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

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Message-ID: <CAHGf_=ojhwPUWJR0r+jVgjNd5h_sRrppzJntSpHzxyv+OuBueg@mail.gmail.com>
Date:	Tue, 1 May 2012 10:37:20 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Michael Kerrisk <mtk.manpages@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-man@...r.kernel.org,
	linux-mm@...ck.org, mgorman@...e.de,
	Jeff Moyer <jmoyer@...hat.com>, npiggin@...il.com
Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers

> 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.
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