Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2791 insertions, 0 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
new file mode 100644
index 000000000..be2d32984
--- /dev/null
+++ b/fs/fs-writeback.c
@@ -0,0 +1,2791 @@
+// 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);
+}
+
+static bool isw_prepare_wbs_switch(struct inode_switch_wbs_context *isw,
+				   struct list_head *list, int *nr)
+{
+	struct inode *inode;
+
+	list_for_each_entry(inode, list, i_io_list) {
+		if (!inode_prepare_wbs_switch(inode, isw->new_wb))
+			continue;
+
+		isw->inodes[*nr] = inode;
+		(*nr)++;
+
+		if (*nr >= WB_MAX_INODES_PER_ISW - 1)
+			return true;
+	}
+	return false;
+}
+
+/**
+ * cleanup_offline_cgwb - detach associated inodes
+ * @wb: target wb
+ *
+ * Switch all inodes attached to @wb to a nearest living ancestor's wb in order
+ * to eventually release the dying @wb.  Returns %true if not all inodes were
+ * switched and the function has to be restarted.
+ */
+bool cleanup_offline_cgwb(struct bdi_writeback *wb)
+{
+	struct cgroup_subsys_state *memcg_css;
+	struct inode_switch_wbs_context *isw;
+	int nr;
+	bool restart = false;
+
+	isw = kzalloc(struct_size(isw, inodes, WB_MAX_INODES_PER_ISW),
+		      GFP_KERNEL);
+	if (!isw)
+		return restart;
+
+	atomic_inc(&isw_nr_in_flight);
+
+	for (memcg_css = wb->memcg_css->parent; memcg_css;
+	     memcg_css = memcg_css->parent) {
+		isw->new_wb = wb_get_create(wb->bdi, memcg_css, GFP_KERNEL);
+		if (isw->new_wb)
+			break;
+	}
+	if (unlikely(!isw->new_wb))
+		isw->new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
+
+	nr = 0;
+	spin_lock(&wb->list_lock);
+	/*
+	 * In addition to the inodes that have completed writeback, also switch
+	 * cgwbs for those inodes only with dirty timestamps. Otherwise, those
+	 * inodes won't be written back for a long time when lazytime is
+	 * enabled, and thus pinning the dying cgwbs. It won't break the
+	 * bandwidth restrictions, as writeback of inode metadata is not
+	 * accounted for.
+	 */
+	restart = isw_prepare_wbs_switch(isw, &wb->b_attached, &nr);
+	if (!restart)
+		restart = isw_prepare_wbs_switch(isw, &wb->b_dirty_time, &nr);
+	spin_unlock(&wb->list_lock);
+
+	/* no attached inodes? bail out */
+	if (nr == 0) {
+		atomic_dec(&isw_nr_in_flight);
+		wb_put(isw->new_wb);
+		kfree(isw);
+		return restart;
+	}
+
+	/*
+	 * In addition to synchronizing among switchers, I_WB_SWITCH tells
+	 * the RCU protected stat update paths to grab the i_page
+	 * lock so that stat transfer can synchronize against them.
+	 * Let's continue after I_WB_SWITCH is guaranteed to be visible.
+	 */
+	INIT_RCU_WORK(&isw->work, inode_switch_wbs_work_fn);
+	queue_rcu_work(isw_wq, &isw->work);
+
+	return restart;
+}
+
+/**
+ * wbc_attach_and_unlock_inode - associate wbc with target inode and unlock it
+ * @wbc: writeback_control of interest
+ * @inode: target inode
+ *
+ * @inode is locked and about to be written back under the control of @wbc.
+ * Record @inode's writeback context into @wbc and unlock the i_lock.  On
+ * writeback completion, wbc_detach_inode() should be called.  This is used
+ * to track the cgroup writeback context.
+ */
+void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
+				 struct inode *inode)
+{
+	if (!inode_cgwb_enabled(inode)) {
+		spin_unlock(&inode->i_lock);
+		return;
+	}
+
+	wbc->wb = inode_to_wb(inode);
+	wbc->inode = inode;
+
+	wbc->wb_id = wbc->wb->memcg_css->id;
+	wbc->wb_lcand_id = inode->i_wb_frn_winner;
+	wbc->wb_tcand_id = 0;
+	wbc->wb_bytes = 0;
+	wbc->wb_lcand_bytes = 0;
+	wbc->wb_tcand_bytes = 0;
+
+	wb_get(wbc->wb);
+	spin_unlock(&inode->i_lock);
+
+	/*
+	 * A dying wb indicates that either the blkcg associated with the
+	 * memcg changed or the associated memcg is dying.  In the first
+	 * case, a replacement wb should already be available and we should
+	 * refresh the wb immediately.  In the second case, trying to
+	 * refresh will keep failing.
