Re: [PATCH v2] btrfs: balance dirty metadata pages in btrfs_finish_ordered_io

[Chris Mason <clm@fb.com>]

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.

-chris