linux-kernel - Re: [PATCH v2] mm: Fix slab->page

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Message-ID: <CALnjE+qGpJCf6FOb37DjJfh5qHfJuF7DBmbqDNYR7KJ3Ux41Uw@mail.gmail.com>
Date:	Mon, 14 May 2012 17:13:17 -0700
From:	Pravin Shelar <pshelar@...ira.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	cl@...ux.com, penberg@...nel.org, mpm@...enic.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jesse@...ira.com,
	abhide@...ira.com
Subject: Re: [PATCH v2] mm: Fix slab->page _count corruption.

On Mon, May 14, 2012 at 3:34 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2012-05-14 at 15:29 -0700, Pravin B Shelar wrote:
>> On arches that do not support this_cpu_cmpxchg_double slab_lock is used
>> to do atomic cmpxchg() on double word which contains page->_count.
>> page count can be changed from get_page() or put_page() without taking
>> slab_lock. That corrupts page counter.
>>
>> Following patch fixes it by moving page->_count out of cmpxchg_double
>> data. So that slub does no change it while updating slub meta-data in
>> struct page.
>
> I say again : Page is owned by slub, so get_page() or put_page() is not
> allowed ?
>
This is already done in multiple subsystem in Linux kernel. e.g.
ocfs, xfs, etc.
So object from slab can be passed to IO using DMA. I don't think this
rule you referring to is enforced anywhere.

Thanks,
Pravin.
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