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

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Message-ID: <20060726101340.GE9592@osiris.boeblingen.de.ibm.com>
Date:	Wed, 26 Jul 2006 12:13:40 +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>
Subject: Re: [patch 2/2] slab: always consider arch mandated alignment

On Wed, Jul 26, 2006 at 01:05:43PM +0300, Pekka J Enberg wrote:
> On Wed, 26 Jul 2006, Heiko Carstens wrote:
> > Since ARCH_KMALLOC_MINALIGN didn't work on s390 I tried ARCH_SLAB_MINALIGN
> > instead, just to find out that it didn't work too.
> > In case of CONFIG_DEBUG_SLAB kmem_cache_create() creates caches with an
> > alignment lesser than ARCH_SLAB_MINALIGN, which it shouldn't according to
> > this comment in mm/slab.c :
> 
> [snip]
> 
> > Index: linux-2.6/mm/slab.c
> > ===================================================================
> > --- linux-2.6.orig/mm/slab.c	2006-07-26 09:55:54.000000000 +0200
> > +++ linux-2.6/mm/slab.c	2006-07-26 09:57:07.000000000 +0200
> > @@ -2103,6 +2103,9 @@
> >  		if (ralign > BYTES_PER_WORD)
> >  			flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
> >  	}
> > +	if (BYTES_PER_WORD < ARCH_SLAB_MINALIGN)
> > +		flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
> > +
> >  	/* 3) caller mandated alignment: disables debug if necessary */
> >  	if (ralign < align) {
> >  		ralign = align;
> 
> This is similar to my patch and should be enough to fix the problem. The 
> first patch seems bogus and I don't really understand why you would need 
> it.

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?
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