Re: Bufferheads & page-cache reference

[Andrew Morton <akpm@osdl.org>]

Badari Pulavarty <pbadari@us.ibm.com> wrote:
>
>  Most of DB2 customers use filesystem for their database. Under the load,
>  they complain that entire memory in the system is used by filesystem
>  pagecache, freememory is very low and system starts swapping crazy OR
>  see lots of memory allocation failures and OOM killer kills db2.
>  slabinfo shows lots of bufferheads and VM folks claim that, bufferheads
>  are holding a ref. on the pages, so they can't use them. So, I want
>  to find the truth in the story and findout what exactly happening here
>  and which one to blame (VM or FS or IO problems) ?
> 
>  BTW, all these on 2.4 kernels and I don't have a reproducible testcase
>  :(
> 
>  Feb 7 05:35:17 nmcopsu41 kernel: ENOMEM in do_get_write_access,
>  retrying.

Do these machines have a large amount of highmem?

If so, yes, you can oom because lots of highmem pages have buffer_heads
attached and you've run out of lowmem.  The 2.4 VM will go off looking for
lowmem pages to reclaim and will ignore the highmem pages because there's
no highmem shortage.  Consequently those buffer_heads don't get freed up
and we're unable to reclaim any lowmem -> oom.

Andrea did a patch along time ago (it'll be in suse 2.4 kernels) which,
under these circumstances, strip the buffers from those highmem pages when
they're encountered on the LRU.  From a quick read it seems that that patch
is not in current 2.4 kernels.

It's harder to do that in 2.6 because we have a separate LR per zone.


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