+	 */
+	if (unlikely(wb_dying(wbc->wb) && !css_is_dying(wbc->wb->memcg_css)))
+		inode_switch_wbs(inode, wbc->wb_id);
+}
+EXPORT_SYMBOL_GPL(wbc_attach_and_unlock_inode);
+
+/**
+ * wbc_detach_inode - disassociate wbc from inode and perform foreign detection
+ * @wbc: writeback_control of the just finished writeback
+ *
+ * To be called after a writeback attempt of an inode finishes and undoes
+ * wbc_attach_and_unlock_inode().  Can be called under any context.
+ *
+ * As concurrent write sharing of an inode is expected to be very rare and
+ * memcg only tracks page ownership on first-use basis severely confining
+ * the usefulness of such sharing, cgroup writeback tracks ownership
+ * per-inode.  While the support for concurrent write sharing of an inode
+ * is deemed unnecessary, an inode being written to by different cgroups at
+ * different points in time is a lot more common, and, more importantly,
+ * charging only by first-use can too readily lead to grossly incorrect
+ * behaviors (single foreign page can lead to gigabytes of writeback to be
+ * incorrectly attributed).
+ *
+ * To resolve this issue, cgroup writeback detects the majority dirtier of
+ * an inode and transfers the ownership to it.  To avoid unnecessary
+ * oscillation, the detection mechanism keeps track of history and gives
+ * out the switch verdict only if the foreign usage pattern is stable over
+ * a certain amount of time and/or writeback attempts.
+ *
+ * On each writeback attempt, @wbc tries to detect the majority writer
+ * using Boyer-Moore majority vote algorithm.  In addition to the byte
+ * count from the majority voting, it also counts the bytes written for the
+ * current wb and the last round's winner wb (max of last round's current
+ * wb, the winner from two rounds ago, and the last round's majority
+ * candidate).  Keeping track of the historical winner helps the algorithm
+ * to semi-reliably detect the most active writer even when it's not the
+ * absolute majority.
+ *
+ * Once the winner of the round is determined, whether the winner is
+ * foreign or not and how much IO time the round consumed is recorded in
+ * inode->i_wb_frn_history.  If the amount of recorded foreign IO time is
+ * over a certain threshold, the switch verdict is given.
+ */
+void wbc_detach_inode(struct writeback_control *wbc)
+{
+	struct bdi_writeback *wb = wbc->wb;
+	struct inode *inode = wbc->inode;
+	unsigned long avg_time, max_bytes, max_time;
+	u16 history;
+	int max_id;
+
+	if (!wb)
+		return;
+
+	history = inode->i_wb_frn_history;
+	avg_time = inode->i_wb_frn_avg_time;
+
+	/* pick the winner of this round */
+	if (wbc->wb_bytes >= wbc->wb_lcand_bytes &&
+	    wbc->wb_bytes >= wbc->wb_tcand_bytes) {
+		max_id = wbc->wb_id;
+		max_bytes = wbc->wb_bytes;
+	} else if (wbc->wb_lcand_bytes >= wbc->wb_tcand_bytes) {
+		max_id = wbc->wb_lcand_id;
+		max_bytes = wbc->wb_lcand_bytes;
+	} else {
+		max_id = wbc->wb_tcand_id;
+		max_bytes = wbc->wb_tcand_bytes;
+	}
+
+	/*
+	 * Calculate the amount of IO time the winner consumed and fold it
+	 * into the running average kept per inode.  If the consumed IO
+	 * time is lower than avag / WB_FRN_TIME_CUT_DIV, ignore it for
+	 * deciding whether to switch or not.  This is to prevent one-off
+	 * small dirtiers from skewing the verdict.
