linux-kernel - Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY

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Message-ID: <20150818195439.GB15739@mtj.duckdns.org>
Date:	Tue, 18 Aug 2015 12:54:39 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.com>,
	Eryu Guan <eguan@...hat.com>, xfs@....sgi.com, axboe@...com,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY_TIME
 inodes

Hello,

On Tue, Aug 18, 2015 at 10:47:18AM -0700, Tejun Heo wrote:
> Hmm... the only possibility I can think of is tot_write_bandwidth
> being zero when it shouldn't be.  I've been staring at the code for a
> while now but nothing rings a bell.  Time for another debug patch, I
> guess.

So, I can now reproduce the bug (it takes a lot of trials but lowering
the number of tested files helps quite a bit) and instrumented all the
early exit paths w/o the fix patch.  bdi_has_dirty_io() and
wb_has_dirty_io() are never out of sync with the actual dirty / io
lists even when the test 048 fails, so the bug at least is not caused
by writeback skipping due to buggy bdi/wb_has_dirty_io() result.
Whenever it skips, all the lists are actually empty (verified while
holding list_lock).

One suspicion I have is that this could be a subtle timing issue which
is being exposed by the new short-cut path.  Anything which adds delay
seems to make the issue go away.  Dave, does anything ring a bell?

As for the proposed I_DIRTY_TIME fix, I think it'd be a good idea to
merge it.  It fixes a clear brekage regardless of this xfs issue.

Thanks.

-- 
tejun
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