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

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Message-ID: <48907F49.7020101@linux-foundation.org>
Date:	Wed, 30 Jul 2008 09:48:41 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, mhalcrow@...ibm.com,
	stable@...nel.org
Subject: Re: [PATCH] eCryptfs - use page_alloc not kmalloc to get a page of
 memory

Pekka Enberg wrote:
>> 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.

It possible because PAGE_CACHE_SIZE is still handled by SLUB and if debugging is on then   kmalloc may return non page aligned objects. The handoff to the page allocator only occurs for objects > 4k. We used to do this also for 4k objects but then we got performance regressions in tbench.



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

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