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

[Ying Han <yinghan@google.com>]

On Wed, Mar 18, 2009 at 3:40 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>
> On Wed, 18 Mar 2009, Andrew Morton wrote:
>
>> On Wed, 18 Mar 2009 12:44:08 -0700 Ying Han <yinghan@google.com> wrote:
>> >
>> > The "bad pages" count differs each time from one digit to 4,5 digit
>> > for 128M ftruncated file. and what i also found that the bad page
>> > number are contiguous for each segment which total bad pages container
>> > several segments. ext "1-4, 9-20, 48-50" (  batch flushing ? )
>
> Yeah, probably the batched write-out.
>
> Can you say what filesystem, and what mount-flags you use? Iirc, last time
> we had MAP_SHARED lost writes it was at least partly triggered by the
> filesystem doing its own flushing independently of the VM (ie ext3 with
> "data=journal", I think), so that kind of thing does tend to matter.
>
> See for example commit ecdfc9787fe527491baefc22dce8b2dbd5b2908d.
>
>> > (The failure is reproduced based on 2.6.29-rc8, also happened on
>> > 2.6.18 kernel. . Here is the simple test case to reproduce it with
>> > memory pressure. )
>>
>> Thanks.  This will be a regression - the testing I did back in the days
>> when I actually wrote stuff would have picked this up.
>>
>> Perhaps it is a 2.6.17 thing.  Which, IIRC, is when we made the changes to
>> redirty pages on each write fault.  Or maybe it was something else.
>
> Hmm. I _think_ that changes went in _after_ 2.6.18, if you're talking
> about Peter's exact dirty page tracking. If I recall correctly, that
> became then 2.6.19, and then had the horrible mm dirty bit loss that
> triggered in librtorrent downloads, which got fixed sometime after 2.6.20
> (and back-ported).
>
> So if 2.6.18 shows the same problem, then it's a _really_ old bug, and not
> related to the exact dirty tracking.
>
> The exact dirty accounting patch I'm talking about is d08b3851da41 ("mm:
> tracking shared dirty pages"), but maybe you had something else in mind?
>
>> Given the amount of time for which this bug has existed, I guess it isn't a
>> 2.6.29 blocker, but once we've found out the cause we should have a little
>> post-mortem to work out how a bug of this nature has gone undetected for so
>> long.
>
> I'm somewhat surprised, because this test-program looks like a very simple
> version of the exact one that I used to track down the 2.6.20 mmap
> corruption problems. And that one got pretty heavily tested back then,
> when people were looking at it (December 2006) and then when trying out my
> fix for it.
>
> 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.

>
>                                Linus
>

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