Chris Mason 於 2018-12-12 22:47 寫到:
On 28 May 2018, at 1:48, Ethan Lien wrote:
It took me a while to trigger, but this actually deadlocks ;) More
below.
[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
produce:
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.
In general, slowing down btrfs_finish_ordered_io isn't ideal because it
adds latency to places we need to finish quickly. Also,
btrfs_finish_ordered_io is used by the free space cache. Even though
this happens from its own workqueue, it means completing free space
cache writeback may end up waiting on balance_dirty_pages, something
like this stack trace:
12260 kworker/u96:16+btrfs-freespace-write D
[<0>] balance_dirty_pages+0x6e6/0x7ad
[<0>] balance_dirty_pages_ratelimited+0x6bb/0xa90
[<0>] btrfs_finish_ordered_io+0x3da/0x770
[<0>] normal_work_helper+0x1c5/0x5a0
[<0>] process_one_work+0x1ee/0x5a0
[<0>] worker_thread+0x46/0x3d0
[<0>] kthread+0xf5/0x130
[<0>] ret_from_fork+0x24/0x30
[<0>] 0xffffffffffffffff
Transaction commit will wait on the freespace cache:
838 btrfs-transacti D
[<0>] btrfs_start_ordered_extent+0x154/0x1e0
[<0>] btrfs_wait_ordered_range+0xbd/0x110
[<0>] __btrfs_wait_cache_io+0x49/0x1a0
[<0>] btrfs_write_dirty_block_groups+0x10b/0x3b0
[<0>] commit_cowonly_roots+0x215/0x2b0
[<0>] btrfs_commit_transaction+0x37e/0x910
[<0>] transaction_kthread+0x14d/0x180
[<0>] kthread+0xf5/0x130
[<0>] ret_from_fork+0x24/0x30
[<0>] 0xffffffffffffffff
And then writepages ends up waiting on transaction commit:
9520 kworker/u96:13+flush-btrfs-1 D
[<0>] wait_current_trans+0xac/0xe0
[<0>] start_transaction+0x21b/0x4b0
[<0>] cow_file_range_inline+0x10b/0x6b0
[<0>] cow_file_range.isra.69+0x329/0x4a0
[<0>] run_delalloc_range+0x105/0x3c0
[<0>] writepage_delalloc+0x119/0x180
[<0>] __extent_writepage+0x10c/0x390
[<0>] extent_write_cache_pages+0x26f/0x3d0
[<0>] extent_writepages+0x4f/0x80
[<0>] do_writepages+0x17/0x60
[<0>] __writeback_single_inode+0x59/0x690
[<0>] writeback_sb_inodes+0x291/0x4e0
[<0>] __writeback_inodes_wb+0x87/0xb0
[<0>] wb_writeback+0x3bb/0x500
[<0>] wb_workfn+0x40d/0x610
[<0>] process_one_work+0x1ee/0x5a0
[<0>] worker_thread+0x1e0/0x3d0
[<0>] kthread+0xf5/0x130
[<0>] ret_from_fork+0x24/0x30
[<0>] 0xffffffffffffffff
Eventually, we have every process in the system waiting on
balance_dirty_pages(), and nobody is able to make progress on page
writeback.
[Reproduce steps]
[ ... ]
V2:
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_put_ordered_extent(ordered_extent);
+ btrfs_btree_balance_dirty_nodelay(fs_info);
+
return ret;
}
The original OOM you describe feels like an MM bug to me, but I'm going
to try the repro steps here. Since the freespace cache has its own
workqueue, we could fix the deadlock just by wrapping the
balance_dirty_pages call in a check for the freespace inode. But, I
think we'll get better performance by nudging the fix outside of
btrfs_finish_ordered_io. I'll see if I can reproduce.
Before this patch, I tried adding a new workqueue for metadata
writeback,
and kick off async flush work on fs_info->btree_inode in
btrfs_finish_ordered_io(). It works, but it can't guarantee we control
dirty
pages under MM's dirty_bytes limit if btrfs_finish_ordered_io() still
running
at full speed.
I haven't been able to trigger the OOM this morning. Ethan, is this
something you can still hit on upstream kernels with the
balance_dirty_pages() removed?
I hit the OOM problem in 4.4 kernel. I'll try if I can trigger it in
uptodate kernel.
-chris
