To: mikulas@artax.karlin.mff.cuni.cz
CC: a.p.zijlstra@chello.nl, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org, wfg@mail.ustc.edu.cn, miklos@szeredi.hu
In-reply-to: <Pine.LNX.4.64.0711110135450.18189@artax.karlin.mff.cuni.cz>
	(message from Mikulas Patocka on Sun, 11 Nov 2007 01:38:27 +0100
	(CET))
Subject: Re: Temporary lockup on loopback block device
References: <Pine.LNX.4.64.0711102025440.17691@artax.karlin.mff.cuni.cz> 
 <20071110145444.a6993df1.akpm@linux-foundation.org> <1194735752.5828.1.camel@lappy> <Pine.LNX.4.64.0711110135450.18189@artax.karlin.mff.cuni.cz>
Message-Id: <E1Ir7al-0004Tm-00@dorka.pomaz.szeredi.hu>
From: Miklos Szeredi <miklos@szeredi.hu>
Date: Sun, 11 Nov 2007 08:50:51 +0100
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1159
Lines: 29

> > > Arguably we just have the wrong backing-device here, and what we should do
> > > is to propagate the real backing device's pointer through up into the
> > > filesystem.  There's machinery for this which things like DM stacks use.
> > > 
> > > I wonder if the post-2.6.23 changes happened to make this problem go away.
> > 
> > The per BDI dirty stuff in 24 should make this work, I just checked and
> > loopback thingies seem to have their own BDI, so all should be well.
> 
> This is not only about loopback (I think the lockup can happen even 
> without loopback) --- the main problem is:
> 
> Why are there over-limit dirty pages that no one is writing?

Please do a sysrq-t, and cat /proc/vmstat during the hang.  Those
will show us what exactly is happening.

I've seen this type of hang many times, and I agree with Peter, that
it's probably about loopback, and is fixed in 2.6.24-rc.

Thanks,
Miklos


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