transfer vmscan pageout works to the flusher thread

RFC PATCH In review Wu Fengguang 5 participants 24 messages Sep 13, 2010 – Nov 5, 2010

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Memory management VFS Tracing MGLRU +2 more subsystems

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Wu Fengguang

author

2010-09-13 13:08 UTC #1 lore raw

PATCH 1/4 writeback: integrated background work

In reply to Wu Fengguang

Check background work whenever the flusher thread wakes up. The page reclaim code may lower the soft dirty limit immediately before sending some work to the flusher thread.

This is also the prerequisite of next patch.

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

fs/fs-writeback.c +18 -0

@@ -716,6 +716,23 @@ get_next_work_item(struct backing_dev_in
 	return work;
 }
 
+static long wb_check_background_flush(struct bdi_writeback *wb)
+{
+	if (over_bground_thresh()) {
+
+		struct wb_writeback_work work = {
+			.nr_pages	= LONG_MAX,
+			.sync_mode	= WB_SYNC_NONE,
+			.for_background	= 1,
+			.range_cyclic	= 1,
+		};
+
+		return wb_writeback(wb, &work);
+	}
+
+	return 0;
+}
+
 static long wb_check_old_data_flush(struct bdi_writeback *wb)
 {
 	unsigned long expired;

@@ -787,6 +804,7 @@ long wb_do_writeback(struct bdi_writebac
 	 * Check for periodic writeback, kupdated() style
 	 */
 	wrote += wb_check_old_data_flush(wb);
+	wrote += wb_check_background_flush(wb);
 	clear_bit(BDI_writeback_running, &wb->bdi->state);
 
 	return wrote;

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Wu Fengguang

author

2010-09-13 13:08 UTC #5 lore raw

PATCH 2/4 writeback: quit background/periodic work when other works are enqueued

In reply to Wu Fengguang

From: Jan Kara <[email protected]>

Background writeback and kupdate-style writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Fix the problem by interrupting background and kupdate writeback if there is some other work to do. We can return to them after completing all the queued work.

Signed-off-by Jan Kara <[email protected]>

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c |    8 ++++++++
 1 file changed, 8 insertions(+)

fs/fs-writeback.c +8 -0

@@ -643,6 +643,14 @@ static long wb_writeback(struct bdi_writ
 			break;
 
 		/*
+		 * Background writeout and kupdate-style writeback are
+		 * easily livelockable. Stop them if there is other work
+		 * to do so that e.g. sync can proceed.
+		 */
+		if ((work->for_background || work->for_kupdate) &&
+		    !list_empty(&wb->bdi->work_list))
+			break;
+		/*
 		 * For background writeout, stop when we are below the
 		 * background dirty threshold
 		 */

Wu Fengguang

author

2010-09-13 13:08 UTC #0 lore raw

cover transfer vmscan pageout works to the flusher thread

Hi,

Some rather early work, for integration with Mel's patches :)

Thanks,
Fengguang

Wu Fengguang

author

2010-09-13 13:08 UTC #20 lore raw

PATCH 3/4 writeback: introduce bdi_start_inode_writeback()

This is to transfer dirty pages encountered in page reclaim to the flusher threads for writeback.

The flusher will piggy back more dirty pages for IO
- it's more IO efficient
- it helps clean more pages, a good number of them may sit in the same LRU list that is being scanned.

To avoid memory allocations at page reclaim, a mempool is created.

Background/periodic works will quit automatically, so as to clean the pages under reclaim ASAP. However the sync work can still block us for long time.

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c           |  103 +++++++++++++++++++++++++++++++++-
 include/linux/backing-dev.h |    2 
 2 files changed, 102 insertions(+), 3 deletions(-)

fs/fs-writeback.c +100 -3

@@ -30,11 +30,20 @@
 #include "internal.h"
 
 /*
+ * When flushing an inode page (for page reclaim), try to piggy back more
+ * nearby pages for IO efficiency. These pages will have good opportunity
+ * to be in the same LRU list.
+ */
+#define WRITE_AROUND_PAGES	(1UL << (20 - PAGE_CACHE_SHIFT))
+
+/*
  * Passed into wb_writeback(), essentially a subset of writeback_control
  */
 struct wb_writeback_work {
 	long nr_pages;
 	struct super_block *sb;
+	struct inode *inode;
+	pgoff_t offset;
 	enum writeback_sync_modes sync_mode;
 	unsigned int for_kupdate:1;
 	unsigned int range_cyclic:1;

@@ -59,6 +68,27 @@ struct wb_writeback_work {
  */
 int nr_pdflush_threads;
 
+static mempool_t *wb_work_mempool;
+
+static void *wb_work_alloc(gfp_t gfp_mask, void *pool_data)
+{
+	/*
+	 * bdi_start_inode_writeback() may be called on page reclaim
+	 */
+	if (current->flags & PF_MEMALLOC)
+		return NULL;
+
+	return kmalloc(sizeof(struct wb_writeback_work), gfp_mask);
+}
+
+static __init int wb_work_init(void)
+{
+	wb_work_mempool = mempool_create(1024,
+					 wb_work_alloc, mempool_kfree, NULL);
+	return wb_work_mempool ? 0 : -ENOMEM;
+}
+fs_initcall(wb_work_init);
+
 /**
  * writeback_in_progress - determine whether there is writeback in progress
  * @bdi: the device's backing_dev_info structure.

