On Mon, 17 Sep 2007 13:11:14 -0400
Rik van Riel <riel@redhat.com> wrote:

quoted text
> > 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.

yeah, this got pretty unavoidably broken when we killed the global LRU
in 2.5.early. It's odd that it took this long for someone to hit it.

quoted text
> I've got code for this in RHEL 3, but never bothered to
> merge it upstream since I thought people with large memory
> systems would be running 64 bit kernels by now.
> 
> Obviously I was wrong.  Andrew, are you interested in a
> fix for this problem?
> 
> IIRC I simply kept a list of all buffer heads and walked
> that to reclaim pages when the number of buffer heads is
> too high (and we need memory).  This list can be maintained
> in places where we already hold the lock for the buffer head
> freelist, so there should be no additional locking overhead
> (again, IIRC).

Christoph's slab defragmentation code should permit us to fix this:
grab a page of buffer_heads off the slab lists, trylock the page,
strip the buffer_heads.  I think that would be a better approach
if we can get it going because it's more general.
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