linux-kernel - Re: ftruncate-mmap: pages are lost after writing to mmaped file.

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Date:	Fri, 20 Mar 2009 00:00:17 -0700
From:	Ying Han <yinghan@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, guichaz@...il.com,
	Alex Khesin <alexk@...gle.com>,
	Mike Waychison <mikew@...gle.com>,
	Rohit Seth <rohitseth@...gle.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: ftruncate-mmap: pages are lost after writing to mmaped file.

On Thu, Mar 19, 2009 at 5:49 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>
> On Thu, 19 Mar 2009, Ying Han wrote:
>> >
>> > Ying Han - since you're all set up for testing this and have reproduced it
>> > on multiple kernels, can you try it on a few more kernel versions? It
>> > would be interesting to both go further back in time (say 2.6.15-ish),
>> > _and_ check something like 2.6.21 which had the exact dirty accounting
>> > fix. Maybe it's not really an old bug - maybe we re-introduced a bug that
>> > was fixed for a while.
>>
>> I tried 2.6.24 for couple of hours and the problem not happening yet. While
>> the same test on 2.6.25, the problem happen right away.
>
> Ok, so 2.6.25 is known bad. Can you test 2.6.24 a lot more, because we
> should not decide that it's bug-free without a _lot_ of testing.
>
> But if it's a bug that has gone away and then re-appeared, it at least
> explains how 2.6.21 (which got a fair amount of mmap testing) didn't have
> lots of reports of mmap corruption.
>
> That said, I can think of nothing obvious in between 2.6.24 and .25 that
> would have re-introduced it. But if some heavy testing really does confirm
> that 2.6.24 doesn't have the problem, that is a good first step to trying
> to narrow down where things started going wrong.
>
> That said, it could _easily_ be some timing-related pattern. One of the
> things in between 2.6.24 and .25 is
>
>  - 8bc3be2751b4f74ab90a446da1912fd8204d53f7: "writeback: speed up
>   writeback of big dirty files"
>
> which is that exact kind of "change the timing patterns, but don't change
> anything fundamental" thing.
>
> Which is why I'd like you to continue testing 2.6.24 just to be _really_
> sure that it really doesn't happen there.

Unfortunately, 2.6.24 is not immune. After running several hours, i triggered
the problem.
>
>                Linus
>
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