linux-kernel - Re: [PATCH] slab: fix slab flags for archs use alignment larger 64-bit

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Date:	Fri, 13 Feb 2009 11:22:58 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Giuseppe CAVALLARO <peppe.cavallaro@...com>
Cc:	Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-mm@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH] slab: fix slab flags for archs use alignment larger 
	64-bit

Hi Giuseppe,

On Fri, Feb 13, 2009 at 12:00 PM, Giuseppe CAVALLARO
<peppe.cavallaro@...com> wrote:
> IIUC, and as you explained above, ralign is already set to the cache
> line size by the following code:
> ...
> /* 3) caller mandated alignment */
> if (ralign < align)
> ralign = align;
>
> Then, there is following check:
> ...
> /* disable debug if necessary */
> if (ralign > _alignof__(unsigned long long))
> flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
>
> In my point of view, just this check appears "incoherent" (please, note
> I'm not familiar with the slab internals).
> It always makes sense in case of x86 where ARCH_KMALLOC_MINALIGN is
> defined as: __alignof__(unsigned long long) as well.

The code is indeed quite cryptic but I don't think the above statement
is correct. Both SLAB_RED_ZONE and SLAB_STORE_USER add padding in
front of an object and the maximum size of that padding is 2 *
sizeof(unsigned long long). That's why we check for
__alignof__(unsigned long) there to make sure the object can begin
immediately after the padding added for red-zone and user pointer.

On Fri, Feb 13, 2009 at 12:00 PM, Giuseppe CAVALLARO
<peppe.cavallaro@...com> wrote:
> In case of sh, we always disable debug for 32 aligned objects. As side
> effect, within the leaks_show function we immediately exit for them.
> Indeed, after applying the patch, I attached, I was able to find size-X
> fields within the slab_allocators proc entry.

That sounds unfortunate. Can you post

  cat /proc/meminfo | grep Slab

results on sh without and with your patch? Bumping the limit up to
ARCH_KMALLOC_MINALIGN does make sense but we'd need to know what kind
of problems it might cause.

                                Pekka
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