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

[Ying Han <yinghan@google.com>]

On Thu, Apr 2, 2009 at 8:51 AM, Nick Piggin <nickpiggin@yahoo.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.

--Ying

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