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

[Eric Sandeen <sandeen@redhat.com>]

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...

-Eric