@@ -101,7 +131,7 @@ __bdi_start_writeback(struct backing_dev
 	 * This is WB_SYNC_NONE writeback, so if allocation fails just
 	 * wakeup the thread for old dirty data writeback
 	 */
-	work = kzalloc(sizeof(*work), GFP_ATOMIC);
+	work = mempool_alloc(wb_work_mempool, GFP_NOWAIT);
 	if (!work) {
 		if (bdi->wb.task) {
 			trace_writeback_nowork(bdi);

@@ -110,6 +140,7 @@ __bdi_start_writeback(struct backing_dev
 		return;
 	}
 
+	memset(work, 0, sizeof(*work));
 	work->sync_mode	= WB_SYNC_NONE;
 	work->nr_pages	= nr_pages;
 	work->range_cyclic = range_cyclic;

@@ -148,6 +179,55 @@ void bdi_start_background_writeback(stru
 	__bdi_start_writeback(bdi, LONG_MAX, true, true);
 }
 
+int bdi_start_inode_writeback(struct backing_dev_info *bdi,
+			      struct inode *inode, pgoff_t offset)
+{
+	struct wb_writeback_work *work;
+
+	spin_lock_bh(&bdi->wb_lock);
+	list_for_each_entry_reverse(work, &bdi->work_list, list) {
+		unsigned long end;
+		if (work->inode != inode)
+			continue;
+		end = work->offset + work->nr_pages;
+		if (work->offset - offset < WRITE_AROUND_PAGES) {
+			work->nr_pages += work->offset - offset;
+			work->offset = offset;
+			inode = NULL;
+			break;
+		} else if (offset - end < WRITE_AROUND_PAGES) {
+			work->nr_pages += offset - end;
+			inode = NULL;
+			break;
+		} else if (offset > work->offset &&
+			   offset < end) {
+			inode = NULL;
+			break;
+		}
+	}
+	spin_unlock_bh(&bdi->wb_lock);
+
+	if (!inode)
+		return 0;
+
+	if (!igrab(inode))
+		return -ENOENT;
+
+	work = mempool_alloc(wb_work_mempool, GFP_NOWAIT);
+	if (!work)
+		return -ENOMEM;
+
+	memset(work, 0, sizeof(*work));
+	work->sync_mode		= WB_SYNC_NONE;
+	work->inode		= inode;
+	work->offset		= offset;
+	work->nr_pages		= 1;
+
+	bdi_queue_work(inode->i_sb->s_bdi, work);
+
+	return 0;
+}
+
 /*
  * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
  * furthest end of its superblock's dirty-inode list.

@@ -724,6 +804,20 @@ get_next_work_item(struct backing_dev_in
 	return work;
 }
 
+static long wb_flush_inode(struct bdi_writeback *wb,
+			   struct wb_writeback_work *work)
+{
+	pgoff_t start = round_down(work->offset, WRITE_AROUND_PAGES);
+	pgoff_t end = round_up(work->offset + work->nr_pages,
+			       WRITE_AROUND_PAGES);
+	int wrote;
+
+	wrote = __filemap_fdatawrite_range(work->inode->i_mapping,
+					   start, end, WB_SYNC_NONE);
+	iput(work->inode);
+	return wrote;
+}
+
 static long wb_check_background_flush(struct bdi_writeback *wb)
 {
 	if (over_bground_thresh()) {

@@ -796,7 +890,10 @@ long wb_do_writeback(struct bdi_writebac
 
 		trace_writeback_exec(bdi, work);
 
-		wrote += wb_writeback(wb, work);
+		if (work->inode)
+			wrote += wb_flush_inode(wb, work);
+		else
+			wrote += wb_writeback(wb, work);
 
 		/*
 		 * Notify the caller of completion if this is a synchronous

@@ -805,7 +902,7 @@ long wb_do_writeback(struct bdi_writebac
 		if (work->done)
 			complete(work->done);
 		else
-			kfree(work);
+			mempool_free(work, wb_work_mempool);
 	}
 
 	/*

include/linux/backing-dev.h +2 -0

@@ -106,6 +106,8 @@ void bdi_unregister(struct backing_dev_i
 int bdi_setup_and_register(struct backing_dev_info *, char *, unsigned int);
 void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages);
 void bdi_start_background_writeback(struct backing_dev_info *bdi);
+int bdi_start_inode_writeback(struct backing_dev_info *bdi,
+			      struct inode *inode, pgoff_t offset);
 int bdi_writeback_thread(void *data);
 int bdi_has_dirty_io(struct backing_dev_info *bdi);
 void bdi_arm_supers_timer(void);

Wu Fengguang

author

2010-09-13 13:08 UTC #23 lore raw

PATCH 4/4 vmscan: transfer async file writeback to the flusher

In reply to Wu Fengguang

This relays all ASYNC file writeback IOs to the flusher threads. The lesser SYNC pageout()s will work as before (as a last resort).

It's a minimal prototype implementation and barely runs without panic. It potentially requires lots of more work to go stable.

TODO: avoid OOM if the LRU list is small and/or the storage is slow, so that the flusher cannot clean enough pages before the LRU is full scanned. One simple way could be to do waits on dirty/writeback pages when priority < 3 for even order 0 allocations.

Signed-off-by Wu Fengguang <[email protected]>

---
 mm/page-writeback.c |    2 +-
 mm/vmscan.c         |   13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

mm/vmscan.c +13 -0

@@ -756,6 +756,19 @@ static unsigned long shrink_page_list(st
 				}
 			}
 
+			if (page_is_file_cache(page) && mapping &&
+			    sync_writeback == PAGEOUT_IO_ASYNC) {
+				if (!bdi_start_inode_writeback(
+					mapping->backing_dev_info,
+					mapping->host, page_index(page))) {
+					SetPageReclaim(page);
+					goto keep_locked;
+				} else if (!current_is_kswapd() &&
+					   printk_ratelimit()) {
+					printk(KERN_INFO "cannot pageout\n");
+				}
+			}
+
 			if (references == PAGEREF_RECLAIM_CLEAN)
 				goto keep_locked;
 			if (!may_enter_fs)

mm/page-writeback.c +1 -1

@@ -1232,6 +1232,7 @@ int set_page_dirty(struct page *page)
 {
 	struct address_space *mapping = page_mapping(page);
 
+	ClearPageReclaim(page);
 	if (likely(mapping)) {
 		int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
 #ifdef CONFIG_BLOCK

@@ -1289,7 +1290,6 @@ int clear_page_dirty_for_io(struct page 
 
 	BUG_ON(!PageLocked(page));
 
-	ClearPageReclaim(page);
 	if (mapping && mapping_cap_account_dirty(mapping)) {
 		/*
 		 * Yes, Virginia, this is indeed insane.

