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

[Linus Torvalds <torvalds@linux-foundation.org>]

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.

				Linus

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