linux-kernel - Re: [PATCH] Describe race of direct read and fork for unaligned buffers

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Message-ID: <x491un3nc7a.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 01 May 2012 11:38:33 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Michael Kerrisk <mtk.manpages@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-man@...r.kernel.org,
	linux-mm@...ck.org, mgorman@...e.de, npiggin@...il.com,
	Andrea Arcangeli <aarcange@...hat.com>,
	Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers

KOSAKI Motohiro <kosaki.motohiro@...il.com> writes:

> On Tue, May 1, 2012 at 11:11 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> KOSAKI Motohiro <kosaki.motohiro@...il.com> writes:
>>
>>>> Hello,
>>>>
>>>> Thank you revisit this. But as far as my remember is correct, this issue is NOT
>>>> unaligned access issue. It's just get_user_pages(_fast) vs fork race issue. i.e.
>>>> DIRECT_IO w/ multi thread process should not use fork().
>>>
>>> The problem is, fork (and its COW logic) assume new access makes cow break,
>>> But page table protection can't detect a DMA write. Therefore DIO may override
>>> shared page data.
>>
>> Hm, I've only seen this with misaligned or multiple sub-page-sized reads
>> in the same page.  AFAIR, aligned, page-sized I/O does not get split.
>> But, I could be wrong...
>
> If my remember is correct, the reproducer of past thread is misleading.
>
> dma_thread.c in
> http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html has
> align parameter. But it doesn't only change align. Because of, every
> worker thread read 4K (pagesize), then
>  - when offset is page aligned
>     -> every page is accessed from only one worker
>  - when offset is not page aligned
>     -> every page is accessed from two workers
>
> But I don't remember why two threads are important things. hmm.. I'm
> looking into the code a while.
> Please don't 100% trust me.

I bet Andrea or Larry would remember the details.

Cheers,
Jeff
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