+	 */
+	max_time = DIV_ROUND_UP((max_bytes >> PAGE_SHIFT) << WB_FRN_TIME_SHIFT,
+				wb->avg_write_bandwidth);
+	if (avg_time)
+		avg_time += (max_time >> WB_FRN_TIME_AVG_SHIFT) -
+			    (avg_time >> WB_FRN_TIME_AVG_SHIFT);
+	else
+		avg_time = max_time;	/* immediate catch up on first run */
+
+	if (max_time >= avg_time / WB_FRN_TIME_CUT_DIV) {
+		int slots;
+
+		/*
+		 * The switch verdict is reached if foreign wb's consume
+		 * more than a certain proportion of IO time in a
+		 * WB_FRN_TIME_PERIOD.  This is loosely tracked by 16 slot
+		 * history mask where each bit represents one sixteenth of
+		 * the period.  Determine the number of slots to shift into
+		 * history from @max_time.
+		 */
+		slots = min(DIV_ROUND_UP(max_time, WB_FRN_HIST_UNIT),
+			    (unsigned long)WB_FRN_HIST_MAX_SLOTS);
+		history <<= slots;
+		if (wbc->wb_id != max_id)
+			history |= (1U << slots) - 1;
+
+		if (history)
+			trace_inode_foreign_history(inode, wbc, history);
+
+		/*
+		 * Switch if the current wb isn't the consistent winner.
+		 * If there are multiple closely competing dirtiers, the
+		 * inode may switch across them repeatedly over time, which
+		 * is okay.  The main goal is avoiding keeping an inode on
+		 * the wrong wb for an extended period of time.
+		 */
+		if (hweight16(history) > WB_FRN_HIST_THR_SLOTS)
+			inode_switch_wbs(inode, max_id);
+	}
+
+	/*
+	 * Multiple instances of this function may race to update the
+	 * following fields but we don't mind occassional inaccuracies.
+	 */
+	inode->i_wb_frn_winner = max_id;
+	inode->i_wb_frn_avg_time = min(avg_time, (unsigned long)U16_MAX);
+	inode->i_wb_frn_history = history;
+
+	wb_put(wbc->wb);
+	wbc->wb = NULL;
+}
+EXPORT_SYMBOL_GPL(wbc_detach_inode);
+
+/**
+ * wbc_account_cgroup_owner - account writeback to update inode cgroup ownership
+ * @wbc: writeback_control of the writeback in progress
+ * @page: page being written out
+ * @bytes: number of bytes being written out
+ *
+ * @bytes from @page are about to written out during the writeback
+ * controlled by @wbc.  Keep the book for foreign inode detection.  See
+ * wbc_detach_inode().
+ */
+void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
+			      size_t bytes)
+{
+	struct 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;
+		}
+
+		/*
+		 * If wb_tryget fails, the wb has been shutdown, skip it.
+		 *
+		 * Pin @wb so that it stays on @bdi->wb_list.  This allows
+		 * continuing iteration from @wb after dropping and
+		 * regrabbing rcu read lock.
+		 */
+		if (!wb_tryget(wb))
+			continue;
+
+		/* alloc failed, execute synchronously using on-stack fallback */
+		work = &fallback_work;
+		*work = *base_work;
+		work->nr_pages = nr_pages;
+		work->auto_free = 0;
+		work->done = &fallback_work_done;
+
+		wb_queue_work(wb, work);
+		last_wb = wb;
+
+		rcu_read_unlock();
+		wb_wait_for_completion(&fallback_work_done);
+		goto restart;
+	}
+	rcu_read_unlock();
+
+	if (last_wb)
+		wb_put(last_wb);
+}
+
+/**
+ * cgroup_writeback_by_id - initiate cgroup writeback from bdi and memcg IDs
+ * @bdi_id: target bdi id
+ * @memcg_id: target memcg css id
+ * @reason: reason why some writeback work initiated
+ * @done: target wb_completion
+ *
+ * Initiate flush of the bdi_writeback identified by @bdi_id and @memcg_id
+ * with the specified parameters.
+ */
+int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
+			   enum wb_reason reason, struct wb_completion *done)
+{
+	struct backing_dev_info *bdi;
+	struct cgroup_subsys_state *memcg_css;
+	struct bdi_writeback *wb;
+	struct wb_writeback_work *work;
+	unsigned long dirty;
+	int ret;
+
+	/* lookup bdi and memcg */
+	bdi = bdi_get_by_id(bdi_id);
+	if (!bdi)
+		return -ENOENT;
+
+	rcu_read_lock();
+	memcg_css = css_from_id(memcg_id, &memory_cgrp_subsys);
+	if (memcg_css && !css_tryget(memcg_css))
+		memcg_css = NULL;
+	rcu_read_unlock();
+	if (!memcg_css) {
+		ret = -ENOENT;
+		goto out_bdi_put;
+	}
+
+	/*
+	 * And find the associated wb.  If the wb isn't there already
+	 * there's nothing to flush, don't create one.
