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

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Date:	Wed, 2 May 2012 15:25:58 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Nick Piggin <npiggin@...il.com>, Jeff Moyer <jmoyer@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.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 2, 2012 at 3:23 PM, Jan Kara <jack@...e.cz> wrote:
> On Wed 02-05-12 15:14:33, KOSAKI Motohiro wrote:
>> Hello,
>>
>> >> I see what you mean.
>> >>
>> >> I'm not sure, though. For most apps it's bad practice I think. If you get into
>> >> realm of sophisticated, performance critical IO/storage managers, it would
>> >> not surprise me if such concurrent buffer modifications could be allowed.
>> >> We allow exactly such a thing in our pagecache layer. Although probably
>> >> those would be using shared mmaps for their buffer cache.
>> >>
>> >> I think it is safest to make a default policy of asking for IOs against private
>> >> cow-able mappings to be quiesced before fork, so there are no surprises
>> >> or reliance on COW details in the mm. Do you think?
>> >    Yes, I agree that (and MADV_DONTFORK) is probably the best thing to have
>> > in documentation. Otherwise it's a bit too hairy...
>>
>> I neglected this issue for years because Linus asked who need this and
>> I couldn't
>> find real world usecase.
>>
>> Ah, no, not exactly correct. Fujitsu proprietary database had such
>> usecase. But they quickly fixed it. Then I couldn't find alternative usecase.
>  One of our customers hit this bug recently which is why I started to look
> at this. But they also modified their application not to hit the problem.
>
>> I'm not sure why you say "hairy". Do you mean you have any use case of this?
>  I meant that if we should describe conditions like "if you have page
> aligned buffer and you don't write to it while the IO is running, the
> problem also won't occur", then it's already too detailed and might
> easily change in future kernels...

ok, thanks.
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