[PATCH v3 2/3] mm: memcontrol: fix memcg id ref counter on swap charge move

[Vladimir Davydov]

Since commit 73f576c04b941 swap entries do not pin memcg->css.refcnt
directly. Instead, they pin memcg->id.ref. So we should adjust the
reference counters accordingly when moving swap charges between cgroups.

Fixes: 73f576c04b941 ("mm: memcontrol: fix cgroup creation failure after many small jobs")
Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[3.19+]
---
 mm/memcontrol.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5be9a6ca7540..18db8a490a93 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4031,9 +4031,9 @@ static struct cftype mem_cgroup_legacy_files[] = {
 
 static DEFINE_IDR(mem_cgroup_idr);
 
-static void mem_cgroup_id_get(struct mem_cgroup *memcg)
+static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
 {
-	atomic_inc(&memcg->id.ref);
+	atomic_add(n, &memcg->id.ref);
 }
 
 static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
@@ -4054,9 +4054,9 @@ static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
 	return memcg;
 }
 
-static void mem_cgroup_id_put(struct mem_cgroup *memcg)
+static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
 {
-	if (atomic_dec_and_test(&memcg->id.ref)) {
+	if (atomic_sub_and_test(n, &memcg->id.ref)) {
 		idr_remove(&mem_cgroup_idr, memcg->id.id);
 		memcg->id.id = 0;
 
@@ -4065,6 +4065,16 @@ static void mem_cgroup_id_put(struct mem_cgroup *memcg)
 	}
 }
 
+static inline void mem_cgroup_id_get(struct mem_cgroup *memcg)
+{
+	mem_cgroup_id_get_many(memcg, 1);
+}
+
+static inline void mem_cgroup_id_put(struct mem_cgroup *memcg)
+{
+	mem_cgroup_id_put_many(memcg, 1);
+}
+
 /**
  * mem_cgroup_from_id - look up a memcg from a memcg id
  * @id: the memcg id to look up
@@ -4699,6 +4709,8 @@ static void __mem_cgroup_clear_mc(void)
 		if (!mem_cgroup_is_root(mc.from))
 			page_counter_uncharge(&mc.from->memsw, mc.moved_swap);
 
+		mem_cgroup_id_put_many(mc.from, mc.moved_swap);
+
 		/*
 		 * we charged both to->memory and to->memsw, so we
 		 * should uncharge to->memory.
@@ -4706,9 +4718,9 @@ static void __mem_cgroup_clear_mc(void)
 		if (!mem_cgroup_is_root(mc.to))
 			page_counter_uncharge(&mc.to->memory, mc.moved_swap);
 
-		css_put_many(&mc.from->css, mc.moved_swap);
+		mem_cgroup_id_get_many(mc.to, mc.moved_swap);
+		css_put_many(&mc.to->css, mc.moved_swap);
 
-		/* we've already done css_get(mc.to) */
 		mc.moved_swap = 0;
 	}
 	memcg_oom_recover(from);
-- 
2.1.4