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

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Message-ID: <604427e00904021725n6bd673baq65771e53750fa913@mail.gmail.com>
Date:	Thu, 2 Apr 2009 17:25:30 -0700
From:	Ying Han <yinghan@...gle.com>
To:	Jan Kara <jack@...e.cz>
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, Apr 2, 2009 at 4:39 PM, Jan Kara <jack@...e.cz> wrote:
> 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.

I just did a quick run after applied the patch, unforturnately, my
problem is still there...

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