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SubjectRe: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim
On Fri, Jul 02, 2010 at 12:51:55PM -0700, Andrew Morton wrote:
> On Tue, 29 Jun 2010 12:34:46 +0100
> Mel Gorman <> wrote:
> > When memory is under enough pressure, a process may enter direct
> > reclaim to free pages in the same manner kswapd does. If a dirty page is
> > encountered during the scan, this page is written to backing storage using
> > mapping->writepage. This can result in very deep call stacks, particularly
> > if the target storage or filesystem are complex. It has already been observed
> > on XFS that the stack overflows but the problem is not XFS-specific.
> >
> > This patch prevents direct reclaim writing back pages by not setting
> > may_writepage in scan_control. Instead, dirty pages are placed back on the
> > LRU lists for either background writing by the BDI threads or kswapd. If
> > in direct lumpy reclaim and dirty pages are encountered, the process will
> > stall for the background flusher before trying to reclaim the pages again.
> >
> > Memory control groups do not have a kswapd-like thread nor do pages get
> > direct reclaimed from the page allocator. Instead, memory control group
> > pages are reclaimed when the quota is being exceeded or the group is being
> > shrunk. As it is not expected that the entry points into page reclaim are
> > deep call chains memcg is still allowed to writeback dirty pages.
> I already had "[PATCH 01/14] vmscan: Fix mapping use after free" and
> I'll send that in for 2.6.35.

Perfect, thanks.

> I grabbed [02/14] up to [11/14]. Including "[PATCH 06/14] vmscan: kill
> prev_priority completely", grumpyouallsuck.
> I wimped out at this, "Do not writeback pages in direct reclaim". It
> really is a profound change and needs a bit more thought, discussion
> and if possible testing which is designed to explore possible pathologies.

Ok, that's reasonable as I'm still working on that patch. For example, the
patch disabled anonymous page writeback which is unnecessary as the stack
usage for anon writeback is less than file writeback. Second, using systemtap,
I was able to see that file-backed dirty pages have a tendency to be near the
end of the LRU even though they are a small percentage of the overall pages
in the LRU. I'm hoping to figure out why this is as it would make avoiding
writeback a lot less controversial.

Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab

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