memcg: fix use_hierarchy css_is_ancestor oops regression
If use_hierarchy is set, reclaim testing soon oopses in css_is_ancestor() called from __mem_cgroup_same_or_subtree() called from page_referenced(): when processes are exiting, it's easy for mm_match_cgroup() to pass along a NULL memcg coming from a NULL mm->owner. Check for that in __mem_cgroup_same_or_subtree(). Return true or false? False because we cannot know if it was in the hierarchy, but also false because it's better not to count a reference from an exiting process. Signed-off-by: Hugh Dickins <hughd@google.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Konstantin Khlebnikov <khlebnikov@openvz.org> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Acked-by: Michal Hocko <mhocko@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1148,7 +1148,7 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
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{
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if (root_memcg == memcg)
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return true;
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if (!root_memcg->use_hierarchy)
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if (!root_memcg->use_hierarchy || !memcg)
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return false;
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return css_is_ancestor(&memcg->css, &root_memcg->css);
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}
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