Minchan Kim

2010-09-13 22:46 UTC #2 lore raw

Re: [PATCH 1/4] writeback: integrated background work

In reply to Wu Fengguang

Hi Wu,

Quoted 3 lines · Wu Fengguang

Check background work whenever the flusher thread wakes up. The page reclaim code may lower the soft dirty limit immediately before sending some work to the flusher thread.

I looked over this series. First impression is the approach is good. :) But let me have a question.
I can't find things about soft dirty limit.
Maybe it's a thing based on your another patch series.
But at least, could you explain it in this series if it is really related to this series?

Quoted 2 lines

This is also the prerequisite of next patch.

I can't understand why is the prerequisite of next patch. Please specify it.

Quoted 2 lines

Signed-off-by Wu Fengguang <[email protected]>

Signature · 8 lines

Kind regards,
Minchan Kim

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Wu Fengguang

author

2010-09-14 00:53 UTC #3 lore raw

Re: [PATCH 1/4] writeback: integrated background work

Quoted 8 lines · Minchan Kim

Hi Wu,
Quoted 3 lines · Wu Fengguang
Check background work whenever the flusher thread wakes up. A The page reclaim code may lower the soft dirty limit immediately before sending some work to the flusher thread.
I looked over this series. First impression is the approach is good. :)

Thanks :)

Quoted 3 lines

But let me have a question.
I can't find things about soft dirty limit. Maybe it's a thing based on your another patch series.

Yes, it's in the series "[RFC] soft and dynamic dirty throttling limits", in particular the patch "[PATCH 15/17] mm: lower soft dirty limits on memory pressure". https://patchwork.kernel.org/patch/173232/

Quoted 2 lines

But at least, could you explain it in this series if it is really related to this series?

The above patch with URL has a chunk:

@@ -745,6 +745,16 @@ static unsigned long shrink_page_list(st

                }
 
                if (PageDirty(page)) {

+
+ if (file && scanning_global_lru(sc)) {
+ int dp = VM_DIRTY_PRESSURE >>
+ (DEF_PRIORITY + 1 - sc->priority);
+ if (vm_dirty_pressure > dp) {
+ vm_dirty_pressure = dp;
+ vm_dirty_pressure_node = numa_node_id();
+ }
+ }
+

which lowers the soft dirty limits. It could explicitly check and start background work, however doesn't do so because this patchset will take care of it, by adding this chunk immediately after the above code:

@@ -756,6 +756,19 @@ static unsigned long shrink_page_list(st

                                }
                        }

+ if (page_is_file_cache(page) && mapping &&
+ sync_writeback == PAGEOUT_IO_ASYNC) {
+ if (!bdi_start_inode_writeback(

+                                       mapping->backing_dev_info,
+                                       mapping->host, page_index(page))) {
+                                       SetPageReclaim(page);
+                                       goto keep_locked;

+ } else if (!current_is_kswapd() &&
+ printk_ratelimit()) {
+ printk(KERN_INFO "cannot pageout\n");
+ }
+ }
+

The bdi_start_inode_writeback() will wake up the flusher thread, which will then check for background writeback (behavior added by this patch).

Quoted 5 lines

Quoted 2 lines
This is also the prerequisite of next patch.
I can't understand why is the prerequisite of next patch. Please specify it.

Because the next patch breaks out of the background work (to serve other works first). The code in this patch will resume the background work when other works are done.

Thanks,
Fengguang

Jan Kara

maintainer

2010-09-14 12:41 UTC #6 lore raw

Re: [PATCH 2/4] writeback: quit background/periodic work when other works are enqueued

In reply to Wu Fengguang

Hi,

Quoted 8 lines · Wu Fengguang

From: Jan Kara <[email protected]>
Background writeback and kupdate-style writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Fix the problem by interrupting background and kupdate writeback if there is some other work to do. We can return to them after completing all the queued work.

I actually have a slightly updated version with a better changelog:

Background writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Generally, when a flusher thread has some work queued, someone submitted the work to achieve a goal more specific than what background writeback does. So it makes sense to give it a priority over a generic page cleaning.

Thus we interrupt background writeback if there is some other work to do. We return to the background writeback after completing all the queued work.

Could you please update it? Thanks.
Honza

PS: I've also attached the full patch if that's more convenient for you.

Quoted 25 lines

Signed-off-by Jan Kara <[email protected]>

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c |    8 ++++++++
 1 file changed, 8 insertions(+)

fs/fs-writeback.c +8 -0

@@ -643,6 +643,14 @@ static long wb_writeback(struct bdi_writ
 			break;
 
 		/*
+		 * Background writeout and kupdate-style writeback are
+		 * easily livelockable. Stop them if there is other work
+		 * to do so that e.g. sync can proceed.
+		 */
+		if ((work->for_background || work->for_kupdate) &&
+		    !list_empty(&wb->bdi->work_list))
+			break;
+		/*
 		 * For background writeout, stop when we are below the
 		 * background dirty threshold
 		 */

Signature · 2 lines

Jan Kara <[email protected]>
SUSE Labs, CR

Jan Kara

maintainer

2010-09-14 12:46 UTC #4 lore raw

Re: [PATCH 1/4] writeback: integrated background work

In reply to Wu Fengguang

Quoted 5 lines · Wu Fengguang

Check background work whenever the flusher thread wakes up. The page reclaim code may lower the soft dirty limit immediately before sending some work to the flusher thread.
This is also the prerequisite of next patch.

I have a patch doing something functionally rather similar but it also
cleans up the code which isn't necessary after this patch. So could you maybe consider using that one?
BTW: What has happened with your patch which started writing back old
inodes?

