[v2 PATCH 3/9] mm: vmscan: guarantee shrinker_slab_memcg() sees valid shrinker_maps for online memcg

[Yang Shi <shy828301@gmail.com>]

The shrink_slab_memcg() races with mem_cgroup_css_online(). A visibility of CSS_ONLINE flag
in shrink_slab_memcg()->mem_cgroup_online() does not guarantee that we will see
memcg->nodeinfo[nid]->shrinker_maps != NULL.  This may occur because of processor reordering
on !x86.

This seems like the below case:

           CPU A          CPU B
store shrinker_map      load CSS_ONLINE
store CSS_ONLINE        load shrinker_map

So the memory ordering could be guaranteed by smp_wmb()/smp_rmb() pair.

The memory barriers pair will guarantee the ordering between shrinker_deferred and CSS_ONLINE
for the following patches as well.

Signed-off-by: Yang Shi <shy828301@gmail.com>
---
 mm/memcontrol.c | 7 +++++++
 mm/vmscan.c     | 8 +++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ed942734235f..3d4ddbb84a01 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5406,6 +5406,13 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 		return -ENOMEM;
 	}
 
+	/*
+	 * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees shirnker_maps
+	 * and shrinker_deferred before CSS_ONLINE. It pairs with the read barrier
+	 * in shrink_slab_memcg().
+	 */
+	smp_wmb();
+
 	/* Online state pins memcg ID, memcg ID pins CSS */
 	refcount_set(&memcg->id.ref, 1);
 	css_get(css);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 912c044301dd..9b31b9c419ec 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -552,13 +552,15 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 	if (!mem_cgroup_online(memcg))
 		return 0;
 
+	/* Pairs with write barrier in mem_cgroup_css_online() */
+	smp_rmb();
+
 	if (!down_read_trylock(&shrinker_rwsem))
 		return 0;
 
+	/* Once memcg is online it can't be NULL */
 	map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,
 					true);
-	if (unlikely(!map))
-		goto unlock;
 
 	for_each_set_bit(i, map->map, shrinker_nr_max) {
 		struct shrink_control sc = {
@@ -612,7 +614,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 			break;
 		}
 	}
-unlock:
+
 	up_read(&shrinker_rwsem);
 	return freed;
 }
-- 
2.26.2