dispatch_queue,

  			       int flags,

  			       struct wb_writeback_work *work)

  {

  	unsigned long *older_than_this = NULL;
  	unsigned long expire_time;

  	LIST_HEAD(tmp);
  	struct list_head *pos, *node;

  	struct super_block *sb = NULL;

  	struct inode *inode;

  	int do_sb_sort = 0;

  	int moved = 0;

  	if ((flags & EXPIRE_DIRTY_ATIME) == 0)
  		older_than_this = work->older_than_this;

  	else if (!work->for_sync) {
  		expire_time = jiffies - (dirtytime_expire_interval * HZ);

  		older_than_this = &expire_time;
  	}

  	while (!list_empty(delaying_queue)) {

  		inode = wb_inode(delaying_queue->prev);

  		if (older_than_this &&
  		    inode_dirtied_after(inode, *older_than_this))

  			break;

  		list_move(&inode->i_io_list, &tmp);

  		moved++;

  		if (flags & EXPIRE_DIRTY_ATIME)
  			set_bit(__I_DIRTY_TIME_EXPIRED, &inode->i_state);

  		if (sb_is_blkdev_sb(inode->i_sb))
  			continue;

  		if (sb && sb != inode->i_sb)
  			do_sb_sort = 1;
  		sb = inode->i_sb;

  	}

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

  		goto out;

  	}

  	/* Move inodes from one superblock together */
  	while (!list_empty(&tmp)) {

  		sb = wb_inode(tmp.prev)->i_sb;

  		list_for_each_prev_safe(pos, node, &tmp) {

  			inode = wb_inode(pos);

  			if (inode->i_sb == sb)

  				list_move(&inode->i_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)

  {

  	int moved;

  	assert_spin_locked(&wb->list_lock);

  	list_splice_init(&wb->b_more_io, &wb->b_io);

  	moved = move_expired_inodes(&wb->b_dirty, &wb->b_io, 0, work);
  	moved += move_expired_inodes(&wb->b_dirty_time, &wb->b_io,
  				     EXPIRE_DIRTY_ATIME, work);

  	if (moved)
  		wb_io_lists_populated(wb);

  	trace_writeback_queue_io(wb, work, moved);

  }

  static int write_inode(struct inode *inode, struct writeback_control *wbc)

  {

  	int ret;
  
  	if (inode->i_sb->s_op->write_inode && !is_bad_inode(inode)) {
  		trace_writeback_write_inode_start(inode, wbc);
  		ret = inode->i_sb->s_op->write_inode(inode, wbc);
  		trace_writeback_write_inode(inode, wbc);
  		return ret;
  	}

  	return 0;

  }

  /*

   * Wait for writeback on an inode to complete. Called with i_lock held.
   * Caller must make sure inode cannot go away when we drop i_lock.

   */

  static void __inode_wait_for_writeback(struct inode *inode)
  	__releases(inode->i_lock)
  	__acquires(inode->i_lock)

  {
  	DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
  	wait_queue_head_t *wqh;
  
  	wqh = bit_waitqueue(&inode->i_state, __I_SYNC);

  	while (inode->i_state & I_SYNC) {
  		spin_unlock(&inode->i_lock);

  		__wait_on_bit(wqh, &wq, bit_wait,
  			      TASK_UNINTERRUPTIBLE);

  		spin_lock(&inode->i_lock);

  	}

  }
  
  /*

   * Wait for writeback on an inode to complete. Caller must have inode pinned.
   */
  void inode_wait_for_writeback(struct inode *inode)
  {
  	spin_lock(&inode->i_lock);
  	__inode_wait_for_writeback(inode);
  	spin_unlock(&inode->i_lock);
  }
  
  /*
   * Sleep until I_SYNC is cleared. This function must be called with i_lock
   * held and drops it. It is aimed for callers not holding any inode reference
   * so once i_lock is dropped, inode can go away.
   */
  static void inode_sleep_on_writeback(struct inode *inode)
  	__releases(inode->i_lock)
  {
  	DEFINE_WAIT(wait);
  	wait_queue_head_t *wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
  	int sleep;
  
  	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
  	sleep = inode->i_state & I_SYNC;
  	spin_unlock(&inode->i_lock);
  	if (sleep)
  		schedule();
  	finish_wait(wqh, &wait);
  }
  
  /*

   * Find proper writeback list for the inode depending on its current state and
   * possibly also change of its state while we were doing writeback.  Here we
   * handle things such as livelock prevention or fairness of writeback among
   * inodes. This function can be called only by flusher thread - noone else
   * processes all inodes in writeback lists and requeueing inodes behind flusher
   * thread's back can have unexpected consequences.
   */
  static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
  			  struct writeback_control *wbc)
  {
  	if (inode->i_state & I_FREEING)
  		return;
  