Honza

Quoted 42 lines

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

fs/fs-writeback.c +18 -0

@@ -716,6 +716,23 @@ get_next_work_item(struct backing_dev_in
 	return work;
 }
 
+static long wb_check_background_flush(struct bdi_writeback *wb)
+{
+	if (over_bground_thresh()) {
+
+		struct wb_writeback_work work = {
+			.nr_pages	= LONG_MAX,
+			.sync_mode	= WB_SYNC_NONE,
+			.for_background	= 1,
+			.range_cyclic	= 1,
+		};
+
+		return wb_writeback(wb, &work);
+	}
+
+	return 0;
+}
+
 static long wb_check_old_data_flush(struct bdi_writeback *wb)
 {
 	unsigned long expired;

@@ -787,6 +804,7 @@ long wb_do_writeback(struct bdi_writebac
 	 * Check for periodic writeback, kupdated() style
 	 */
 	wrote += wb_check_old_data_flush(wb);
+	wrote += wb_check_background_flush(wb);
 	clear_bit(BDI_writeback_running, &wb->bdi->state);
 
 	return wrote;

Signature · 2 lines

Jan Kara <[email protected]>
SUSE Labs, CR

Jan Kara

maintainer

2010-09-14 13:37 UTC #21 lore raw

Re: [PATCH 3/4] writeback: introduce bdi_start_inode_writeback()

In reply to Wu Fengguang

Quoted 20 lines · Wu Fengguang

This is to transfer dirty pages encountered in page reclaim to the flusher threads for writeback.
The flusher will piggy back more dirty pages for IO
- it's more IO efficient
- it helps clean more pages, a good number of them may sit in the same LRU list that is being scanned.
To avoid memory allocations at page reclaim, a mempool is created.
Background/periodic works will quit automatically, so as to clean the pages under reclaim ASAP. However the sync work can still block us for long time.
Signed-off-by Wu Fengguang <[email protected]>
---
 fs/fs-writeback.c           |  103 +++++++++++++++++++++++++++++++++-
 include/linux/backing-dev.h |    2 
 2 files changed, 102 insertions(+), 3 deletions(-)

...

fs/fs-writeback.c void bdi_start_background_writeback(stru · line 191

Quoted 10 lines

fs/fs-writeback.c +10 -0

+int bdi_start_inode_writeback(struct backing_dev_info *bdi,
+			      struct inode *inode, pgoff_t offset)
+{
+	struct wb_writeback_work *work;
+
+	spin_lock_bh(&bdi->wb_lock);
+	list_for_each_entry_reverse(work, &bdi->work_list, list) {
+		unsigned long end;
+		if (work->inode != inode)
+			continue;

Hmm, this looks rather inefficient. I can imagine the list of work items
can grow rather large on memory stressed machine and the linear scan does not play well with that (and contention on wb_lock would make it even worse). I'm not sure how to best handle your set of intervals... RB tree attached to an inode is an obvious choice but it seems too expensive (memory spent for every inode) for such a rare use. Maybe you could have a per-bdi mapping (hash table) from ino to it's tree of intervals for reclaim... But before going for this, probably measuring how many intervals are we going to have under memory pressure would be good.

fs/fs-writeback.c void bdi_start_background_writeback(stru · line 193

Quoted 2 lines

fs/fs-writeback.c +2 -0

+		end = work->offset + work->nr_pages;
+		if (work->offset - offset < WRITE_AROUND_PAGES) {

It's slightly unclear what's intended here when offset >
work->offset. Could you make that explicit?

Honza

Signature · 8 lines

Jan Kara <[email protected]>
SUSE Labs, CR

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Wu Fengguang

author

2010-11-01 13:15 UTC #8 lore raw

PATCH 2/2 writeback: stop background/kupdate works from livelocking other works

In reply to Wu Fengguang

From: Jan Kara <[email protected]>

Thus we interrupt background writeback if there is some other work to do. We return to the background writeback after completing all the queued work.

Signed-off-by Jan Kara <[email protected]>

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c |    9 +++++++++
 1 file changed, 9 insertions(+)

fs/fs-writeback.c +9 -0

@@ -664,6 +664,15 @@ static long wb_writeback(struct bdi_writ
 			break;
 
 		/*
+		 * Background writeout and kupdate-style writeback are
+		 * easily livelockable. Stop them if there is other work
+		 * to do so that e.g. sync can proceed.
+		 */
+		if ((work->for_background || work->for_kupdate) &&
+		    !list_empty(&wb->bdi->work_list))
+			break;
+
+		/*
 		 * For background writeout, stop when we are below the
 		 * background dirty threshold
 		 */

Wu Fengguang

author

2010-11-01 13:15 UTC #17 lore raw

Re: [PATCH 2/4] writeback: quit background/periodic work when other works are enqueued

In reply to Jan Kara

Hi Jan,

Sorry for the long delay!

Quoted 20 lines · Jan Kara

Hi,
Quoted 8 lines · Wu Fengguang
From: Jan Kara <[email protected]>
Background writeback and kupdate-style writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Fix the problem by interrupting background and kupdate writeback if there is some other work to do. We can return to them after completing all the queued work.
I actually have a slightly updated version with a better changelog:
Background writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Generally, when a flusher thread has some work queued, someone submitted the work to achieve a goal more specific than what background writeback does. So it makes sense to give it a priority over a generic page cleaning.

Quoted 7 lines

Thus we interrupt background writeback if there is some other work to do. We return to the background writeback after completing all the queued work.
Could you please update it? Thanks.
Honza
PS: I've also attached the full patch if that's more convenient for you.

You patches are more complete than mine, so let's use them. However I do prefer to have a standalone wb_check_background_flush() that is called _after_ wb_check_old_data_flush(). This helps make the writeout a bit more ordered and the separation itself looks a bit more clean to me.

Followed are the slightly updated patches. IMHO they are straightforward fixes that could be merged before other writeback changes.

Thanks,
Fengguang

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Wu Fengguang

author

2010-11-01 13:15 UTC #7 lore raw

PATCH 1/2 writeback: integrated background writeback work

In reply to Jan Kara

From: Jan Kara <[email protected]>

Check whether background writeback is needed after finishing each work.

When bdi flusher thread finishes doing some work check whether any kind of background writeback needs to be done (either because dirty_background_ratio is exceeded or because we need to start flushing old inodes). If so, just do background write back.

This way, bdi_start_background_writeback() just needs to wake up the flusher thread. It will do background writeback as soon as there is no other work.

This is a preparatory patch for the next patch which stops background writeback as soon as there is other work to do.

