[RFC PATCH 3/3] memcg: allocate pages for swap cgroup until the first child memcg is alive

[Jeff Liu <jeff.liu@xxxxxxxxxx>]

- Call swap_cgroup_init() when the first child memcg was created.
- Free pages from swap cgroup once the latest child memcg was removed.
- Teach swap_cgroup_record()/swap_cgroup_cmpxchg()/swap_cgroup_lookup_id()
  to aware of the new static variable. i.e, swap_cgroup_initialized, so that
  they would not perform further jobs if swap cgroup is not active.

Signed-off-by: Jie Liu <jeff.liu@xxxxxxxxxx>
CC: Glauber Costa <glommer@xxxxxxxxxxxxx>
CC: Michal Hocko <mhocko@xxxxxxx>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
CC: Johannes Weiner <hannes@xxxxxxxxxxx>
CC: Mel Gorman <mgorman@xxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 mm/memcontrol.c  |    3 +++
 mm/page_cgroup.c |    9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dd39ba0..1f14375 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4982,6 +4982,8 @@ mem_cgroup_create(struct cgroup *cont)
 		}
 		hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
 	} else {
+		if (swap_cgroup_init())
+			goto free_out;
 		parent = mem_cgroup_from_cont(cont->parent);
 		memcg->use_hierarchy = parent->use_hierarchy;
 		memcg->oom_kill_disable = parent->oom_kill_disable;
@@ -5046,6 +5048,7 @@ static void mem_cgroup_destroy(struct cgroup *cont)
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
 
 	kmem_cgroup_destroy(memcg);
+	swap_cgroup_destroy();
 
 	mem_cgroup_put(memcg);
 }
diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index f1b257b..63c6789 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -429,6 +429,9 @@ unsigned short swap_cgroup_cmpxchg(swp_entry_t ent,
 	unsigned long flags;
 	unsigned short retval;
 
+	if (!atomic_read(&swap_cgroup_initialized))
+		return 0;
+
 	sc = lookup_swap_cgroup(ent, &ctrl);
 
 	spin_lock_irqsave(&ctrl->lock, flags);
@@ -456,6 +459,9 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
 	unsigned short old;
 	unsigned long flags;
 
+	if (!atomic_read(&swap_cgroup_initialized))
+		return 0;
+
 	sc = lookup_swap_cgroup(ent, &ctrl);
 
 	spin_lock_irqsave(&ctrl->lock, flags);
@@ -474,6 +480,9 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
  */
 unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
 {
+	if (!atomic_read(&swap_cgroup_initialized))
+		return 0;
+
 	return lookup_swap_cgroup(ent, NULL)->id;
 }
 
-- 
1.7.9.5

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