[PATCH 2/5] mm: prepare to premature release of per-node lruvec_stat_cpu

[Roman Gushchin <guroan@gmail.com>]

Similar to the memcg's vmstats_percpu, per-memcg per-node stats
consists of percpu- and atomic counterparts, and we do expect
that both coexist during the whole life-cycle of the memcg.

To prepare for a premature release of percpu per-node data,
let's pretend that lruvec_stat_cpu is a rcu-protected pointer,
which can be NULL. This patch adds corresponding checks whenever
required.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/memcontrol.h | 21 +++++++++++++++------
 mm/memcontrol.c            | 11 +++++++++--
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 05ca77767c6a..8ac04632002a 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -126,7 +126,7 @@ struct memcg_shrinker_map {
 struct mem_cgroup_per_node {
 	struct lruvec		lruvec;
 
-	struct lruvec_stat __percpu *lruvec_stat_cpu;
+	struct lruvec_stat __rcu /* __percpu */ *lruvec_stat_cpu;
 	atomic_long_t		lruvec_stat[NR_VM_NODE_STAT_ITEMS];
 
 	unsigned long		lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
@@ -682,6 +682,7 @@ static inline unsigned long lruvec_page_state(struct lruvec *lruvec,
 static inline void __mod_lruvec_state(struct lruvec *lruvec,
 				      enum node_stat_item idx, int val)
 {
+	struct lruvec_stat __percpu *lruvec_stat_cpu;
 	struct mem_cgroup_per_node *pn;
 	long x;
 
@@ -697,12 +698,20 @@ static inline void __mod_lruvec_state(struct lruvec *lruvec,
 	__mod_memcg_state(pn->memcg, idx, val);
 
 	/* Update lruvec */
-	x = val + __this_cpu_read(pn->lruvec_stat_cpu->count[idx]);
-	if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
-		atomic_long_add(x, &pn->lruvec_stat[idx]);
-		x = 0;
+	rcu_read_lock();
+	lruvec_stat_cpu = (struct lruvec_stat __percpu *)
+		rcu_dereference(pn->lruvec_stat_cpu);
+	if (likely(lruvec_stat_cpu)) {
+		x = val + __this_cpu_read(lruvec_stat_cpu->count[idx]);
+		if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
+			atomic_long_add(x, &pn->lruvec_stat[idx]);
+			x = 0;
+		}
+		__this_cpu_write(lruvec_stat_cpu->count[idx], x);
+	} else {
+		atomic_long_add(val, &pn->lruvec_stat[idx]);
 	}
-	__this_cpu_write(pn->lruvec_stat_cpu->count[idx], x);
+	rcu_read_unlock();
 }
 
 static inline void mod_lruvec_state(struct lruvec *lruvec,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 803c772f354b..8f3cac02221a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2122,6 +2122,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 static int memcg_hotplug_cpu_dead(unsigned int cpu)
 {
 	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
+	struct lruvec_stat __percpu *lruvec_stat_cpu;
 	struct memcg_stock_pcp *stock;
 	struct mem_cgroup *memcg;
 
@@ -2152,7 +2153,12 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
 				struct mem_cgroup_per_node *pn;
 
 				pn = mem_cgroup_nodeinfo(memcg, nid);
-				x = this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
+
+				lruvec_stat_cpu = (struct lruvec_stat __percpu*)
+					rcu_dereference(pn->lruvec_stat_cpu);
+				if (!lruvec_stat_cpu)
+					continue;
+				x = this_cpu_xchg(lruvec_stat_cpu->count[i], 0);
 				if (x)
 					atomic_long_add(x, &pn->lruvec_stat[i]);
 			}
@@ -4430,7 +4436,8 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
 	if (!pn)
 		return 1;
 
-	pn->lruvec_stat_cpu = alloc_percpu(struct lruvec_stat);
+	rcu_assign_pointer(pn->lruvec_stat_cpu,
+			   alloc_percpu(struct lruvec_stat));
 	if (!pn->lruvec_stat_cpu) {
 		kfree(pn);
 		return 1;
-- 
2.20.1