linux-kernel - Re: [PATCH] eCryptfs - use page_alloc not kmalloc to get a page of memory

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Message-ID: <488E3643.8000300@cs.helsinki.fi>
Date:	Tue, 29 Jul 2008 00:12:35 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, mhalcrow@...ibm.com,
	stable@...nel.org, Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH] eCryptfs - use page_alloc not kmalloc to get a page of
 memory

Hi Eric,

Eric Sandeen wrote:
> Pekka Enberg wrote:
>> On Mon, Jul 28, 2008 at 11:35 PM, Andrew Morton
>> <akpm@...ux-foundation.org> wrote:
>>> On Mon, 28 Jul 2008 11:13:03 -0500
>>> Eric Sandeen <sandeen@...hat.com> wrote:
>>>
>>>> With SLUB debugging turned on in 2.6.26, I was getting memory corruption
>>>> when testing eCryptfs.  The root cause turned out to be that eCryptfs
>>>> was doing kmalloc(PAGE_CACHE_SIZE); virt_to_page() and treating that
>>>> as a nice page-aligned chunk of memory.  But at least with SLUB debugging
>>>> on, this is not always true, and the page we get from virt_to_page does
>>>> not necessarily match the PAGE_CACHE_SIZE worth of memory we got from
>>>> kmalloc.
>>>>
>>>> My simple testcase was 2 loops doing "rm -f fileX; cp /tmp/fileX ." for
>>>> 2 different multi-megabyte files.  With this change I no longer see
>>>> the corruption.
>>> The fix applies to both 2.6.25 and to 2.6.26 and appears to be needed
>>> in both kernel versions, so I have tagged it for backporting into both.
>> Hmm, SLUB will use the page allocator directly for PAGE_CACHE_SIZE
>> regadless of whether debugging is enabled or not...
> 
> For whatever reason, I did see non-page-aligned memory returned from
> kmalloc(PAGE_CACHE_SIZE), and I think this is what caused the problem
> once virt_to_page() was used to get hold of a page to pass around in the
> ecryptfs/crypto code...

With SLUB? I can't see how that's possible. I can see this with SLAB, 
though, for 4K pages.

In any case, the patch, of course, make sense as kmalloc() behavior 
varies between allocators.

		Pekka
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