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Date: Wed, 26 Jul 2006 11:59:57 -0700 (PDT)
From: Christoph Lameter <clameter@sgi.com>
To: Manfred Spraul <manfred@colorfullife.com>
cc: Pekka J Enberg <penberg@cs.helsinki.fi>,
       Heiko Carstens <heiko.carstens@de.ibm.com>,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [patch 2/2] slab: always consider arch mandated alignment
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On Wed, 26 Jul 2006, Manfred Spraul wrote:

> > 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?

So Redzoning etc will now be diabled regardless even if  
ARCH_SLAB_MINALIGN is not set but another alignment is given to 
kmem_cache_alloc?

So we sacrifice the ability to worsen the performance of slabs by 
misaligning them for debugging purposes.