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

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Date:	Wed, 26 Jul 2006 12:52:04 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Pekka J Enberg <penberg@...Helsinki.FI>
Cc:	Christoph Lameter <clameter@....com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Martin Schwidefsky <schwidefsky@...ibm.com>,
	manfred@...orfullife.com
Subject: Re: [patch 2/2] slab: always consider arch mandated alignment

On Wed, Jul 26, 2006 at 01:37:42PM +0300, Pekka J Enberg wrote:
> On Wed, 26 Jul 2006, Heiko Carstens wrote:
> > It's enough to fix the ARCH_SLAB_MINALIGN problem. But it does _not_ fix the
> > ARCH_KMALLOC_MINALIGN problem. s390 currently only uses ARCH_KMALLOC_MINALIGN
> > since that should be good enough and it doesn't disable as much debugging
> > as ARCH_SLAB_MINALIGN does.
> > What exactly isn't clear from the description of the first patch? Or why do
> > you consider it bogus?
> 
> Now I am confused. What do you mean by "doesn't disable as much debugging 
> as ARCH_SLAB_MINALIGN does"? AFAICT, the SLAB_RED_ZONE and SLAB_STORE_USER 
> options _require_ BYTES_PER_WORD alignment, so if s390 requires 8 
> byte alignment, you can't have them debugging anyhow...

We only specify ARCH_KMALLOC_MINALIGN, since that aligns only the kmalloc
caches, but it doesn't disable debugging on other caches that are created
via kmem_cache_create() where an alignment of e.g. 0 is specified.

The point of the first patch is: why should the slab cache be allowed to chose
an aligment that is less than what the caller specified? This does very likely
break things.
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