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

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Date:	Fri, 3 Apr 2009 01:39:09 +0200
From:	Jan Kara <jack@...e.cz>
To:	Ying Han <yinghan@...gle.com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	"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 Thu 02-04-09 15:52:19, Ying Han wrote:
> On Thu, Apr 2, 2009 at 10:44 AM, Ying Han <yinghan@...gle.com> wrote:
> > On Thu, Apr 2, 2009 at 8:51 AM, Nick Piggin <nickpiggin@...oo.com.au> wrote:
> >> 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.
> >>
> >
> > Here is how i reproduce it:
> > Filesystem is ext2 with blocksize 4096
> > Fill up the ram with 95% anon memory and mlockall ( put enough memory
> > pressure which will trigger page reclaim and background writeout)
> > Run one thread of the test program
> >
> > and i will see "bad pages" within few minutes.
> 
> And here is the "top" and stdout while it is getting "bad pages"
> top
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  3487 root      20   0 52616  50m  284 R   95  0.3   3:58.85 usemem
>  3810 root      20   0  129m  99m  99m D   41  0.6   0:01.87 ftruncate_mmap
>   261 root      15  -5     0    0    0 D    4  0.0   0:31.08 kswapd0
>   262 root      15  -5     0    0    0 D    3  0.0   0:10.26 kswapd1
> 
> stdout:
> 
> while true; do
>     ./ftruncate_mmap;
> done
> Running 852 bad page
> Running 315 bad page
> Running 999 bad page
> Running 482 bad page
> Running 24 bad page
  Thanks, for the help. I've debugged the problem to a bug in
ext2_get_block(). I've already sent out a patch which should fix the issue
(at least it fixes the problem for me).
  The fix is also attached if you want to try it.

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "0001-ext2-Fix-data-corruption-for-racing-writes.patch" of type "text/x-patch" (4341 bytes)