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| author | Michal Hocko <mhocko@suse.cz> | 2015-02-27 15:51:46 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2015-02-28 09:57:51 -0800 |
| commit | 4e54dede38b45052a941bcf709f7d29f2e18174d (patch) | |
| tree | 4640490752366d061856ce0632fa94ae8a45b1fc /mm/memcontrol.c | |
| parent | da616534ed7f6e8ffaab699258b55c8d78d0b4ea (diff) | |
| download | lwn-4e54dede38b45052a941bcf709f7d29f2e18174d.tar.gz lwn-4e54dede38b45052a941bcf709f7d29f2e18174d.zip | |
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>
Diffstat (limited to 'mm/memcontrol.c')
| -rw-r--r-- | mm/memcontrol.c | 4 |
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; |