Signed-off-by Jan Kara <[email protected]>

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c |   61 +++++++++++++++++++++++++++++++++-----------
 1 file changed, 46 insertions(+), 15 deletions(-)

fs/fs-writeback.c +46 ~~-15~~

@@ -79,13 +79,9 @@ static inline struct backing_dev_info *i
 	return sb->s_bdi;
 }
 
-static void bdi_queue_work(struct backing_dev_info *bdi,
-		struct wb_writeback_work *work)
+/* Wakeup flusher thread or forker thread to fork it. Requires bdi->wb_lock. */
+static void _bdi_wakeup_flusher(struct backing_dev_info *bdi)
 {
-	trace_writeback_queue(bdi, work);
-
-	spin_lock_bh(&bdi->wb_lock);
-	list_add_tail(&work->list, &bdi->work_list);
 	if (bdi->wb.task) {
 		wake_up_process(bdi->wb.task);
 	} else {

@@ -93,15 +89,26 @@ static void bdi_queue_work(struct backin
 		 * The bdi thread isn't there, wake up the forker thread which
 		 * will create and run it.
 		 */
-		trace_writeback_nothread(bdi, work);
 		wake_up_process(default_backing_dev_info.wb.task);
 	}
+}
+
+static void bdi_queue_work(struct backing_dev_info *bdi,
+			   struct wb_writeback_work *work)
+{
+	trace_writeback_queue(bdi, work);
+
+	spin_lock_bh(&bdi->wb_lock);
+	list_add_tail(&work->list, &bdi->work_list);
+	if (!bdi->wb.task)
+		trace_writeback_nothread(bdi, work);
+	_bdi_wakeup_flusher(bdi);
 	spin_unlock_bh(&bdi->wb_lock);
 }
 
 static void
 __bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
-		bool range_cyclic, bool for_background)
+		      bool range_cyclic)
 {
 	struct wb_writeback_work *work;

@@ -121,7 +128,6 @@ __bdi_start_writeback(struct backing_dev
 	work->sync_mode	= WB_SYNC_NONE;
 	work->nr_pages	= nr_pages;
 	work->range_cyclic = range_cyclic;
-	work->for_background = for_background;
 
 	bdi_queue_work(bdi, work);
 }

@@ -139,7 +145,7 @@ __bdi_start_writeback(struct backing_dev
  */
 void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages)
 {
-	__bdi_start_writeback(bdi, nr_pages, true, false);
+	__bdi_start_writeback(bdi, nr_pages, true);
 }
 
 /**

@@ -147,13 +153,20 @@ void bdi_start_writeback(struct backing_
  * @bdi: the backing device to write from
  *
  * Description:
- *   This does WB_SYNC_NONE background writeback. The IO is only
- *   started when this function returns, we make no guarentees on
- *   completion. Caller need not hold sb s_umount semaphore.
+ *   This makes sure WB_SYNC_NONE background writeback happens. When
+ *   this function returns, it is only guaranteed that for given BDI
+ *   some IO is happening if we are over background dirty threshold.
+ *   Caller need not hold sb s_umount semaphore.
  */
 void bdi_start_background_writeback(struct backing_dev_info *bdi)
 {
-	__bdi_start_writeback(bdi, LONG_MAX, true, true);
+	/*
+	 * We just wake up the flusher thread. It will perform background
+	 * writeback as soon as there is no other work to do.
+	 */
+	spin_lock_bh(&bdi->wb_lock);
+	_bdi_wakeup_flusher(bdi);
+	spin_unlock_bh(&bdi->wb_lock);
 }
 
 /*

@@ -724,6 +737,23 @@ get_next_work_item(struct backing_dev_in
 	return work;
 }
 
+static long wb_check_background_flush(struct bdi_writeback *wb)
+{
+	if (over_bground_thresh()) {
+
+		struct wb_writeback_work work = {
+			.nr_pages	= LONG_MAX,
+			.sync_mode	= WB_SYNC_NONE,
+			.for_background	= 1,
+			.range_cyclic	= 1,
+		};
+
+		return wb_writeback(wb, &work);
+	}
+
+	return 0;
+}
+
 static long wb_check_old_data_flush(struct bdi_writeback *wb)
 {
 	unsigned long expired;

@@ -795,6 +825,7 @@ long wb_do_writeback(struct bdi_writebac
 	 * Check for periodic writeback, kupdated() style
 	 */
 	wrote += wb_check_old_data_flush(wb);
+	wrote += wb_check_background_flush(wb);
 	clear_bit(BDI_writeback_running, &wb->bdi->state);
 
 	return wrote;

@@ -881,7 +912,7 @@ void wakeup_flusher_threads(long nr_page
 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
 		if (!bdi_has_dirty_io(bdi))
 			continue;
-		__bdi_start_writeback(bdi, nr_pages, false, false);
+		__bdi_start_writeback(bdi, nr_pages, false);
 	}
 	rcu_read_unlock();
 }

Wu Fengguang

author

2010-11-01 13:15 UTC #22 lore raw

Re: [PATCH 3/4] writeback: introduce bdi_start_inode_writeback()

In reply to Jan Kara

Quoted 33 lines · Jan Kara

Quoted 20 lines · Wu Fengguang
This is to transfer dirty pages encountered in page reclaim to the flusher threads for writeback.
The flusher will piggy back more dirty pages for IO
- it's more IO efficient
- it helps clean more pages, a good number of them may sit in the same LRU list that is being scanned.
To avoid memory allocations at page reclaim, a mempool is created.
Background/periodic works will quit automatically, so as to clean the pages under reclaim ASAP. However the sync work can still block us for long time.
Signed-off-by Wu Fengguang <[email protected]>
---
 fs/fs-writeback.c           |  103 +++++++++++++++++++++++++++++++++-
 include/linux/backing-dev.h |    2 
 2 files changed, 102 insertions(+), 3 deletions(-)
...
Review
Quoted 10 lines
(quoted diff) +10 -0
+int bdi_start_inode_writeback(struct backing_dev_info *bdi,
+			      struct inode *inode, pgoff_t offset)
+{
+	struct wb_writeback_work *work;
+
+	spin_lock_bh(&bdi->wb_lock);
+	list_for_each_entry_reverse(work, &bdi->work_list, list) {
+		unsigned long end;
+		if (work->inode != inode)
+			continue;
Hmm, this looks rather inefficient. I can imagine the list of work items

Quoted 8 lines

can grow rather large on memory stressed machine and the linear scan does not play well with that (and contention on wb_lock would make it even worse). I'm not sure how to best handle your set of intervals... RB tree attached to an inode is an obvious choice but it seems too expensive (memory spent for every inode) for such a rare use. Maybe you could have a per-bdi mapping (hash table) from ino to it's tree of intervals for reclaim... But before going for this, probably measuring how many intervals are we going to have under memory pressure would be good.

