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

[Ying Han <yinghan@google.com>]

On Thu, Apr 2, 2009 at 10:44 AM, Ying Han <yinghan@google.com> wrote:
> 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.

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

--Ying

>
> --Ying
>

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