linux-kernel - Re: [GIT PULL] SLAB changes for v3.10

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Date:	Wed, 8 May 2013 15:26:09 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Tony Lindgren <tony@...mide.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [GIT PULL] SLAB changes for v3.10

On Wed, May 8, 2013 at 2:58 PM, Glauber Costa <glommer@...allels.com> wrote:
> My first guess is that it hit a NULL cache. Being a NULL pointer
> dereference, the thing among all that has the biggest chances of being
> NULL and accessed unconditionally is the cache pointer itself.
>
> Due to the size being too big. But if that were the case, he would have
> hit the WARN_ON recently introduced:
>
>                 if (WARN_ON_ONCE(size > KMALLOC_MAX_SIZE))
>                         return NULL;
>
>
> Is this WARN hit ?

I doubt it:

[    0.000000] r7 : 00000000  r6 : 600001d3  r5 : 00000000  r4 : 00008000
[    0.000000] r3 : 00000050  r2 : c06ec000  r1 : c06f77c8  r0 : c00eda9c

[    0.000000] [<c00edab4>] (kmem_cache_alloc_trace+0x50/0x178) from
[<c0086958>] (alloc_desc+0x24/0xb4)

It's the kzalloc_node() in kernel/irq/irqdesc.c::alloc_desc() and
AFAICT based on r4 it's a 32 KB allocation. It's more likely that
KMALLOC_SHIFT_HIGH is less than 25 and because kmalloc_index() doesn't
respect it, we return a pointer to an uninitialized kmalloc cache.

                          Pekka
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