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

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Date:	Wed, 18 Mar 2009 16:36:57 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ying Han <yinghan@...gle.com>
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 Wed, 18 Mar 2009, Ying Han wrote:
> >
> > 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.
> 
> /etc/fstab
> "/dev/hda1 / ext2 defaults 1 0"

Sadly, /etc/fstab is not necessarily accurate for the root filesystem. At 
least Fedora will ignore the flags in it.

What does /proc/mounts say? That should be a more reliable indication of 
what the kernel actually does. 

That said, I assume the ext2 part is accurate. Maybe that's why people 
haven't seen it - I guess most testing was done on ext3. It certainly was 
for me.

> > 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 will give a try.

Thanks,

		Linus
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