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

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Date:	Wed, 2 May 2012 15:14:33 -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>,
	KOSAKI Motohiro <kosaki.motohiro@...il.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

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.

I'm not sure why you say "hairy". Do you mean you have any use case of this?

Thank you.
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