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Re: [PATCH v3 1/2] xfs: replace global xfslogd wq with per-mount wq

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: [PATCH v3 1/2] xfs: replace global xfslogd wq with per-mount wq
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 28 Nov 2014 13:49:45 +1100
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1415906641-43587-2-git-send-email-bfoster@xxxxxxxxxx>
References: <1415906641-43587-1-git-send-email-bfoster@xxxxxxxxxx> <1415906641-43587-2-git-send-email-bfoster@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Nov 13, 2014 at 02:24:00PM -0500, Brian Foster wrote:
> The xfslogd workqueue is a global, single-job workqueue for buffer ioend
> processing. This means we allow for a single work item at a time for all
> possible XFS mounts on a system. fsstress testing in loopback XFS over
> XFS configurations has reproduced xfslogd deadlocks due to the single
> threaded nature of the queue and dependencies introduced between the
> separate XFS instances by online discard (-o discard).
> 
> Discard over a loopback device converts the discard request to a hole
> punch (fallocate) on the underlying file. Online discard requests are
> issued synchronously and from xfslogd context in XFS, hence the xfslogd
> workqueue is blocked in the upper fs waiting on a hole punch request to
> be servied in the lower fs. If the lower fs issues I/O that depends on
> xfslogd to complete, both filesystems end up hung indefinitely. This is
> reproduced reliabily by generic/013 on XFS->loop->XFS test devices with
> the '-o discard' mount option.
> 
> Further, docker implementations appear to use this kind of configuration
> for container instance filesystems by default (container fs->dm->
> loop->base fs) and therefore are subject to this deadlock when running
> on XFS.
> 
> Replace the global xfslogd workqueue with a per-mount variant. This
> guarantees each mount access to a single worker and prevents deadlocks
> due to inter-fs dependencies introduced by discard. Since the queue is
> only responsible for buffer iodone processing at this point in time,
> rename xfslogd to xfs-buf.
> 
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>

Looks good. I'll take this as is and we can refine the way we point
to the workqueue in the patches that separate the log buffer
completions...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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