[PATCH v2 3/7] memcg: provide online test for memcg

[Glauber Costa <glommer@parallels.com>]

Since we are now splitting the memcg creation in two parts, following
the cgroup standard, it would be helpful to be able to determine if a
created memcg is already online.

We can do this by initially forcing the refcnt to 0, and waiting until
the last minute to flip it to 1. During memcg's lifetime, this value
will vary. But if it ever reaches 0 again, memcg will be destructed. We
can therefore be sure that any value different than 0 will mean that
our group is online.

Signed-off-by: Glauber Costa <glommer@parallels.com>
---
 mm/memcontrol.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2229945..2ac2808 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -475,6 +475,11 @@ enum res_type {
 static void mem_cgroup_get(struct mem_cgroup *memcg);
 static void mem_cgroup_put(struct mem_cgroup *memcg);
 
+static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
+{
+	return atomic_read(&memcg->refcnt) > 0;
+}
+
 static inline
 struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *s)
 {
@@ -6098,7 +6103,7 @@ mem_cgroup_css_alloc(struct cgroup *cont)
 
 	memcg->last_scanned_node = MAX_NUMNODES;
 	INIT_LIST_HEAD(&memcg->oom_notify);
-	atomic_set(&memcg->refcnt, 1);
+	atomic_set(&memcg->refcnt, 0);
 	memcg->move_charge_at_immigrate = 0;
 	mutex_init(&memcg->thresholds_lock);
 	spin_lock_init(&memcg->move_lock);
@@ -6116,10 +6121,13 @@ mem_cgroup_css_online(struct cgroup *cont)
 	struct mem_cgroup *memcg, *parent;
 	int error = 0;
 
-	if (!cont->parent)
+	memcg = mem_cgroup_from_cont(cont);
+	if (!cont->parent) {
+		/* no need to lock, since this is the root cgroup */
+		atomic_set(&memcg->refcnt, 1);
 		return 0;
+	}
 
-	memcg = mem_cgroup_from_cont(cont);
 	parent = mem_cgroup_from_cont(cont->parent);
 
 	memcg->use_hierarchy = parent->use_hierarchy;
@@ -6151,6 +6159,7 @@ mem_cgroup_css_online(struct cgroup *cont)
 	}
 
 	memcg->swappiness = mem_cgroup_swappiness(parent);
+	atomic_set(&memcg->refcnt, 1);
 
 	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
 	if (error) {
-- 
1.7.11.7

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