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

[Pekka Enberg]

Hi Eric,

Eric Sandeen wrote:
> Pekka Enberg wrote:
> > On Mon, Jul 28, 2008 at 11:35 PM, Andrew Morton
> > <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > On Mon, 28 Jul 2008 11:13:03 -0500
> > > Eric Sandeen <sandeen@xxxxxxxxxx> 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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