linux-ext4 - Re: upcoming kerneloops.org item: get_page_from

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Date:	Fri, 26 Jun 2009 11:07:41 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Theodore Tso <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>,
	Andreas Dilger <adilger@....com>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	arjan@...radead.org, linux-kernel@...r.kernel.org,
	cl@...ux-foundation.org, linux-ext4@...r.kernel.org
Subject: Re: upcoming kerneloops.org item: get_page_from_freelist

On Fri, Jun 26, 2009 at 11:58:01AM +0300, Pekka Enberg wrote:
> On Fri, 2009-06-26 at 10:56 +0200, Nick Piggin wrote:
> > On Fri, Jun 26, 2009 at 08:16:18AM +0300, Pekka Enberg wrote:
> > > Hi Ted,
> > > 
> > > On Thu, Jun 25, 2009 at 05:11:25PM -0500, Eric Sandeen wrote:
> > > > > ecryptfs used to do kmalloc(PAGE_CACHE_SIZE) & virt_to_page on that, and
> > > > > with SLUB + slub debug, that gave back non-aligned memory, causing
> > > > > eventual corruption ...
> > > 
> > > On Thu, 25 Jun 2009, Theodore Tso wrote:
> > > > Grumble.  Any chance we could add an kmem_cache option which requires
> > > > the memory to be aligned?  Otherwise we could rewrite our own sub-page
> > > > allocator in ext4 that only handled aligned filesystem block sizes
> > > > (i.e., 1k, 2k, 4k, etc.) but that would be really silly and be extra
> > > > code that really should be done once at core functionality.
> > > 
> > > We alredy have SLAB_HW_ALIGN but I wonder if this is a plain old bug in 
> > > SLUB. Christoph, Nick, don't we need something like this in the allocator? 
> > > Eric, does this fix your case?
> > 
> > Well I don't understand Ted's complaint. kmem_cache_create takes an
> > alignment argument.
> 
> Yes, but AFAICT, SLUB_DEBUG doesn't respect the given alignment without
> my patch.

Well your patch doesn't hurt (doesn't seem to make much sense to
use objsize rather than size when calculating alignment). But I
think alignment should still be calculated OK: calculate_alignemnt
will always return >= align, and so the next statement will
round up size to the correct alignment AFAICT.
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