linux-ext4 - Re: [BUG] fatal hang untarring 90GB file, possibly writeback related.

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Date:	Thu, 28 Apr 2011 14:59:27 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Jan Kara <jack@...e.cz>, colin.king@...onical.com,
	Chris Mason <chris.mason@...cle.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>, mgorman@...ell.com
Subject: Re: [BUG] fatal hang untarring 90GB file, possibly writeback
 related.

On Thu, 2011-04-28 at 20:21 +0100, Mel Gorman wrote:
> On Thu, Apr 28, 2011 at 01:30:36PM -0500, James Bottomley wrote:
> > > Way hey, cgroups are also in the mix. How jolly.
> > > 
> > > Is systemd a common element of the machines hitting this bug by any
> > > chance?
> > 
> > Well, yes, the bug report is against FC15, which needs cgroups for
> > systemd.
> > 
> 
> Ok although we do not have direct evidence that it's the problem yet. A
> broken shrinker could just mean we are also trying to aggressively
> reclaim in cgroups.
> 
> > > The remaining traces seem to be follow-on damage related to the three
> > > issues of "shrinkers are bust in some manner" causing "we are not
> > > getting over the min watermark" and as a side-show "we are spending lots
> > > of time doing something unspecified but unhelpful in cgroups".
> > 
> > Heh, well find a way for me to verify this: I can't turn off cgroups
> > because systemd then won't work and the machine won't boot ...
> > 
> 
> Same testcase, same kernel but a distro that is not using systemd to
> verify if cgroups are the problem. Not ideal I know. When I'm back
> online Tuesday, I'll try reproducing this on a !Fedora distribution. In
> the meantime, the following untested hatchet job might spit out
> which shrinker we are getting stuck in. It is also breaking out of
> the shrink_slab loop so it'd even be interesting to see if the bug
> is mitigated in any way.

Actually, talking to Chris, I think I can get the system up using
init=/bin/bash without systemd, so I can try the no cgroup config.

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c74a501..ed99104 100644

In the mean time, this patch produces:

(that's nothing ... apparently the trace doesn't activate when kswapd
goes mad).

James


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