linux-kernel - Re: [linux-next-20130422] Bug in SLAB?

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Message-ID: <0000013e564e0e5a-121c52f9-e489-470f-99d5-67a5ad42eb75-000000@email.amazonses.com>
Date:	Mon, 29 Apr 2013 14:59:16 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Glauber Costa <glommer@...allels.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [linux-next-20130422] Bug in SLAB?

On Mon, 29 Apr 2013, Glauber Costa wrote:

> >> causes no warning at compile time and returns NULL at runtime. But
> >>
> >>   unsigned int size = 8 * 1024 * 1024;
> >>   kmalloc(size, GFP_KERNEL);
> >>
> >> causes compile time warning
> >>
> >>   include/linux/slab_def.h:136: warning: array subscript is above array bounds
> >>
> >> and runtime bug.

SLAB should have support up to 2 << 25 = 1 mb << 5 = 32M

> I believe this is because the code now always assume that the cache is
> found when a constant is passed. Before this patch, we had a "found"
> statement that was mistakenly removed.

The code in kmalloc_index() creates a BUG() and preferentially should
create a compile time failure when a number that is too big is passed to it.

What is MAX_ORDER on the architecture?

An allocation size of more than MAX_ORDER is not supported by the page
allocator or by slab. It is safe to return NULL in that case.

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