Re: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 29 Jun 2010 12:34:46 +0100
Mel Gorman <mel@csn.ul.ie> 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.

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.