linux-kernel - Re: repeatable slab corruption with LTP msgctl08

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Date:	Thu, 12 Jun 2008 13:35:20 +0300 (EEST)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Nadia Derbey <Nadia.Derbey@...l.net>,
	Manfred Spraul <manfred@...orfullife.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Lameter <clameter@....com>
Subject: Re: repeatable slab corruption with LTP msgctl08

Hi Andrew,

On Wed, 11 Jun 2008, Andrew Morton wrote:
> version is ltp-full-20070228 (lots of retro-computing there).
> 
> Config is at http://userweb.kernel.org/~akpm/config-vmm.txt
> 
> ./testcases/bin/msgctl08 crashes after ten minutes or so:
> 
> slab: Internal list corruption detected in cache 'size-128'(26), slabp f2905000(20). Hexdump:
> 
> 000: 00 e0 12 f2 88 32 c0 f7 88 00 00 00 88 50 90 f2
> 010: 14 00 00 00 0f 00 00 00 00 00 00 00 ff ff ff ff
> 020: fd ff ff ff fd ff ff ff fd ff ff ff fd ff ff ff
> 030: fd ff ff ff fd ff ff ff fd ff ff ff fd ff ff ff
> 040: fd ff ff ff fd ff ff ff 00 00 00 00 fd ff ff ff
> 050: fd ff ff ff fd ff ff ff 19 00 00 00 17 00 00 00
> 060: fd ff ff ff fd ff ff ff 0b 00 00 00 fd ff ff ff
> 070: fd ff ff ff fd ff ff ff fd ff ff ff fd ff ff ff
> 080: 10 00 00 00

Looking at the above dump, slabp->free is 0x0f and the bufctl it points to 
is 0xff ("BUFCTL_END") which marks the last element in the chain. This is 
wrong as the total number of objects in the slab (cachep->num) is 26 but 
the number of objects in use (slabp->inuse) is 20. So somehow you have 
managed to lost 6 objects from the bufctl chain.

I really don't understand how your bufctl chains has so many BUFCTL_END 
elements in the first place. It's doesn't look like the memory has been 
stomped on (slab->s_mem, for example, is 0xf2906088), so I'd look for a 
double kfree() of size 128 somewhere...

		Pekka
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