From 4e54dede38b45052a941bcf709f7d29f2e18174d Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Fri, 27 Feb 2015 15:51:46 -0800 Subject: [PATCH] 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 Signed-off-by: Michal Hocko Acked-by: Johannes Weiner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 4 ++-- 1 file 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;