linux-kernel - Re: Temporary lockup on loopback block device

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Date:	Sun, 11 Nov 2007 08:50:51 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	mikulas@...ax.karlin.mff.cuni.cz
CC:	a.p.zijlstra@...llo.nl, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, wfg@...l.ustc.edu.cn,
	miklos@...redi.hu
Subject: Re: Temporary lockup on loopback block device

> > > 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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