+	 */
+	wb = wb_get_lookup(bdi, memcg_css);
+	if (!wb) {
+		ret = -ENOENT;
+		goto out_css_put;
+	}
+
+	/*
+	 * The caller is attempting to write out most of
+	 * the currently dirty pages.  Let's take the current dirty page
+	 * count and inflate it by 25% which should be large enough to
+	 * flush out most dirty pages while avoiding getting livelocked by
+	 * concurrent dirtiers.
+	 *
+	 * BTW the memcg stats are flushed periodically and this is best-effort
+	 * estimation, so some potential error is ok.
+	 */
+	dirty = memcg_page_state(mem_cgroup_from_css(memcg_css), NR_FILE_DIRTY);
+	dirty = dirty * 10 / 8;
+
+	/* issue the writeback work */
+	work = kzalloc(sizeof(*work), GFP_NOWAIT | __GFP_NOWARN);
+	if (work) {
+		work->nr_pages = dirty;
+		work->sync_mode = WB_SYNC_NONE;
+		work->range_cyclic = 1;
+		work->reason = reason;
+		work->done = done;
+		work->auto_free = 1;
+		wb_queue_work(wb, work);
+		ret = 0;
+	} else {
+		ret = -ENOMEM;
+	}
+
+	wb_put(wb);
+out_css_put:
+	css_put(memcg_css);
+out_bdi_put:
+	bdi_put(bdi);
+	return ret;
+}
+
+/**
+ * cgroup_writeback_umount - flush inode wb switches for umount
+ *
+ * This function is called when a super_block is about to be destroyed and
+ * flushes in-flight inode wb switches.  An inode wb switch goes through
+ * RCU and then workqueue, so the two need to be flushed in order to ensure
+ * that all previously scheduled switches are finished.  As wb switches are
+ * rare occurrences and synchronize_rcu() can take a while, perform
+ * flushing iff wb switches are in flight.
+ */
+void cgroup_writeback_umount(void)
+{
+	/*
+	 * SB_ACTIVE should be reliably cleared before checking
+	 * isw_nr_in_flight, see generic_shutdown_super().
+	 */
+	smp_mb();
+
+	if (atomic_read(&isw_nr_in_flight)) {
+		/*
+		 * Use rcu_barrier() to wait for all pending callbacks to
+		 * ensure that all in-flight wb switches are in the workqueue.
+		 */
+		rcu_barrier();
+		flush_workqueue(isw_wq);
+	}
+}
+
+static int __init cgroup_writeback_init(void)
+{
+	isw_wq = alloc_workqueue("inode_switch_wbs", 0, 0);
+	if (!isw_wq)
+		return -ENOMEM;
+	return 0;
+}
+fs_initcall(cgroup_writeback_init);
+
+#else	/* CONFIG_CGROUP_WRITEBACK */
+
+static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }
+static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi) { }
+
+static void inode_cgwb_move_to_attached(struct inode *inode,
+					struct bdi_writeback *wb)
+{
+	assert_spin_locked(&wb->list_lock);
+	assert_spin_locked(&inode->i_lock);
+
+	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. Although having skipped pages
+		 * is odd for clean inodes, it can happen for some
+		 * filesystems so handle that gracefully.
+		 */
+		if (inode->i_state & I_DIRTY_ALL)
+			redirty_tail_locked(inode, wb);
+		else
+			inode_cgwb_move_to_attached(inode, wb);
+		return;
+	}
+
+	if (mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY)) {
+		/*
+		 * We didn't write back all the pages.  nfs_writepages()
+		 * sometimes bales out without doing anything.
+		 */
+		if (wbc->nr_to_write <= 0) {
+			/* 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 freeing, its i_io_list shoudn't be updated
+	 * as it can be finally deleted at this moment.
+	 */
+	if (!(inode->i_state & I_FREEING)) {
+		/*
+		 * If the inode is now fully clean, then it can be safely
+		 * removed from its writeback list (if any). Otherwise the
+		 * flusher threads are responsible for the writeback lists.
