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

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Message-ID: <CAHGf_=qdE3yNw=htuRssfav2pECO1Q0+gWMRTuNROd_3tVrd6Q@mail.gmail.com>
Date:	Tue, 1 May 2012 10:31:44 -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

On Mon, Apr 30, 2012 at 5:30 AM, Jan Kara <jack@...e.cz> wrote:
> This is a long standing problem (or a surprising feature) in our implementation
> of get_user_pages() (used by direct IO). Since several attempts to fix it
> failed (e.g.
> http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg06542.html, or
> http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html refused in
> http://comments.gmane.org/gmane.linux.kernel.mm/31569) and it's not completely
> clear whether we really want to fix it given the costs, let's at least document
> it.
>
> CC: mgorman@...e.de
> CC: Jeff Moyer <jmoyer@...hat.com>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>
> --- a/man2/open.2       2012-04-27 00:07:51.736883092 +0200
> +++ b/man2/open.2       2012-04-27 00:29:59.489892980 +0200
> @@ -769,7 +769,12 @@
>  and the file offset must all be multiples of the logical block size
>  of the file system.
>  Under Linux 2.6, alignment to 512-byte boundaries
> -suffices.
> +suffices. However, if the user buffer is not page aligned and direct read
> +runs in parallel with a
> +.BR fork (2)
> +of the reader process, it may happen that the read data is split between
> +pages owned by the original process and its child. Thus effectively read
> +data is corrupted.
>  .LP
>  The
>  .B O_DIRECT

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().
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