Commit c2b42d3c authored by Tejun Heo's avatar Tejun Heo Committed by Linus Torvalds
Browse files

memcg: convert mem_cgroup->under_oom from atomic_t to int 



memcg->under_oom tracks whether the memcg is under OOM conditions and is
an atomic_t counter managed with mem_cgroup_[un]mark_under_oom().  While
atomic_t appears to be simple synchronization-wise, when used as a
synchronization construct like here, it's trickier and more error-prone
due to weak memory ordering rules, especially around atomic_read(), and
false sense of security.

For example, both non-trivial read sites of memcg->under_oom are a bit
problematic although not being actually broken.

* mem_cgroup_oom_register_event()

  It isn't explicit what guarantees the memory ordering between event
  addition and memcg->under_oom check.  This isn't broken only because
  memcg_oom_lock is used for both event list and memcg->oom_lock.

* memcg_oom_recover()

  The lockless test doesn't have any explanation why this would be
  safe.

mem_cgroup_[un]mark_under_oom() are very cold paths and there's no point
in avoiding locking memcg_oom_lock there.  This patch converts
memcg->under_oom from atomic_t to int, puts their modifications under
memcg_oom_lock and documents why the lockless test in
memcg_oom_recover() is safe.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f4b90b70

mm/memcontrol.c

+21 −8
Original line number Diff line number Diff line
@@ -285,8 +285,9 @@ struct mem_cgroup { 
	 */

	bool use_hierarchy;



	/* protected by memcg_oom_lock */

	bool		oom_lock;

	atomic_t	under_oom;

	int		under_oom;



	int	swappiness;

	/* OOM-Killer disable */
@@ -1809,8 +1810,10 @@ static void mem_cgroup_mark_under_oom(struct mem_cgroup *memcg) 
{

	struct mem_cgroup *iter;



	spin_lock(&memcg_oom_lock);

	for_each_mem_cgroup_tree(iter, memcg)

		atomic_inc(&iter->under_oom);

		iter->under_oom++;

	spin_unlock(&memcg_oom_lock);

}



static void mem_cgroup_unmark_under_oom(struct mem_cgroup *memcg)
@@ -1819,11 +1822,13 @@ static void mem_cgroup_unmark_under_oom(struct mem_cgroup *memcg) 


	/*

	 * When a new child is created while the hierarchy is under oom,

	 * mem_cgroup_oom_lock() may not be called. We have to use

	 * atomic_add_unless() here.

	 * mem_cgroup_oom_lock() may not be called. Watch for underflow.

	 */

	spin_lock(&memcg_oom_lock);

	for_each_mem_cgroup_tree(iter, memcg)

		atomic_add_unless(&iter->under_oom, -1, 0);

		if (iter->under_oom > 0)

			iter->under_oom--;

	spin_unlock(&memcg_oom_lock);

}



static DECLARE_WAIT_QUEUE_HEAD(memcg_oom_waitq);
@@ -1851,7 +1856,15 @@ static int memcg_oom_wake_function(wait_queue_t *wait, 


static void memcg_oom_recover(struct mem_cgroup *memcg)

{

	if (memcg && atomic_read(&memcg->under_oom))

	/*

	 * For the following lockless ->under_oom test, the only required

	 * guarantee is that it must see the state asserted by an OOM when

	 * this function is called as a result of userland actions

	 * triggered by the notification of the OOM.  This is trivially

	 * achieved by invoking mem_cgroup_mark_under_oom() before

	 * triggering notification.

	 */

	if (memcg && memcg->under_oom)

		__wake_up(&memcg_oom_waitq, TASK_NORMAL, 0, memcg);

}
@@ -3860,7 +3873,7 @@ static int mem_cgroup_oom_register_event(struct mem_cgroup *memcg, 
	list_add(&event->list, &memcg->oom_notify);



	/* already in OOM ? */

	if (atomic_read(&memcg->under_oom))

	if (memcg->under_oom)

		eventfd_signal(eventfd, 1);

	spin_unlock(&memcg_oom_lock);
@@ -3889,7 +3902,7 @@ static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v) 
	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf));



	seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);

	seq_printf(sf, "under_oom %d\n", (bool)atomic_read(&memcg->under_oom));

	seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);

	return 0;

}

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