+		 */
+		if (!(inode->i_state & I_DIRTY_ALL))
+			inode_cgwb_move_to_attached(inode, wb);
+		else if (!(inode->i_state & I_SYNC_QUEUED)) {
+			if ((inode->i_state & I_DIRTY))
+				redirty_tail_locked(inode, wb);
+			else if (inode->i_state & I_DIRTY_TIME) {
+				inode->dirtied_when = jiffies;
+				inode_io_list_move_locked(inode,
+							  wb,
+							  &wb->b_dirty_time);
+			}
+		}
+	}
+
+	spin_unlock(&wb->list_lock);
+	inode_sync_complete(inode);
+out:
+	spin_unlock(&inode->i_lock);
+	return ret;
+}
+
+static long writeback_chunk_size(struct bdi_writeback *wb,
+				 struct wb_writeback_work *work)
+{
+	long pages;
+
+	/*
+	 * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
+	 * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
+	 * here avoids calling into writeback_inodes_wb() more than once.
+	 *
+	 * The intended call sequence for WB_SYNC_ALL writeback is:
+	 *
+	 *      wb_writeback()
+	 *          writeback_sb_inodes()       <== called only once
+	 *              write_cache_pages()     <== called once for each inode
+	 *                   (quickly) tag currently dirty pages
+	 *                   (maybe slowly) sync all tagged pages
+	 */
+	if (work->sync_mode == WB_SYNC_ALL || work->tagged_writepages)
+		pages = LONG_MAX;
+	else {
+		pages = min(wb->avg_write_bandwidth / 2,
+			    global_wb_domain.dirty_limit / DIRTY_SCOPE);
+		pages = min(pages, work->nr_pages);
+		pages = round_down(pages + MIN_WRITEBACK_PAGES,
+				   MIN_WRITEBACK_PAGES);
+	}
+
+	return pages;
+}
+
+/*
+ * Write a portion of b_io inodes which belong to @sb.
+ *
+ * Return the number of pages and/or inodes written.
+ *
+ * NOTE! This is called with wb->list_lock held, and will
+ * unlock and relock that for each inode it ends up doing
+ * IO for.
+ */
+static long writeback_sb_inodes(struct super_block *sb,
+				struct bdi_writeback *wb,
+				struct wb_writeback_work *work)
+{
+	struct writeback_control wbc = {
+		.sync_mode		= work->sync_mode,
+		.tagged_writepages	= work->tagged_writepages,
+		.for_kupdate		= work->for_kupdate,
+		.for_background		= work->for_background,
+		.for_sync		= work->for_sync,
+		.range_cyclic		= work->range_cyclic,
+		.range_start		= 0,
+		.range_end		= LLONG_MAX,
+	};
+	unsigned long start_time = jiffies;
+	long write_chunk;
+	long total_wrote = 0;  /* count both pages and inodes */
+
+	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) {
+		/*
+		 * Inode timestamp update will piggback on this dirtying.
+		 * We tell ->dirty_inode callback that timestamps need to
+		 * be updated by setting I_DIRTY_TIME in flags.
+		 */
+		if (inode->i_state & I_DIRTY_TIME) {
+			spin_lock(&inode->i_lock);
+			if (inode->i_state & I_DIRTY_TIME) {
+				inode->i_state &= ~I_DIRTY_TIME;
+				flags |= I_DIRTY_TIME;
+			}
+			spin_unlock(&inode->i_lock);
+		}
+
+		/*
+		 * Notify the filesystem about the inode being dirtied, so that
+		 * (if needed) it can update on-disk fields and journal the
+		 * inode.  This is only needed when the inode itself is being
+		 * dirtied now.  I.e. it's only needed for I_DIRTY_INODE, not
+		 * for just I_DIRTY_PAGES or I_DIRTY_TIME.
+		 */
+		trace_writeback_dirty_inode_start(inode, flags);
+		if (sb->s_op->dirty_inode)
+			sb->s_op->dirty_inode(inode,
+				flags & (I_DIRTY_INODE | I_DIRTY_TIME));
+		trace_writeback_dirty_inode(inode, flags);
+
+		/* I_DIRTY_INODE supersedes I_DIRTY_TIME. */
+		flags &= ~I_DIRTY_TIME;
+	} else {
+		/*
+		 * Else it's either I_DIRTY_PAGES, I_DIRTY_TIME, or nothing.
