Re: VM/VFS bug with large amount of memory and file systems?

Peter Zijlstra
2007-09-17T14:33:19Z

On Mon, 17 Sep 2007 15:04:05 +0100 Anton Altaparmakov <aia21@cam.ac.uk>
wrote:

> They files
> are attached this time rather than inlined so people don't complain
> about line wrapping! (No doubt people will not complain about them
> being attached! )-:)

I switched mailer after I learnt about flowed stuffs. Still,
appreciated.

> If I read it correctly it appears all of low memory is eaten up by
> buffer_heads.
>
> <quote>
> # name <active_objs> <num_objs> <objsize> <objperslab>
> <pagesperslab>
> : tunables <limit> <batchcount> <sharedfactor> : slabdata
> <active_slabs> <num_s
> labs> <sharedavail>
> buffer_head 12569528 12569535 56 67 1 : tunables
> 120 60 8 :
> slabdata 187605 187605 0
> </quote>
>
> That is 671MiB of low memory in buffer_heads.
>
> But why is the kernel not reclaiming them by getting rid of the page
> cache pages they are attached to or even leaving the pages around but
> killing their buffers?

Well, you see, you have this very odd configuration where:

11GB highmem
1GB normal

pagecache pages go into highmem
buggerheads go into normal

I'm guessing there is no pressure at all on zone_highmem so the
kernel will not try to reclaim pagecache. And because the pagecache
pages are happily sitting there, the buggerheads are pinned and do not
get reclaimed.