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Subject[PATCH 0/4] [RFC] Functional fix to zone_reclaim() and bring behaviour more in line with expectations V2
Date
Changelog since V1
o Rebase to mmotm
o Add various acks
o Documentation and patch leader fixes
o Use Kosaki's method for calculating the number of unmapped pages
o Consider the zone full in more situations than all pages being unreclaimable
o Add a counter to detect when scan-avoidance heuristics are failing
o Handle jiffie wraps for zone_reclaim_interval
o Move zone_reclaim_interval to the end of the set with the view to dropping
it. If Kosaki's calculation is accurate, then the problem being dealt with
should also be addressed

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 is 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%.

This patchset intends to address that bug and bring the behaviour of
zone_reclaim() more in line with expectations. It is based on top of mmotm
and takes advantage of Kosaki's work with respect to zone_reclaim().

Patch 1 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 and historically it could not deal with
tmpfs pages at all. This fixes up the heuristic so that the scan
is more likely to be correctly avoided.

Patch 2 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 were unsuitable.

Patch 3 introduces a counter zreclaim_failed that will increment each
time the zone_reclaim scan-avoidance heuristics fail. If that
counter is rapidly increasing, then zone_reclaim_mode should be
set to 0 as a temporarily resolution and a bug reported.

Patch 4 reintroduces zone_reclaim_interval to catch the situation where
zone_reclaim() cannot tell in advance that the scan is a waste of
time. This is a brute force catch-all. I've asked the bug reporter
to test with just patch 1. If that works, then this patch will be
dropped and patch 3 will be enough to tell us if/when the situation
occured again. Even with this patch applied, the counter will
increase slowly so it's still possible to detect the problem.

Documentation/sysctl/vm.txt | 15 +++++++
include/linux/mmzone.h | 9 ++++
include/linux/swap.h | 1 +
include/linux/vmstat.h | 3 +
kernel/sysctl.c | 9 ++++
mm/internal.h | 4 ++
mm/page_alloc.c | 26 ++++++++++--
mm/vmscan.c | 91 ++++++++++++++++++++++++++++++++++---------
mm/vmstat.c | 3 +
9 files changed, 138 insertions(+), 23 deletions(-)



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