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From: Ethan Lien <>
Cc: Ethan Lien <>
Subject: [PATCH v2] btrfs: balance dirty metadata pages in btrfs_finish_ordered_io
Date: Mon, 28 May 2018 13:48:20 +0800
Message-ID: <> (raw)

[Problem description and how we fix it]
We should balance dirty metadata pages at the end of
btrfs_finish_ordered_io, since a small, unmergeable random write can
potentially produce dirty metadata which is multiple times larger than
the data itself. For example, a small, unmergeable 4KiB write may

    16KiB dirty leaf (and possibly 16KiB dirty node) in subvolume tree
    16KiB dirty leaf (and possibly 16KiB dirty node) in checksum tree
    16KiB dirty leaf (and possibly 16KiB dirty node) in extent tree

Although we do call balance dirty pages in write side, but in the
buffered write path, most metadata are dirtied only after we reach the
dirty background limit (which by far only counts dirty data pages) and
wakeup the flusher thread. If there are many small, unmergeable random
writes spread in a large btree, we'll find a burst of dirty pages
exceeds the dirty_bytes limit after we wakeup the flusher thread - which
is not what we expect. In our machine, it caused out-of-memory problem
since a page cannot be dropped if it is marked dirty.

Someone may worry about we may sleep in btrfs_btree_balance_dirty_nodelay,
but since we do btrfs_finish_ordered_io in a separate worker, it will not
stop the flusher consuming dirty pages. Also, we use different worker for
metadata writeback endio, sleep in btrfs_finish_ordered_io help us throttle
the size of dirty metadata pages.

[Reproduce steps]
To reproduce the problem, we need to do 4KiB write randomly spread in a
large btree. In our 2GiB RAM machine:
1) Create 4 subvolumes.
2) Run fio on each subvolume:


3) Take snapshot on each subvolume and repeat fio on existing files.
4) Repeat step (3) until we get large btrees.
   In our case, by observing btrfs_root_item->bytes_used, we have 2GiB of
   metadata in each subvolume tree and 12GiB of metadata in extent tree.
5) Stop all fio, take snapshot again, and wait until all delayed work is
6) Start all fio. Few seconds later we hit OOM when the flusher starts
   to work.

It can be reproduced even when using nocow write.

Signed-off-by: Ethan Lien <>

	Replace btrfs_btree_balance_dirty with btrfs_btree_balance_dirty_nodelay.
	Add reproduce steps.

 fs/btrfs/inode.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 8e604e7071f1..e54547df24ee 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3158,6 +3158,8 @@ static int btrfs_finish_ordered_io(struct btrfs_ordered_extent *ordered_extent)
 	/* once for the tree */
+	btrfs_btree_balance_dirty_nodelay(fs_info);
 	return ret;

             reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-28  5:48 Ethan Lien [this message]
2018-05-29 15:33 ` David Sterba
2018-12-12 14:47 ` Chris Mason
2018-12-12 15:22   ` Martin Raiber
2018-12-12 15:36     ` David Sterba
2018-12-12 17:55       ` Chris Mason
2018-12-14  8:07     ` ethanlien
2018-12-17 14:00       ` Martin Raiber
2018-12-19 10:33         ` ethanlien
2018-12-19 14:22           ` Chris Mason
2018-12-13  8:38   ` ethanlien
2019-01-04 15:59     ` David Sterba
2019-01-09 10:07       ` ethanlien

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