On Tue, Feb 17, 2015 at 09:33:27AM +0100, Michal Hocko wrote: > On Tue 17-02-15 14:24:59, Joonsoo Kim wrote: > > It can be possible to return NULL in parent_mem_cgroup() > > if use_hierarchy is 0. > > This alone is not sufficient because the low limit is present only in > the unified hierarchy API and there is no use_hierarchy there. The > primary issue here is that the memcg has 0 usage so the previous > check for usage will not stop us. And that is bug IMO. Yes, empty groups shouldn't be considered low. > From f5d74671d30e44c50b45b4464c92f536f1dbdff6 Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@xxxxxxx> > Date: Tue, 17 Feb 2015 08:02:12 +0100 > 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 <js1304@xxxxxxxxx> > Signed-off-by: Michal Hocko <mhocko@xxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>