linux-kernel - Re: [RFC PATCH 2/6] slub: add kmalloc

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Date:	Tue, 20 Mar 2012 09:21:24 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
cc:	Pekka Enberg <penberg@...nel.org>, Matt Mackall <mpm@...enic.com>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 2/6] slub: add kmalloc_align()

On Tue, 20 Mar 2012, Christoph Lameter wrote:

> > diff --git a/mm/slub.c b/mm/slub.c
> > index 4907563..01cf99d 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3238,7 +3238,7 @@ static struct kmem_cache *__init create_kmalloc_cache(const char *name,
> >  	 * This function is called with IRQs disabled during early-boot on
> >  	 * single CPU so there's no need to take slub_lock here.
> >  	 */
> > -	if (!kmem_cache_open(s, name, size, ARCH_KMALLOC_MINALIGN,
> > +	if (!kmem_cache_open(s, name, size, ALIGN_OF_LAST_BIT(size),
> >  								flags, NULL))
> >  		goto panic;
>
> Why does the alignment of struct kmem_cache change? I'd rather have a
> __alignof__(struct kmem_cache) here with alignment specified with the
> struct definition.

Ok this aligns the data not the cache . Ok I see what is going on here.
So the kmalloc array now has a higher alignment. That means you can align
up to that limit within the structure.

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