linux-kernel - Re: [patch] slab: always follow arch requested alignments

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Date:	Wed, 26 Jul 2006 20:24:18 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Christoph Lameter <clameter@....com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch] slab: always follow arch requested alignments

Pekka Enberg wrote:

> Hi Christoph,
>
> On 7/26/06, Christoph Lameter <clameter@....com> wrote:
>
>> We intentionally discard the caller mandated alignment for debugging
>> purposes.
>
>
There are two different types of alignment:
- SLAB_HWCACHE_ALIGN: it's a recommendation, it's regularly ignored.
- the align parameter, or ARCH_SLAB_MINALIGN: It's mandatory. For 
example the pgd structures must be 4 kB aligned, it's required by the 
hardware. And I think there was (is?) a structure where ptr & ~(size-1) 
was used to find the start of the structure.

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.

--
    Manfred
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