Re: [PATCH v3] Btrfs: fix abnormal long waiting in fsync

[Chris Mason <clm@fb.com>]

On 07/17/2014 04:08 AM, Liu Bo wrote:
> xfstests generic/127 detected this problem.
> 
> With commit 7fc34a62ca4434a79c68e23e70ed26111b7a4cf8, now fsync will only flush
> data within the passed range.  This is the cause of the above problem,
> -- btrfs's fsync has a stage called 'sync log' which will wait for all the
> ordered extents it've recorded to finish.
> 
> In xfstests/generic/127, with mixed operations such as truncate, fallocate,
> punch hole, and mapwrite, we get some pre-allocated extents, and mapwrite will
> mmap, and then msync.  And I find that msync will wait for quite a long time
> (about 20s in my case), thanks to ftrace, it turns out that the previous
> fallocate calls 'btrfs_wait_ordered_range()' to flush dirty pages, but as the
> range of dirty pages may be larger than 'btrfs_wait_ordered_range()' wants,
> there can be some ordered extents created but not getting corresponding pages
> flushed, then they're left in memory until we fsync which runs into the
> stage 'sync log', and fsync will just wait for the system writeback thread
> to flush those pages and get ordered extents finished, so the latency is
> inevitable.
> 
> This adds a flush similar to btrfs_start_ordered_extent() in
> btrfs_wait_logged_extents() to fix that.

I was able to trigger the stalls with plain fsx as well.  Thanks!

-chris