mm/memcontrol.c: fix the margin computation in mem_cgroup_margin()

mem_cgroup_margin() might return (memory.limit - memory_count) when the
memsw.limit is in excess.  This doesn't happen usually because we do not
allow excess on hard limits and (memory.limit <= memsw.limit), but
__GFP_NOFAIL charges can force the charge and cause the excess when no
memory is really swappable (swap is full or no anonymous memory is
left).

[mhocko@suse.com: rewrote changelog]
Link: http://lkml.kernel.org/r/20160525155122.GK20132@dhcp22.suse.cz
Link: http://lkml.kernel.org/r/1464068266-27736-1-git-send-email-roy.qing.li@gmail.com
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f6477a9..485c688 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1108,6 +1108,8 @@ 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;