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Date: Mon, 17 Sep 2007 16:32:57 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Anton Altaparmakov <aia21@cam.ac.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Linux Memory Management List <linux-mm@kvack.org>,
       marc.smith@esmail.mcc.edu
Subject: Re: VM/VFS bug with large amount of memory and file systems?
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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.