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

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Date:	Thu, 12 Jun 2008 20:33:23 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Pekka J Enberg <penberg@...helsinki.fi>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Nadia Derbey <Nadia.Derbey@...l.net>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Lameter <clameter@....com>
Subject: Re: repeatable slab corruption with LTP msgctl08

Pekka J Enberg wrote:
> 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.
>
>   
Hmm. double kfree() should be cached by the redzone code.
And I disagree with your link interpretation:

000: 00 e0 12 f2 88 32 c0 f7 88 00 00 00 88 50 90 f2
010:
inuse: 14 00 00 00 (20 entries in use, 6 should be free)
free:  0f 00 00 00
nodeid: 00 00 00 00
bufctl[0x00] ff ff ff ff 020: fd ff ff ff fd ff ff ff fd ff ff ff
bufctl[0x4] fd ff ff ff  030: fd ff ff ff fd ff ff ff fd ff ff ff
bufctl[0x8] fd ff ff ff  040: fd ff ff ff fd ff ff ff 00 00 00 00
bufctl[0x0c] fd ff ff ff 050: fd ff ff ff fd ff ff ff 19 00 00 00
bufctl[0x10] 17 00 00 00 060: fd ff ff ff fd ff ff ff 0b 00 00 00
bufctl[0x14] fd ff ff ff 070: fd ff ff ff fd ff ff ff fd ff ff ff
bufctl[0x18] fd ff ff ff 080: 10 00 00 00

free: points to entry 0x0f.
bufctl[0x0f] is 0x19, i.e. it points to entry 0x19
0x19 points to 0x10
0x10 points to 0x17
0x17 is a BUFCTL_ACTIVE - that's a bug.
but: 0x13 is a valid link entry, is points to 0x0b
0x0b points to 0x00, which is BUFCTL_END.

IMHO the most probable bug is a single bit error:
bufctl[0x10] should be 0x13 instead of 0x17.

What about printing all redzone words? That would allow us to validate the bufctl chain.

Andrew: Could you post the new oops?

--
	Manfred

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