Commit 9c567512 authored Oct 16, 2013 by David Rientjes Committed by Linus Torvalds Oct 16, 2013

mm, memcg: protect mem_cgroup_read_events for cpu hotplug



for_each_online_cpu() needs the protection of {get,put}_online_cpus() so
cpu_online_mask doesn't change during the iteration.

cpu_hotplug.lock is held while a cpu is going down, it's a coarse lock
that is used kernel-wide to synchronize cpu hotplug activity.  Memcg has
a cpu hotplug notifier, called while there may not be any cpu hotplug
refcounts, which drains per-cpu event counts to memcg->nocpu_base.events
to maintain a cumulative event count as cpus disappear.  Without
get_online_cpus() in mem_cgroup_read_events(), it's possible to account
for the event count on a dying cpu twice, and this value may be
significantly large.

In fact, all memcg->pcp_counter_lock use should be nested by
{get,put}_online_cpus().

This fixes that issue and ensures the reported statistics are not vastly
over-reported during cpu hotplug.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 34ec4de4

mm/memcontrol.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -866,6 +866,7 @@ static unsigned long mem_cgroup_read_events(struct mem_cgroup *memcg,
		unsigned long val = 0;
		int cpu;

		get_online_cpus();
		for_each_online_cpu(cpu)
		val += per_cpu(memcg->stat->events[idx], cpu);
		#ifdef CONFIG_HOTPLUG_CPU
		@@ -873,6 +874,7 @@ static unsigned long mem_cgroup_read_events(struct mem_cgroup *memcg,
		val += memcg->nocpu_base.events[idx];
		spin_unlock(&memcg->pcp_counter_lock);
		#endif
		put_online_cpus();
		return val;
		}

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