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

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Date:	Tue, 1 May 2012 11:34:04 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Jeff Moyer <jmoyer@...hat.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
Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers

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.
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