  	/*
  	 * Sync livelock prevention. Each inode is tagged and synced in one
  	 * shot. If still dirty, it will be redirty_tail()'ed below.  Update
  	 * the dirty time to prevent enqueue and sync it again.
  	 */
  	if ((inode->i_state & I_DIRTY) &&
  	    (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages))
  		inode->dirtied_when = jiffies;

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

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

  	} else if (inode->i_state & I_DIRTY_TIME) {

  		inode->dirtied_when = jiffies;

  		inode_io_list_move_locked(inode, wb, &wb->b_dirty_time);

  	} else {
  		/* The inode is clean. Remove from writeback lists. */

  		inode_io_list_del_locked(inode, wb);

  	}
  }
  
  /*

   * Write out an inode and its dirty pages. Do not update the writeback list
   * linkage. That is left to the caller. The caller is also responsible for
   * setting I_SYNC flag and calling inode_sync_complete() to clear it.

   */
  static int

  __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)

  {

  	struct address_space *mapping = inode->i_mapping;

  	long nr_to_write = wbc->nr_to_write;

  	unsigned dirty;

  	int ret;

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

  	trace_writeback_single_inode_start(inode, wbc, nr_to_write);

  	ret = do_writepages(mapping, wbc);

  	/*
  	 * Make sure to wait on the data before writing out the metadata.
  	 * This is important for filesystems that modify metadata on data

  	 * I/O completion. We don't do it for sync(2) writeback because it has a
  	 * separate, external IO completion path and ->sync_fs for guaranteeing
  	 * inode metadata is written back correctly.

  	 */

  	if (wbc->sync_mode == WB_SYNC_ALL && !wbc->for_sync) {

  		int err = filemap_fdatawait(mapping);

  		if (ret == 0)
  			ret = err;
  	}

  	/*
  	 * Some filesystems may redirty the inode during the writeback
  	 * due to delalloc, clear dirty metadata flags right before
  	 * write_inode()
  	 */

  	spin_lock(&inode->i_lock);

  	dirty = inode->i_state & I_DIRTY;

  	if (inode->i_state & I_DIRTY_TIME) {

  		if ((dirty & I_DIRTY_INODE) ||

  		    wbc->sync_mode == WB_SYNC_ALL ||

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

  	inode->i_state &= ~dirty;

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

  	spin_unlock(&inode->i_lock);

  	if (dirty & I_DIRTY_TIME)
  		mark_inode_dirty_sync(inode);

  	/* Don't write the inode if only I_DIRTY_PAGES was set */

  	if (dirty & ~I_DIRTY_PAGES) {

  		int err = write_inode(inode, wbc);

  		if (ret == 0)
  			ret = err;
  	}

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

  static int writeback_single_inode(struct inode *inode,
  				  struct 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) {
  		if (wbc->sync_mode != WB_SYNC_ALL)
  			goto out;
  		/*

  		 * It's a data-integrity sync. We must wait. Since callers hold
  		 * inode reference or inode has I_WILL_FREE set, it cannot go
  		 * away under us.

  		 */

  		__inode_wait_for_writeback(inode);

  	}
  	WARN_ON(inode->i_state & I_SYNC);
  	/*

  	 * Skip inode if it is clean and we have no outstanding writeback in
  	 * WB_SYNC_ALL mode. We don't want to mess with writeback lists in this
  	 * function since flusher thread may be doing for example sync in
  	 * parallel and if we move the inode, it could get skipped. So here we
  	 * make sure inode is on some writeback list and leave it there unless
  	 * we have completely cleaned the inode.

  	 */

  	if (!(inode->i_state & I_DIRTY_ALL) &&

  	    (wbc->sync_mode != WB_SYNC_ALL ||
  	     !mapping_tagged(inode->i_mapping, PAGECACHE_TAG_WRITEBACK)))

  		goto out;
  	inode->i_state |= I_SYNC;

  	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 inode is clean, remove it from writeback lists. Otherwise don't
  	 * touch it. See comment above for explanation.
  	 */

  	if (!(inode->i_state & I_DIRTY_ALL))

  		inode_io_list_del_locked(inode, wb);

  	spin_unlock(&wb->list_lock);

  	inode_sync_complete(inode);

  out:
  	spin_unlock(&inode->i_lock);

  	return ret;
  }

  static long writeback_chunk_size(struct bdi_writeback *wb,

  				 struct wb_writeback_work *work)

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

  	else {

  		pages = min(wb->avg_write_bandwidth / 2,

  			    global_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 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;

  
  		if (inode->i_sb != sb) {

  			if (work->sb) {

  				/*
  				 * We only want to write back data for this
  				 * superblock, move all inodes not belonging
  				 * to it back onto the dirty list.
  				 */

  				redirty_tail(inode, wb);

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

  			break;

  		}

  		/*

  		 * Don't bother with new inodes or inodes being freed, first
  		 * kind does not need periodic writeout yet, and for the latter

