[v3 PATCH 03/11] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation

[Yang Shi]

Since memcg_shrinker_map_size just can be changd under holding shrinker_rwsem
exclusively, the read side can be protected by holding read lock, so it sounds
superfluous to have a dedicated mutex.  This should not exacerbate the contention
to shrinker_rwsem since just one read side critical section is added.

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 mm/vmscan.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9db7b4d6d0ae..ddb9f972f856 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -187,7 +187,6 @@ static DECLARE_RWSEM(shrinker_rwsem);
 #ifdef CONFIG_MEMCG
 
 static int memcg_shrinker_map_size;
-static DEFINE_MUTEX(memcg_shrinker_map_mutex);
 
 static void memcg_free_shrinker_map_rcu(struct rcu_head *head)
 {
@@ -200,8 +199,6 @@ static int memcg_expand_one_shrinker_map(struct mem_cgroup *memcg,
 	struct memcg_shrinker_map *new, *old;
 	int nid;
 
-	lockdep_assert_held(&memcg_shrinker_map_mutex);
-
 	for_each_node(nid) {
 		old = rcu_dereference_protected(
 			mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true);
@@ -250,7 +247,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
 	if (mem_cgroup_is_root(memcg))
 		return 0;
 
-	mutex_lock(&memcg_shrinker_map_mutex);
+	down_read(&shrinker_rwsem);
 	size = memcg_shrinker_map_size;
 	for_each_node(nid) {
 		map = kvzalloc(sizeof(*map) + size, GFP_KERNEL);
@@ -261,7 +258,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
 		}
 		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map);
 	}
-	mutex_unlock(&memcg_shrinker_map_mutex);
+	up_read(&shrinker_rwsem);
 
 	return ret;
 }
@@ -276,9 +273,8 @@ static int memcg_expand_shrinker_maps(int new_id)
 	if (size <= old_size)
 		return 0;
 
-	mutex_lock(&memcg_shrinker_map_mutex);
 	if (!root_mem_cgroup)
-		goto unlock;
+		goto out;
 
 	memcg = mem_cgroup_iter(NULL, NULL, NULL);
 	do {
@@ -287,13 +283,13 @@ static int memcg_expand_shrinker_maps(int new_id)
 		ret = memcg_expand_one_shrinker_map(memcg, size, old_size);
 		if (ret) {
 			mem_cgroup_iter_break(NULL, memcg);
-			goto unlock;
+			goto out;
 		}
 	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
-unlock:
+out:
 	if (!ret)
 		memcg_shrinker_map_size = size;
-	mutex_unlock(&memcg_shrinker_map_mutex);
+
 	return ret;
 }
 
-- 
2.26.2