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

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Date:	Wed, 26 Jul 2006 04:29:54 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	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

On Wed, 26 Jul 2006, Pekka Enberg wrote:

> Hi Heiko,
> 
> On 7/22/06, Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> > Sorry, I should have mentioned it: on s390 (32 bit) we set
> > #define ARCH_KMALLOC_MINALIGN 8.
> > This is needed since our common I/O layer allocates data structures that
> > need
> > to have an eight byte alignment. Now, if I turn on DEBUG_SLAB, nothing works
> > anymore, simply because the slab cache code ignores ARCH_KMALLOC_MINALIGN
> > and
> > uses an BYTES_PER_WORD alignment instead, which it shouldn't:
> 
> This is the bit I missed, sorry. I thought that the s390 hardware
> mandates 8 byte alignment, but it really doesn't. So you're absolutely
> right, you don't need to set ARCH_SLAB_MINALIGN and the alignment
> calculation in slab is indeed broken for both, architecture and caller
> mandated alignments.

Well that is a bit far reaching. What is broken is that SLAB_RED_ZONE and
SLAB_STORE_USER ignore any given alignment. If you want to fix that then 
you need to modify how both debugging methods work.


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