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

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Message-ID: <CAKgNAkh5TkwSzFVKVo5JUvkDWkzY8EaQNxJSQnv3fTHTdj0+FQ@mail.gmail.com>
Date:	Wed, 9 May 2012 17:35:21 +1200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Nick Piggin <npiggin@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Jan Kara <jack@...e.cz>, Jeff Moyer <jmoyer@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-man@...r.kernel.org,
	linux-mm@...ck.org, mgorman@...e.de,
	Andrea Arcangeli <aarcange@...hat.com>,
	Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers

On Wed, May 9, 2012 at 11:10 AM, Nick Piggin <npiggin@...il.com> wrote:
> On 6 May 2012 01:29, KOSAKI Motohiro <kosaki.motohiro@...il.com> wrote:
>>> So, am I correct to assume that right text to add to the page is as below?
>>>
>>> Nick, can you clarify what you mean by "quiesced"?
>>
>> finished?
>
> Yes exactly. That might be a simpler word. Thanks!

Thanks.

But see below. I realize the text is still ambiguous.

>>> [[
>>> O_DIRECT IOs should never be run concurrently with fork(2) system call,
>>> when the memory buffer is anonymous memory, or comes from mmap(2)
>>> with MAP_PRIVATE.
>>>
>>> Any such IOs, whether submitted with asynchronous IO interface or from
>>> another thread in the process, should be quiesced before fork(2) is called.
>>> Failure to do so can result in data corruption and undefined behavior in
>>> parent and child processes.
>>>
>>> This restriction does not apply when the memory buffer for the O_DIRECT
>>> IOs comes from mmap(2) with MAP_SHARED or from shmat(2).
>>> Nor does this restriction apply when the memory buffer has been advised
>>> as MADV_DONTFORK with madvise(2), ensuring that it will not be available
>>> to the child after fork(2).
>>> ]]

In the above, the status of a MAP_SHARED MAP_ANONYMOUS buffer is
unclear. The first paragraph implies that such a buffer is unsafe,
while the third paragraph implies that it *is* safe, thus
contradicting the first paragraph. Which is correct?

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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