Re: abnormal OOM killer message

[Nitin Gupta]

On 08/19/2009 08:14 AM, Minchan Kim wrote:
> On Wed, 19 Aug 2009 10:41:51 +0900
> ììê<chungki.woo@xxxxxxxxx>  wrote:
>
> > Hi all~
> > I have got a log message with OOM below. I don't know why this
> > phenomenon was happened.
> > When direct reclaim routine(try_to_free_pages) in __alloc_pages which
> > allocates kernel memory was failed,
> > one last chance is given to allocate memory before OOM routine is executed.
> > And that time, allocator uses ALLOC_WMARK_HIGH to limit watermark.
> > Then, zone_watermark_ok function test this value with current memory
> > state and decide 'can allocate' or 'cannot allocate'.
> >
> > Here is some kernel source code in __alloc_pages function to understand easily.
> > Kernel version is 2.6.18 for arm11. Memory size is 32Mbyte. And I use
> > compcache(0.5.2).

<snip>

> > In my case, you can see free pages(6804KB) is much more higher than
> > high watermark value(1084KB) in OOM message.
> > And order of allocating is also zero.(order=0)
> > In buddy system, the number of 4kbyte page is 867.
> > So, I think OOM can't be happend.
>
> Yes. I think so.
>
> In that case, even we can also avoid zone defensive algorithm.
>
> > How do you think about this?
> > Is this side effect of compcache?

compcache can be storing lot of stale data and this memory space cannot be
reclaimed (unless overwritten by some other swap data). This is because
compcache does not know when a swap slot has been freed and hence does not know 
when its safe to free corresponding memory. You can check current memory usage 
with /proc/ramzswap (see MemUsedTotal).

BTW, with compcache-0.6 there is an experimental kernel patch that gets rid of 
all this stale data:
http://patchwork.kernel.org/patch/41083/

However, this compcache version needs at least kernel 2.6.28. This version also 
fixes all known problems on ARM. compcache-0.5.3 or earlier is known to crash on 
ARM (see: http://code.google.com/p/compcache/issues/detail?id=33).

Thanks,
Nitin
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