+		 * (We don't support setting both I_DIRTY_PAGES and I_DIRTY_TIME
+		 * in one call to __mark_inode_dirty().)
+		 */
+		dirtytime = flags & I_DIRTY_TIME;
+		WARN_ON_ONCE(dirtytime && flags != I_DIRTY_TIME);
+	}
+
+	/*
+	 * Paired with smp_mb() in __writeback_single_inode() for the
+	 * following lockless i_state test.  See there for details.
+	 */
+	smp_mb();
+
+	if ((inode->i_state & flags) == flags)
+		return;
+
+	spin_lock(&inode->i_lock);
+	if ((inode->i_state & flags) != flags) {
+		const int was_dirty = inode->i_state & I_DIRTY;
+
+		inode_attach_wb(inode, NULL);
+
+		inode->i_state |= flags;
+
+		/*
+		 * Grab inode's wb early because it requires dropping i_lock and we
+		 * need to make sure following checks happen atomically with dirty
+		 * list handling so that we don't move inodes under flush worker's
+		 * hands.
+		 */
+		if (!was_dirty) {
+			wb = locked_inode_to_wb_and_lock_list(inode);
+			spin_lock(&inode->i_lock);
+		}
+
+		/*
+		 * If the inode is queued for writeback by flush worker, just
+		 * update its dirty state. Once the flush worker is done with
+		 * the inode it will place it on the appropriate superblock
+		 * list, based upon its state.
+		 */
+		if (inode->i_state & I_SYNC_QUEUED)
+			goto out_unlock;
+
+		/*
+		 * Only add valid (hashed) inodes to the superblock's
+		 * dirty list.  Add blockdev inodes as well.
+		 */
+		if (!S_ISBLK(inode->i_mode)) {
+			if (inode_unhashed(inode))
+				goto out_unlock;
+		}
+		if (inode->i_state & I_FREEING)
+			goto out_unlock;
+
+		/*
+		 * If the inode was already on b_dirty/b_io/b_more_io, don't
+		 * reposition it (that would break b_dirty time-ordering).
+		 */
+		if (!was_dirty) {
+			struct list_head *dirty_list;
+			bool wakeup_bdi = false;
+
+			inode->dirtied_when = jiffies;
+			if (dirtytime)
+				inode->dirtied_time_when = jiffies;
+
+			if (inode->i_state & I_DIRTY)
+				dirty_list = &wb->b_dirty;
+			else
+				dirty_list = &wb->b_dirty_time;
+
+			wakeup_bdi = inode_io_list_move_locked(inode, wb,
+							       dirty_list);
+
+			spin_unlock(&wb->list_lock);
+			spin_unlock(&inode->i_lock);
+			trace_writeback_dirty_inode_enqueue(inode);
+
+			/*
+			 * If this is the first dirty inode for this bdi,
+			 * we have to wake-up the corresponding bdi thread
+			 * to make sure background write-back happens
+			 * later.
+			 */
+			if (wakeup_bdi &&
+			    (wb->bdi->capabilities & BDI_CAP_WRITEBACK))
+				wb_wakeup_delayed(wb);
+			return;
+		}
+	}
+out_unlock:
+	if (wb)
+		spin_unlock(&wb->list_lock);
+	spin_unlock(&inode->i_lock);
+}
+EXPORT_SYMBOL(__mark_inode_dirty);
+
+/*
+ * The @s_sync_lock is used to serialise concurrent sync operations
+ * to avoid lock contention problems with concurrent wait_sb_inodes() calls.
+ * Concurrent callers will block on the s_sync_lock rather than doing contending
+ * walks. The queueing maintains sync(2) required behaviour as all the IO that
+ * has been issued up to the time this function is enter is guaranteed to be
+ * completed by the time we have gained the lock and waited for all IO that is
+ * in progress regardless of the order callers are granted the lock.
+ */
+static void wait_sb_inodes(struct super_block *sb)
+{
+	LIST_HEAD(sync_list);
+
+	/*
+	 * We need to be protected against the filesystem going from
+	 * r/o to r/w or vice versa.
+	 */
+	WARN_ON(!rwsem_is_locked(&sb->s_umount));
+
+	mutex_lock(&sb->s_sync_lock);
+
+	/*
+	 * Splice the writeback list onto a temporary list to avoid waiting on
+	 * inodes that have started writeback after this point.
