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

[Pekka Enberg <penberg@cs.helsinki.fi>]

Hi Eric,

Eric Sandeen wrote:
> Pekka Enberg wrote:
>> On Mon, Jul 28, 2008 at 11:35 PM, Andrew Morton
>> <akpm@linux-foundation.org> wrote:
>>> On Mon, 28 Jul 2008 11:13:03 -0500
>>> Eric Sandeen <sandeen@redhat.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