[PATCH mm-unstable v4 3/5] mm: increment gen # before restarting traversal

[Kinsey Ho <kinseyho@google.com>]

The generation number in struct mem_cgroup_reclaim_iter should be
incremented on every round-trip. Currently, it is possible for a
concurrent reclaimer to jump in at the end of the hierarchy, causing a
traversal restart (resetting the iteration position) without
incrementing the generation number.

By resetting the position without incrementing the generation, it's
possible for another ongoing mem_cgroup_iter() thread to walk the tree
twice.

Move the traversal restart such that the generation number is
incremented before the restart.

Signed-off-by: Kinsey Ho <kinseyho@google.com>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Hugh Dickins <hughd@google.com>
---
 mm/memcontrol.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cdd324812b55..c24ef6a106e0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -999,7 +999,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		root = root_mem_cgroup;
 
 	rcu_read_lock();
-
+restart:
 	if (reclaim) {
 		struct mem_cgroup_per_node *mz;
 
@@ -1026,14 +1026,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 	for (;;) {
 		css = css_next_descendant_pre(css, &root->css);
 		if (!css) {
-			/*
-			 * Reclaimers share the hierarchy walk, and a
-			 * new one might jump in right at the end of
-			 * the hierarchy - make sure they see at least
-			 * one group and restart from the beginning.
-			 */
-			if (!prev)
-				continue;
 			break;
 		}
 
@@ -1056,8 +1048,18 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		 */
 		(void)cmpxchg(&iter->position, pos, memcg);
 
-		if (!memcg)
+		if (!memcg) {
 			iter->generation++;
+
+			/*
+			 * Reclaimers share the hierarchy walk, and a
+			 * new one might jump in right at the end of
+			 * the hierarchy - make sure they see at least
+			 * one group and restart from the beginning.
+			 */
+			if (!prev)
+				goto restart;
+		}
 	}
 
 out_unlock:
-- 
2.46.0.469.g59c65b2a67-goog