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From: Dave Chinner <>
To: Brian Foster <>
Cc: Martin Svec <>,
Subject: Re: Quota-enabled XFS hangs during mount
Date: Sun, 29 Jan 2017 09:42:42 +1100	[thread overview]
Message-ID: <20170128224242.GD316@dastard> (raw)
In-Reply-To: <20170127170734.GA49571@bfoster.bfoster>

On Fri, Jan 27, 2017 at 12:07:34PM -0500, Brian Foster wrote:
> The problem looks like a race between dquot reclaim and quotacheck. The
> high level sequence of events is as follows:
>  - During quotacheck, xfs_qm_dqiterate() walks the physical dquot
>    buffers and queues them to the delwri queue.
>  - Next, kswapd kicks in and attempts to reclaim a dquot that is backed
>    by a buffer on the quotacheck delwri queue. xfs_qm_dquot_isolate()
>    acquires the flush lock and attempts to queue to the reclaim delwri
>    queue. This silently fails because the buffer is already queued.
>    From this point forward, the dquot flush lock is not going to be
>    released until the buffer is submitted for I/O and completed via
>    quotacheck.
>  - Quotacheck continues on to the xfs_qm_flush_one() pass, hits the
>    dquot in question and waits on the flush lock to issue the flush of
>    the recalculated values. *deadlock*
> There are at least a few ways to deal with this. We could do something
> granular to fix up the reclaim path to check whether the buffer is
> already queued or something of that nature before we actually invoke the
> flush. I think this is effectively pointless, however, because the first
> part of quotacheck walks and queues all physical dquot buffers anyways.
> In other words, I think dquot reclaim during quotacheck should probably
> be bypassed.

> Note that I think this does mean that you could still have low memory
> issues if you happen to have a lot of quotas defined..

Hmmm..... Really needs fixing.

I think submitting the buffer list after xfs_qm_dqiterate() and
waiting for completion will avoid this problem.

However, I suspect reclaim can still race with flushing, so we need
to detect "stuck" dquots, submit the delwri buffer queue and wait,
then flush the dquot again.


Dave Chinner

  parent reply	other threads:[~2017-01-28 22:48 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-11-01 16:45 Martin Svec
2016-11-01 21:58 ` Dave Chinner
2016-11-02 16:31   ` Martin Svec
2016-11-03  1:31     ` Dave Chinner
2016-11-03 12:04       ` Martin Svec
2016-11-03 20:40         ` Dave Chinner
2017-01-23  9:44           ` Martin Svec
2017-01-23 13:44             ` Brian Foster
2017-01-23 22:06               ` Dave Chinner
2017-01-24 13:17               ` Martin Svec
2017-01-25 15:36                 ` Brian Foster
2017-01-25 22:17                 ` Brian Foster
2017-01-26 17:46                   ` Martin Svec
2017-01-26 19:12                     ` Brian Foster
2017-01-27 13:06                       ` Martin Svec
2017-01-27 17:07                         ` Brian Foster
2017-01-27 20:49                           ` Martin Svec
2017-01-27 21:00                             ` Martin Svec
2017-01-27 23:17                               ` Darrick J. Wong
2017-01-28 22:42                           ` Dave Chinner [this message]
2017-01-30 15:31                             ` Brian Foster

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