linux-ext4 - Re: ftruncate-mmap: pages are lost after writing to mmaped file.

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Date:	Fri, 3 Apr 2009 02:51:20 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jan Kara <jack@...e.cz>
Cc:	Ying Han <yinghan@...gle.com>,
	"Martin J. Bligh" <mbligh@...igh.org>, linux-ext4@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: ftruncate-mmap: pages are lost after writing to mmaped file.

On Thursday 02 April 2009 22:34:01 Jan Kara wrote:
> On Thu 02-04-09 22:24:29, Nick Piggin wrote:
> > On Thursday 02 April 2009 09:36:13 Ying Han wrote:
> > > Hi Jan:
> > >     I feel that the problem you saw is kind of differnt than mine. As
> > > you mentioned that you saw the PageError() message, which i don't see
> > > it on my system. I tried you patch(based on 2.6.21) on my system and
> > > it runs ok for 2 days, Still, since i don't see the same error message
> > > as you saw, i am not convineced this is the root cause at least for
> > > our problem. I am still looking into it.
> > >     So, are you seeing the PageError() every time the problem happened?
> > 
> > So I asked if you could test with my workaround of taking truncate_mutex
> > at the start of ext2_get_blocks, and report back. I never heard of any
> > response after that.
> > 
> > To reiterate: I was able to reproduce a problem with ext2 (I was testing
> > on brd to get IO rates high enough to reproduce it quite frequently).
> > I think I narrowed the problem down to block allocation or inode block
> > tree corruption because I was unable to reproduce it with that hack in
> > place.
>   Nick, what load did you use for reproduction? I'll try to reproduce it
> here so that I can debug ext2...

OK, I set up the filesystem like this:

modprobe rd rd_size=$[3*1024*1024]   #almost fill memory so we reclaim buffers
dd if=/dev/zero of=/dev/ram0 bs=4k   #prefill brd so we don't get alloc deadlock
mkfs.ext2 -b1024 /dev/ram0           #1K buffers

Test is basically unmodified except I use 64MB files, and start 8 of them
at once to (8 core system, so improve chances of hitting the bug). Although I
do see it with only 1 running it takes longer to trigger.

I also run a loop doing 'sync ; echo 3 > /proc/sys/vm/drop_caches' but I don't
know if that really helps speed up reproducing it. It is quite random to hit,
but I was able to hit it IIRC in under a minute with that setup.
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