[PATCH v3 7/8] memcg: warn for unexpected events and stats

[Shakeel Butt <shakeel.butt@linux.dev>]

To reduce memory usage by the memcg events and stats, the kernel uses
indirection table and only allocate stats and events which are being
used by the memcg code. To make this more robust, let's add warnings
where unexpected stats and events indexes are used.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---

Changes since v2:
- Based on feedback from Johannes, switched to WARN_ONCE() from
  pr_warn_once().

 mm/memcontrol.c | 55 ++++++++++++++++++++++++++++---------------------
 1 file changed, 32 insertions(+), 23 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index df94abc0088f..72e36977a96e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -664,17 +664,18 @@ struct lruvec_stats {
 unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx)
 {
 	struct mem_cgroup_per_node *pn;
-	long x = 0;
+	long x;
 	int i;
 
 	if (mem_cgroup_disabled())
 		return node_page_state(lruvec_pgdat(lruvec), idx);
 
 	i = memcg_stats_index(idx);
-	if (i >= 0) {
-		pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
-		x = READ_ONCE(pn->lruvec_stats->state[i]);
-	}
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, idx))
+		return 0;
+
+	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
+	x = READ_ONCE(pn->lruvec_stats->state[i]);
 #ifdef CONFIG_SMP
 	if (x < 0)
 		x = 0;
@@ -686,17 +687,18 @@ unsigned long lruvec_page_state_local(struct lruvec *lruvec,
 				      enum node_stat_item idx)
 {
 	struct mem_cgroup_per_node *pn;
-	long x = 0;
+	long x;
 	int i;
 
 	if (mem_cgroup_disabled())
 		return node_page_state(lruvec_pgdat(lruvec), idx);
 
 	i = memcg_stats_index(idx);
-	if (i >= 0) {
-		pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
-		x = READ_ONCE(pn->lruvec_stats->state_local[i]);
-	}
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, idx))
+		return 0;
+
+	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
+	x = READ_ONCE(pn->lruvec_stats->state_local[i]);
 #ifdef CONFIG_SMP
 	if (x < 0)
 		x = 0;
@@ -921,7 +923,7 @@ unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
 	long x;
 	int i = memcg_stats_index(idx);
 
-	if (i < 0)
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, idx))
 		return 0;
 
 	x = READ_ONCE(memcg->vmstats->state[i]);
@@ -958,7 +960,10 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val)
 {
 	int i = memcg_stats_index(idx);
 
-	if (mem_cgroup_disabled() || i < 0)
+	if (mem_cgroup_disabled())
+		return;
+
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, idx))
 		return;
 
 	__this_cpu_add(memcg->vmstats_percpu->state[i], val);
@@ -971,7 +976,7 @@ static unsigned long memcg_page_state_local(struct mem_cgroup *memcg, int idx)
 	long x;
 	int i = memcg_stats_index(idx);
 
-	if (i < 0)
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, idx))
 		return 0;
 
 	x = READ_ONCE(memcg->vmstats->state_local[i]);
@@ -990,7 +995,7 @@ static void __mod_memcg_lruvec_state(struct lruvec *lruvec,
 	struct mem_cgroup *memcg;
 	int i = memcg_stats_index(idx);
 
-	if (i < 0)
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, idx))
 		return;
 
 	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
@@ -1104,34 +1109,38 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
 void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 			  unsigned long count)
 {
-	int index = memcg_events_index(idx);
+	int i = memcg_events_index(idx);
 
-	if (mem_cgroup_disabled() || index < 0)
+	if (mem_cgroup_disabled())
+		return;
+
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, idx))
 		return;
 
 	memcg_stats_lock();
-	__this_cpu_add(memcg->vmstats_percpu->events[index], count);
+	__this_cpu_add(memcg->vmstats_percpu->events[i], count);
 	memcg_rstat_updated(memcg, count);
 	memcg_stats_unlock();
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
 {
-	int index = memcg_events_index(event);
+	int i = memcg_events_index(event);
 
-	if (index < 0)
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, event))
 		return 0;
-	return READ_ONCE(memcg->vmstats->events[index]);
+
+	return READ_ONCE(memcg->vmstats->events[i]);
 }
 
 static unsigned long memcg_events_local(struct mem_cgroup *memcg, int event)
 {
-	int index = memcg_events_index(event);
+	int i = memcg_events_index(event);
 
-	if (index < 0)
+	if (WARN_ONCE(i < 0, "%s: missing stat item %d\n", __func__, event))
 		return 0;
 
-	return READ_ONCE(memcg->vmstats->events_local[index]);
+	return READ_ONCE(memcg->vmstats->events_local[i]);
 }
 
 static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
-- 
2.43.0