+	 *
+	 * Use rcu_read_lock() to keep the inodes around until we have a
+	 * reference. s_inode_wblist_lock protects sb->s_inodes_wb as well as
+	 * the local list because inodes can be dropped from either by writeback
+	 * completion.
+	 */
+	rcu_read_lock();
+	spin_lock_irq(&sb->s_inode_wblist_lock);
+	list_splice_init(&sb->s_inodes_wb, &sync_list);
+
+	/*
+	 * Data integrity sync. Must wait for all pages under writeback, because
+	 * there may have been pages dirtied before our sync call, but which had
+	 * writeout started before we write it out.  In which case, the inode
+	 * may not be on the dirty list, but we still have to wait for that
+	 * writeout.
+	 */
+	while (!list_empty(&sync_list)) {
+		struct inode *inode = list_first_entry(&sync_list, struct inode,
+						       i_wb_list);
+		struct address_space *mapping = inode->i_mapping;
+
+		/*
+		 * Move each inode back to the wb list before we drop the lock
+		 * to preserve consistency between i_wb_list and the mapping
+		 * writeback tag. Writeback completion is responsible to remove
+		 * the inode from either list once the writeback tag is cleared.
+		 */
+		list_move_tail(&inode->i_wb_list, &sb->s_inodes_wb);
+
+		/*
+		 * The mapping can appear untagged while still on-list since we
+		 * do not have the mapping lock. Skip it here, wb completion
+		 * will remove it.
+		 */
+		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
+			continue;
+
+		spin_unlock_irq(&sb->s_inode_wblist_lock);
+
+		spin_lock(&inode->i_lock);
+		if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) {
+			spin_unlock(&inode->i_lock);
+
+			spin_lock_irq(&sb->s_inode_wblist_lock);
+			continue;
+		}
+		__iget(inode);
+		spin_unlock(&inode->i_lock);
+		rcu_read_unlock();
+
+		/*
+		 * We keep the error status of individual mapping so that
+		 * applications can catch the writeback error using fsync(2).
+		 * See filemap_fdatawait_keep_errors() for details.
+		 */
+		filemap_fdatawait_keep_errors(mapping);
+
+		cond_resched();
+
+		iput(inode);
+
+		rcu_read_lock();
+		spin_lock_irq(&sb->s_inode_wblist_lock);
+	}
+	spin_unlock_irq(&sb->s_inode_wblist_lock);
+	rcu_read_unlock();
+	mutex_unlock(&sb->s_sync_lock);
+}
+
+static void __writeback_inodes_sb_nr(struct super_block *sb, unsigned long nr,
+				     enum wb_reason reason, bool skip_if_busy)
+{
+	struct backing_dev_info *bdi = sb->s_bdi;
+	DEFINE_WB_COMPLETION(done, bdi);
+	struct wb_writeback_work work = {
+		.sb			= sb,
+		.sync_mode		= WB_SYNC_NONE,
+		.tagged_writepages	= 1,
+		.done			= &done,
+		.nr_pages		= nr,
+		.reason			= reason,
+	};
+
+	if (!bdi_has_dirty_io(bdi) || bdi == &noop_backing_dev_info)
+		return;
+	WARN_ON(!rwsem_is_locked(&sb->s_umount));
+
+	bdi_split_work_to_wbs(sb->s_bdi, &work, skip_if_busy);
+	wb_wait_for_completion(&done);
+}
+
+/**
+ * writeback_inodes_sb_nr -	writeback dirty inodes from given super_block
+ * @sb: the superblock
+ * @nr: the number of pages to write
+ * @reason: reason why some writeback work initiated
+ *
+ * Start writeback on some inodes on this super_block. No guarantees are made
+ * on how many (if any) will be written, and this function does not wait
+ * for IO completion of submitted IO.
+ */
+void writeback_inodes_sb_nr(struct super_block *sb,
+			    unsigned long nr,
+			    enum wb_reason reason)
+{
+	__writeback_inodes_sb_nr(sb, nr, reason, false);
+}
+EXPORT_SYMBOL(writeback_inodes_sb_nr);
+
+/**
+ * writeback_inodes_sb	-	writeback dirty inodes from given super_block
+ * @sb: the superblock
+ * @reason: reason why some writeback work was initiated
+ *
+ * Start writeback on some inodes on this super_block. No guarantees are made
+ * on how many (if any) will be written, and this function does not wait
+ * for IO completion of submitted IO.
+ */
+void writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
+{
+	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);