[ 33/40] memcg: fix multiple large threshold notifications

Greg Kroah-Hartman
2013-09-25T00:15:24Z

From: Greg Thelen <gthelen@google.com>

commit 2bff24a3707093c435ab3241c47dcdb5f16e432b upstream.

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>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/memcontrol.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4349,7 +4349,13 @@ static int compare_thresholds(const void
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)