  		 * kind writeout is handled by the freer.
  		 */

  		spin_lock(&inode->i_lock);

  		if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {

  			spin_unlock(&inode->i_lock);

  			redirty_tail(inode, wb);

  			continue;
  		}

  		if ((inode->i_state & I_SYNC) && wbc.sync_mode != WB_SYNC_ALL) {
  			/*
  			 * If this inode is locked for writeback and we are not
  			 * doing writeback-for-data-integrity, move it to
  			 * b_more_io so that writeback can proceed with the
  			 * other inodes on s_io.
  			 *
  			 * We'll have another go at writing back this inode
  			 * when we completed a full scan of b_io.
  			 */
  			spin_unlock(&inode->i_lock);
  			requeue_io(inode, wb);
  			trace_writeback_sb_inodes_requeue(inode);
  			continue;
  		}

  		spin_unlock(&wb->list_lock);

  		/*
  		 * We already requeued the inode if it had I_SYNC set and we
  		 * are doing WB_SYNC_NONE writeback. So this catches only the
  		 * WB_SYNC_ALL case.
  		 */

  		if (inode->i_state & I_SYNC) {
  			/* Wait for I_SYNC. This function drops i_lock... */
  			inode_sleep_on_writeback(inode);
  			/* Inode may be gone, start again */

  			spin_lock(&wb->list_lock);

  			continue;
  		}

  		inode->i_state |= I_SYNC;

  		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;

  
  		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);
  			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))

  			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 (wrote) {
  			if (time_is_before_jiffies(start_time + HZ / 10UL))
  				break;
  			if (work->nr_pages <= 0)
  				break;

  		}

  	}

  	return wrote;

  }

  static long __writeback_inodes_wb(struct bdi_writeback *wb,
  				  struct wb_writeback_work *work)

  {

  	unsigned long start_time = jiffies;
  	long wrote = 0;

  	while (!list_empty(&wb->b_io)) {

  		struct inode *inode = wb_inode(wb->b_io.prev);

  		struct super_block *sb = inode->i_sb;

  		if (!trylock_super(sb)) {

  			/*

  			 * trylock_super() may fail consistently due to

  			 * s_umount being grabbed by someone else. Don't use
  			 * requeue_io() to avoid busy retrying the inode/sb.
  			 */
  			redirty_tail(inode, wb);

  			continue;

  		}

  		wrote += writeback_sb_inodes(sb, wb, work);

  		up_read(&sb->s_umount);

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

  	}

  	/* Leave any unwritten inodes on b_io */

  	return wrote;

  }

  static long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages,

  				enum wb_reason reason)

  {

  	struct wb_writeback_work work = {
  		.nr_pages	= nr_pages,
  		.sync_mode	= WB_SYNC_NONE,
  		.range_cyclic	= 1,

  		.reason		= reason,

  	};

  	struct blk_plug plug;

  	blk_start_plug(&plug);

  	spin_lock(&wb->list_lock);

  	if (list_empty(&wb->b_io))

  		queue_io(wb, &work);

  	__writeback_inodes_wb(wb, &work);

  	spin_unlock(&wb->list_lock);

  	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.

   *

   * older_than_this takes precedence over nr_to_write.  So we'll only write back
   * all dirty pages if they are all attached to "old" mappings.

   */

  static long wb_writeback(struct bdi_writeback *wb,

  			 struct wb_writeback_work *work)

  {

  	unsigned long wb_start = jiffies;

  	long nr_pages = work->nr_pages;

  	unsigned long oldest_jif;

  	struct inode *inode;

  	long progress;

  	struct blk_plug plug;

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

  	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) {

  			oldest_jif = jiffies -

  				msecs_to_jiffies(dirty_expire_interval * 10);

  		} else if (work->for_background)

  			oldest_jif = jiffies;

  		trace_writeback_start(wb, work);

  		if (list_empty(&wb->b_io))

  			queue_io(wb, work);

  		if (work->sb)

  			progress = writeback_sb_inodes(work->sb, wb, work);

  		else

  			progress = __writeback_inodes_wb(wb, work);

  		trace_writeback_written(wb, work);

  		wb_update_bandwidth(wb, wb_start);

  
  		/*

  		 * Did we write something? Try for more
  		 *
  		 * Dirty inodes are moved to b_io for writeback in batches.
  		 * The completion of the current batch does not necessarily
  		 * mean the overall work is done. So we keep looping as long
  		 * as made some progress on cleaning pages or inodes.

  		 */

  		if (progress)

  			continue;
  		/*

  		 * No more inodes for IO, bail

  		 */

  		if (list_empty(&wb->b_more_io))

  			break;

  		/*

  		 * Nothing written. Wait for some inode to
  		 * become available for writeback. Otherwise
  		 * we'll just busyloop.
  		 */

  		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);