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authorMichal Hocko <mhocko@suse.cz>2015-02-27 15:51:46 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2015-02-28 09:57:51 -0800
commit4e54dede38b45052a941bcf709f7d29f2e18174d (patch)
tree4640490752366d061856ce0632fa94ae8a45b1fc
parentda616534ed7f6e8ffaab699258b55c8d78d0b4ea (diff)
memcg: fix low limit calculation
A memcg is considered low limited even when the current usage is equal to the low limit. This leads to interesting side effects e.g. groups/hierarchies with no memory accounted are considered protected and so the reclaim will emit MEMCG_LOW event when encountering them. Another and much bigger issue was reported by Joonsoo Kim. He has hit a NULL ptr dereference with the legacy cgroup API which even doesn't have low limit exposed. The limit is 0 by default but the initial check fails for memcg with 0 consumption and parent_mem_cgroup() would return NULL if use_hierarchy is 0 and so page_counter_read would try to dereference NULL. I suppose that the current implementation is just an overlook because the documentation in Documentation/cgroups/unified-hierarchy.txt says: "The memory.low boundary on the other hand is a top-down allocated reserve. A cgroup enjoys reclaim protection when it and all its ancestors are below their low boundaries" Fix the usage and the low limit comparision in mem_cgroup_low accordingly. Fixes: 241994ed8649 (mm: memcontrol: default hierarchy interface for memory) Reported-by: Joonsoo Kim <js1304@gmail.com> Signed-off-by: Michal Hocko <mhocko@suse.cz> 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>
-rw-r--r--mm/memcontrol.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d18d3a6e7337..76c5a1b1dd21 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5426,7 +5426,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
if (memcg == root_mem_cgroup)
return false;
- if (page_counter_read(&memcg->memory) > memcg->low)
+ if (page_counter_read(&memcg->memory) >= memcg->low)
return false;
while (memcg != root) {
@@ -5435,7 +5435,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
if (memcg == root_mem_cgroup)
break;
- if (page_counter_read(&memcg->memory) > memcg->low)
+ if (page_counter_read(&memcg->memory) >= memcg->low)
return false;
}
return true;