Good point. For now I'll just limit the search to 100 recent works. Could be further improved later.

Quoted 4 lines

Review
Quoted 2 lines
(quoted diff) +2 -0
+		end = work->offset + work->nr_pages;
+		if (work->offset - offset < WRITE_AROUND_PAGES) {
It's slightly unclear what's intended here when offset >
work->offset. Could you make that explicit?

It implicitly takes advantage of the "unsigned" compare. When offset

Quoted 1 line

work->offset, "work->offset - offset" will normally be a

huge _positive_ value. I added a comment for it in the following updated version. Is this still way too hacky?

Thanks,
Fengguang
---
Subject: vmscan: transfer async file writeback to the flusher

This is to transfer dirty pages encountered in page reclaim to the flusher threads for writeback.

Only ASYNC pageout() is relayed to the flusher threads, the less frequent SYNC pageout()s will work as before as a last resort. This helps to avoid OOM when the LRU list is small and/or the storage is slow, and the flusher cannot clean enough pages before the LRU is full scanned.

The flusher will piggy back more dirty pages for IO
- it's more IO efficient
- it helps clean more pages, a good number of them may sit in the same LRU list that is being scanned.

To avoid memory allocations at page reclaim, a mempool is created.

Background/periodic works will quit automatically (as done in another patch), so as to clean the pages under reclaim ASAP. However for now the sync work can still block us for long time.

Jan Kara: limit the search scope.

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c           |  118 +++++++++++++++++++++++++++++++++-
 include/linux/backing-dev.h |    2 
 2 files changed, 117 insertions(+), 3 deletions(-)

mm/vmscan.c +10 -0

@@ -752,6 +754,16 @@ static unsigned long shrink_page_list(st
 				}
 			}
 
+			if (page_is_file_cache(page) && mapping &&
+			    sync_writeback == PAGEOUT_IO_ASYNC) {
+				if (!bdi_start_inode_writeback(
+					mapping->backing_dev_info,
+					mapping->host, page_index(page))) {
+					SetPageReclaim(page);
+					goto keep_locked;
+				}
+			}
+
 			if (references == PAGEREF_RECLAIM_CLEAN)
 				goto keep_locked;
 			if (!may_enter_fs)

fs/fs-writeback.c +115 -3

171 lines not on this page · raw message

include/linux/backing-dev.h +2 -0

@@ -107,6 +107,8 @@ void bdi_unregister(struct backing_dev_i
 int bdi_setup_and_register(struct backing_dev_info *, char *, unsigned int);
 void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages);
 void bdi_start_background_writeback(struct backing_dev_info *bdi);
+int bdi_start_inode_writeback(struct backing_dev_info *bdi,
+			      struct inode *inode, pgoff_t offset);
 int bdi_writeback_thread(void *data);
 int bdi_has_dirty_io(struct backing_dev_info *bdi);
 void bdi_arm_supers_timer(void);

Jan Kara

maintainer

2010-11-01 15:08 UTC #18 lore raw

Re: [PATCH 2/4] writeback: quit background/periodic work when other works are enqueued

In reply to Wu Fengguang

Quoted 16 lines · Wu Fengguang

Quoted 7 lines · Jan Kara
Background writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Generally, when a flusher thread has some work queued, someone submitted the work to achieve a goal more specific than what background writeback does. So it makes sense to give it a priority over a generic page cleaning.
Quoted 7 lines
Thus we interrupt background writeback if there is some other work to do. We return to the background writeback after completing all the queued work.
Could you please update it? Thanks.
Honza
PS: I've also attached the full patch if that's more convenient for you.

Quoted 8 lines

You patches are more complete than mine, so let's use them. However I do prefer to have a standalone wb_check_background_flush() that is called _after_ wb_check_old_data_flush(). This helps make the writeout a bit more ordered and the separation itself looks a bit more clean to me.
Followed are the slightly updated patches. IMHO they are straightforward fixes that could be merged before other writeback changes.

Yes, the updated patch looks OK to me. Thanks for picking the patches up.

Honza

Signature · 9 lines

Jan Kara <[email protected]>
SUSE Labs, CR

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Jan Kara

maintainer

2010-11-01 15:13 UTC #19 lore raw

Re: [PATCH 2/4] writeback: quit background/periodic work when other works are enqueued

In reply to Wu Fengguang

Quoted 8 lines · Wu Fengguang

You patches are more complete than mine, so let's use them. However I do prefer to have a standalone wb_check_background_flush() that is called _after_ wb_check_old_data_flush(). This helps make the writeout a bit more ordered and the separation itself looks a bit more clean to me.
Followed are the slightly updated patches. IMHO they are straightforward fixes that could be merged before other writeback changes.

One more question - who's going to merge it with Linus? Would you send
the patches to Jens' to merge them via the block tree (note that Jens has a new email address since he joined FusionIO)? Or maybe to Andrew?

