[PATCH][V2] mm: memcontrol: fix the margin computation in mem_cgroup_margin

[roy.qing.li@gmail.com]

From: Li RongQing <roy.qing.li@gmail.com>

The margin may be set to the difference value between memory limit and
memory count firstly. which maybe returned wrongly if memsw.count excess
memsw.limit, because try_charge forces charging __GFP_NOFAIL allocations,
which may result in memsw.limit excess. If we are below memory.limit
and there's nothing to reclaim to reduce memsw.usage, might end up
looping in try_charge forever.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.com>
---
 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 00981d2..12aaadd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1090,6 +1090,8 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
 		limit = READ_ONCE(memcg->memsw.limit);
 		if (count <= limit)
 			margin = min(margin, limit - count);
+		else
+			margin = 0;
 	}
 
 	return margin;
-- 
2.1.4

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