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

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Message-ID: <x49ehr4lyw1.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 01 May 2012 11:11:26 -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
Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers

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...

Cheers,
Jeff
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