Honza

Signature · 9 lines

Jan Kara <[email protected]>
SUSE Labs, CR

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To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to [email protected].  For more info on Linux MM,
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Christoph Hellwig

2010-11-01 15:21 UTC #12 lore raw

Re: [PATCH 1/2] writeback: integrated background writeback work

In reply to Wu Fengguang

Quoted 1 line

+static void _bdi_wakeup_flusher(struct backing_dev_info *bdi)

Remove the leading underscore, please.

fs/fs-writeback.c void bdi_start_writeback(struct backing_ · line 169

Quoted 10 lines

void bdi_start_background_writeback(struct backing_dev_info *bdi) {

fs/fs-writeback.c +7 -1

-	__bdi_start_writeback(bdi, LONG_MAX, true, true);
+	/*
+	 * We just wake up the flusher thread. It will perform background
+	 * writeback as soon as there is no other work to do.
+	 */
+	spin_lock_bh(&bdi->wb_lock);
+	_bdi_wakeup_flusher(bdi);
+	spin_unlock_bh(&bdi->wb_lock);

We probably want a trace point here, too.

Otherwise the patch looks good to me. Thanks for bringing it up again.

Christoph Hellwig

2010-11-01 15:22 UTC #9 lore raw

Re: [PATCH 2/2] writeback: stop background/kupdate works from livelocking other works

In reply to Wu Fengguang

Looks good.

Wu Fengguang

author

2010-11-01 20:37 UTC #13 lore raw

Re: [PATCH 1/2] writeback: integrated background writeback work

In reply to Christoph Hellwig

Quoted 3 lines · Christoph Hellwig

Quoted 1 line
+static void _bdi_wakeup_flusher(struct backing_dev_info *bdi)
Remove the leading underscore, please.

OK, makes sense. The updated patch will follow.

Quoted 13 lines

Review
Quoted 10 lines
void bdi_start_background_writeback(struct backing_dev_info *bdi) {
(quoted diff) +7 -1
-	__bdi_start_writeback(bdi, LONG_MAX, true, true);
+	/*
+	 * We just wake up the flusher thread. It will perform background
+	 * writeback as soon as there is no other work to do.
+	 */
+	spin_lock_bh(&bdi->wb_lock);
+	_bdi_wakeup_flusher(bdi);
+	spin_unlock_bh(&bdi->wb_lock);
We probably want a trace point here, too.

Quoted 1 line

Otherwise the patch looks good to me. Thanks for bringing it up again.

Thanks. It's trivial to add the trace point, here is the incremental patch.

Thanks,
Fengguang

---
writeback: trace wakeup event for background writeback

This tracks when balance_dirty_pages() tries to wakeup the flusher thread for background writeback (if it was not started already).

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c                |    1 +
 include/trace/events/writeback.h |    1 +
 2 files changed, 2 insertions(+)

include/trace/events/writeback.h +1 -0

@@ -81,6 +81,7 @@ DEFINE_EVENT(writeback_class, name, \
 	TP_ARGS(bdi))
 
 DEFINE_WRITEBACK_EVENT(writeback_nowork);
+DEFINE_WRITEBACK_EVENT(writeback_wake_background);
 DEFINE_WRITEBACK_EVENT(writeback_wake_thread);
 DEFINE_WRITEBACK_EVENT(writeback_wake_forker_thread);
 DEFINE_WRITEBACK_EVENT(writeback_bdi_register);

fs/fs-writeback.c +1 -0

@@ -164,6 +164,7 @@ void bdi_start_background_writeback(stru
 	 * 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(bdi);
 	spin_lock_bh(&bdi->wb_lock);
 	bdi_wakeup_flusher(bdi);
 	spin_unlock_bh(&bdi->wb_lock);

Wu Fengguang

author

2010-11-01 20:39 UTC #14 lore raw

PATCH v2 1/2 writeback: integrated background writeback work

In reply to Christoph Hellwig

From: Jan Kara <[email protected]>

Check whether background writeback is needed after finishing each work.

This way, bdi_start_background_writeback() just needs to wake up the flusher thread. It will do background writeback as soon as there is no other work.

This is a preparatory patch for the next patch which stops background writeback as soon as there is other work to do.

Signed-off-by Jan Kara <[email protected]>

Signed-off-by Wu Fengguang <[email protected]>

---
 fs/fs-writeback.c |   61 +++++++++++++++++++++++++++++++++-----------
 1 file changed, 46 insertions(+), 15 deletions(-)

fs/fs-writeback.c +46 ~~-15~~

@@ -79,13 +79,9 @@ static inline struct backing_dev_info *i
 	return sb->s_bdi;
 }
 
-static void bdi_queue_work(struct backing_dev_info *bdi,
-		struct wb_writeback_work *work)
+/* Wakeup flusher thread or forker thread to fork it. Requires bdi->wb_lock. */
+static void bdi_wakeup_flusher(struct backing_dev_info *bdi)
 {
-	trace_writeback_queue(bdi, work);
-
-	spin_lock_bh(&bdi->wb_lock);
-	list_add_tail(&work->list, &bdi->work_list);
 	if (bdi->wb.task) {
 		wake_up_process(bdi->wb.task);
 	} else {

@@ -93,15 +89,26 @@ static void bdi_queue_work(struct backin
 		 * The bdi thread isn't there, wake up the forker thread which
 		 * will create and run it.
 		 */
-		trace_writeback_nothread(bdi, work);
 		wake_up_process(default_backing_dev_info.wb.task);
 	}
+}
+
+static void bdi_queue_work(struct backing_dev_info *bdi,
+			   struct wb_writeback_work *work)
+{
+	trace_writeback_queue(bdi, work);
+
+	spin_lock_bh(&bdi->wb_lock);
+	list_add_tail(&work->list, &bdi->work_list);
+	if (!bdi->wb.task)
+		trace_writeback_nothread(bdi, work);
+	bdi_wakeup_flusher(bdi);
 	spin_unlock_bh(&bdi->wb_lock);
 }
 
 static void
 __bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
-		bool range_cyclic, bool for_background)
+		      bool range_cyclic)
 {
 	struct wb_writeback_work *work;

@@ -121,7 +128,6 @@ __bdi_start_writeback(struct backing_dev
 	work->sync_mode	= WB_SYNC_NONE;
 	work->nr_pages	= nr_pages;
 	work->range_cyclic = range_cyclic;
-	work->for_background = for_background;
 
 	bdi_queue_work(bdi, work);
 }

@@ -139,7 +145,7 @@ __bdi_start_writeback(struct backing_dev
  */
 void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages)
 {
-	__bdi_start_writeback(bdi, nr_pages, true, false);
+	__bdi_start_writeback(bdi, nr_pages, true);
 }
 
