[PATCH] memcg: fix multiple large threshold notifications

[Greg Thelen <gthelen@google.com>]

A memory cgroup with (1) multiple threshold notifications and (2) at
least one threshold >=2G was not reliable.  Specifically the
notifications would either not fire or would not fire in the proper
order.

The __mem_cgroup_threshold() signaling logic depends on keeping 64 bit
thresholds in sorted order.  mem_cgroup_usage_register_event() sorts
them with compare_thresholds(), which returns the difference of two 64
bit thresholds as an int.  If the difference is positive but has
bit[31] set, then sort() treats the difference as negative and breaks
sort order.

This fix compares the two arbitrary 64 bit thresholds returning the
classic -1, 0, 1 result.

The test below sets two notifications (at 0x1000 and 0x81001000):
  cd /sys/fs/cgroup/memory
  mkdir x
  for x in 4096 2164264960; do
    cgroup_event_listener x/memory.usage_in_bytes $x | sed "s/^/$x listener:/" &
  done
  echo $$ > x/cgroup.procs
  anon_leaker 500M

v3.11-rc7 fails to signal the 4096 event listener:
  Leaking...
  Done leaking pages.

Patched v3.11-rc7 properly notifies:
  Leaking...
  4096 listener:2013:8:31:14:13:36
  Done leaking pages.

The fixed bug is old.  It appears to date back to the introduction of
memcg threshold notifications in v2.6.34-rc1-116-g2e72b6347c94 "memcg:
implement memory thresholds"

Signed-off-by: Greg Thelen <gthelen@google.com>
---
 mm/memcontrol.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0878ff7..aa44621 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5616,7 +5616,13 @@ static int compare_thresholds(const void *a, const void *b)
 	const struct mem_cgroup_threshold *_a = a;
 	const struct mem_cgroup_threshold *_b = b;
 
-	return _a->threshold - _b->threshold;
+	if (_a->threshold > _b->threshold)
+		return 1;
+
+	if (_a->threshold < _b->threshold)
+		return -1;
+
+	return 0;
 }
 
 static int mem_cgroup_oom_notify_cb(struct mem_cgroup *memcg)
-- 
1.8.4

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