linux-kernel - Re: [patch 2/2] slab: always consider arch mandated alignment

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Date:	Wed, 26 Jul 2006 20:45:22 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Christoph Lameter <clameter@....com>
CC:	Pekka J Enberg <penberg@...helsinki.fi>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [patch 2/2] slab: always consider arch mandated alignment

Christoph Lameter wrote:

>On Wed, 26 Jul 2006, Manfred Spraul wrote:
>
>  
>
>>Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
>>    
>>
>
>Good bye to all those cacheline contentions that helped us find so many 
>race conditions in the past if we switched on SLAB_DEBUG. I thought this 
>was intentional?
>
>  
>
Relax, align is nearly never set.

- kmalloc uses align==0, except if the architecture requests it 
(ARCH_KMALLOC_MINALIGN not 0)
- on my i386 system, the following users explicitely use align:
* the pmd structure (4096: hardware requirement)
* the pgd structure (32 bytes: hardware requirement)
* the task structure (16 byte. fxsave)
* sigqueue, pid: both request 4 byte alignment (based on __alignof__()). 
Doesn't affect debugging.

 From the other mail:

>Thus the patch is correct, it's a bug in the slab allocator. If HWCACHE_ALIGN
>> is set, then the allocator ignores align or ARCH_SLAB_MINALIGN.
>  
>
>
>But then Heiko does not want to set ARCH_SLAB_MINALIGN at all. This is not 
>the issue we are discussing. In the DEBUG case he wants 
>ARCH_KMALLOC_MINALIGN to be enforced even if ARCH_SLAB_MINALIGN is not 
>set.
>  
>
The kmalloc caches are allocated with 
HWCACHE_ALIGN+ARCH_KMALLOC_MINALIGN. The logic in kmem_cache_create 
didn't handle that case correctly.
On most architectures, ARCH_KMALLOC_MINALIGN is 0. Thus SLAB_DEBUG 
redzones everything.
On s390, ARCH_KMALLOC_MINALIGN is 8. This disables redzoning.

Ok?

--
    Manfred
-
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