[RFC] Functional fix to zone_reclaim() and bring behaviour more in line with expectations

A bug was brought to my attention against a distro kernel but it affects
mainline and I believe problems like this have been reported in various guises
on the mailing lists although I don't have specific examples at the moment.

The problem that was reported that led to this patchset was that malloc()
stalled for a long time (minutes in some cases) if a large tmpfs mount
was occupying a large percentage of memory overall. The pages did not get
cleaned or reclaimed by zone_reclaim() because the zone_reclaim_mode was
unsuitable, but the lists are uselessly scanned frequencly making the CPU
spin at near 100%.

I do not have the bug resolved yet although I believe patch 1 of this series
addresses it and am waiting to hear back from the bug reporter. However,
the fix should work two other patches in this series also should bring
zone_reclaim() more in line with expectations.

Patch 1 reintroduces zone_reclaim_interval to catch the situation where
	zone_reclaim() cannot tell in advance that the scan is a waste
	of time.

Patch 2 alters the heuristics that zone_reclaim() uses to determine if the
	scan should go ahead. Currently, it is basically assuming
	zone_reclaim_mode is 1

Patch 3 notes that zone_reclaim() returning a failure automatically means
	the zone is marked full. This is not always true. It could have failed
	because the GFP mask or zone_reclaim_mode are unsuitable. The patch
	makes zone_reclaim() more careful about marking zones temporarily full

Note, this patchset has not been tested heavily.

Comments?

 Documentation/sysctl/vm.txt |   13 +++++++++++
 include/linux/mmzone.h      |    9 ++++++++
 include/linux/swap.h        |    1 +
 kernel/sysctl.c             |    9 ++++++++
 mm/internal.h               |    4 +++
 mm/page_alloc.c             |   26 +++++++++++++++++++---
 mm/vmscan.c                 |   48 +++++++++++++++++++++++++++++++++++++-----
 7 files changed, 100 insertions(+), 10 deletions(-)

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