 /**

@@ -147,13 +153,20 @@ void bdi_start_writeback(struct backing_
  * @bdi: the backing device to write from
  *
  * Description:
- *   This does WB_SYNC_NONE background writeback. The IO is only
- *   started when this function returns, we make no guarentees on
- *   completion. Caller need not hold sb s_umount semaphore.
+ *   This makes sure WB_SYNC_NONE background writeback happens. When
+ *   this function returns, it is only guaranteed that for given BDI
+ *   some IO is happening if we are over background dirty threshold.
+ *   Caller need not hold sb s_umount semaphore.
  */
 void bdi_start_background_writeback(struct backing_dev_info *bdi)
 {
-	__bdi_start_writeback(bdi, LONG_MAX, true, true);
+	/*
+	 * We just wake up the flusher thread. It will perform background
+	 * writeback as soon as there is no other work to do.
+	 */
+	spin_lock_bh(&bdi->wb_lock);
+	bdi_wakeup_flusher(bdi);
+	spin_unlock_bh(&bdi->wb_lock);
 }
 
 /*

@@ -724,6 +737,23 @@ get_next_work_item(struct backing_dev_in
 	return work;
 }
 
+static long wb_check_background_flush(struct bdi_writeback *wb)
+{
+	if (over_bground_thresh()) {
+
+		struct wb_writeback_work work = {
+			.nr_pages	= LONG_MAX,
+			.sync_mode	= WB_SYNC_NONE,
+			.for_background	= 1,
+			.range_cyclic	= 1,
+		};
+
+		return wb_writeback(wb, &work);
+	}
+
+	return 0;
+}
+
 static long wb_check_old_data_flush(struct bdi_writeback *wb)
 {
 	unsigned long expired;

@@ -795,6 +825,7 @@ long wb_do_writeback(struct bdi_writebac
 	 * Check for periodic writeback, kupdated() style
 	 */
 	wrote += wb_check_old_data_flush(wb);
+	wrote += wb_check_background_flush(wb);
 	clear_bit(BDI_writeback_running, &wb->bdi->state);
 
 	return wrote;

@@ -881,7 +912,7 @@ void wakeup_flusher_threads(long nr_page
 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
 		if (!bdi_has_dirty_io(bdi))
 			continue;
-		__bdi_start_writeback(bdi, nr_pages, false, false);
+		__bdi_start_writeback(bdi, nr_pages, false);
 	}
 	rcu_read_unlock();
 }

Minchan Kim

Reviewed-by on v2 1/2

2010-11-02 01:55 UTC #15 lore raw

Re: [PATCH 1/2 v2] writeback: integrated background writeback work

Quoted 13 lines · Wu Fengguang

From: Jan Kara <[email protected]>
Check whether background writeback is needed after finishing each work.
When bdi flusher thread finishes doing some work check whether any kind of background writeback needs to be done (either because dirty_background_ratio is exceeded or because we need to start flushing old inodes). If so, just do background write back.
This way, bdi_start_background_writeback() just needs to wake up the flusher thread. It will do background writeback as soon as there is no other work.

Quoted 5 lines

This is a preparatory patch for the next patch which stops background writeback as soon as there is other work to do.
Signed-off-by Jan Kara <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>

Reviewed-by Minchan Kim <[email protected]>

Signature · 2 lines

Kind regards,
Minchan Kim

Minchan Kim

Reviewed-by on v1 2/2

2010-11-02 01:57 UTC #10 lore raw

Re: [PATCH 2/2] writeback: stop background/kupdate works from livelocking other works

In reply to Wu Fengguang

Quoted 9 lines · Wu Fengguang

From: Jan Kara <[email protected]>
Background writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Generally, when a flusher thread has some work queued, someone submitted the work to achieve a goal more specific than what background writeback does. So it makes sense to give it a priority over a generic page cleaning.

Quoted 5 lines

Thus we interrupt background writeback if there is some other work to do. We return to the background writeback after completing all the queued work.
Signed-off-by Jan Kara <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>

Reviewed-by Minchan Kim <[email protected]>

Signature · 2 lines

Kind regards,
Minchan Kim

Johannes Weiner

maintainer Reviewed-by on v2 1/2

2010-11-05 12:01 UTC #16 lore raw

Re: [PATCH 1/2 v2] writeback: integrated background writeback work

In reply to Wu Fengguang

Quoted 13 lines · Wu Fengguang

From: Jan Kara <[email protected]>
Check whether background writeback is needed after finishing each work.
When bdi flusher thread finishes doing some work check whether any kind of background writeback needs to be done (either because dirty_background_ratio is exceeded or because we need to start flushing old inodes). If so, just do background write back.
This way, bdi_start_background_writeback() just needs to wake up the flusher thread. It will do background writeback as soon as there is no other work.

Quoted 5 lines

This is a preparatory patch for the next patch which stops background writeback as soon as there is other work to do.
Signed-off-by Jan Kara <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>

Reviewed-by Johannes Weiner <[email protected]>

Johannes Weiner

maintainer Reviewed-by on v1 2/2

2010-11-05 12:15 UTC #11 lore raw

Re: [PATCH 2/2] writeback: stop background/kupdate works from livelocking other works

In reply to Wu Fengguang

Quoted 9 lines · Wu Fengguang

From: Jan Kara <[email protected]>
Background writeback are easily livelockable (from a definition of their target). This is inconvenient because it can make sync(1) stall forever waiting on its queued work to be finished. Generally, when a flusher thread has some work queued, someone submitted the work to achieve a goal more specific than what background writeback does. So it makes sense to give it a priority over a generic page cleaning.

Quoted 5 lines

Thus we interrupt background writeback if there is some other work to do. We return to the background writeback after completing all the queued work.
Signed-off-by Jan Kara <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>

Reviewed-by Johannes Weiner <[email protected]>