[Patch v2 3/3] mm/memcg: add next_mz back to soft limit tree if not reclaimed yet

[Wei Yang <richard.weiyang@gmail.com>]

When memory reclaim failed for a maximum number of attempts and we bail
out of the reclaim loop, we forgot to put the target mem_cgroup chosen
for next reclaim back to the soft limit tree. This prevented pages in
the mem_cgroup from being reclaimed in the future even though the
mem_cgroup exceeded its soft limit.

Let's say there are two mem_cgroup and both of them exceed the soft
limit, while the first one is more active then the second. Since we add
a mem_cgroup to soft limit tree every 1024 event, the second one just
get a rare chance to be put on soft limit tree even it exceeds the
limit.

As time goes on, the first mem_cgroup was kept close to its soft limit
due to reclaim activities, while the memory usage of the second
mem_cgroup keeps growing over the soft limit for a long time due to its
relatively rare occurrence.

This patch adds next_mz back to prevent this sceanrio.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
---
 mm/memcontrol.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 344a7e891bc5..e803ff02aae2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3493,8 +3493,13 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 			loop > MEM_CGROUP_MAX_SOFT_LIMIT_RECLAIM_LOOPS))
 			break;
 	} while (!nr_reclaimed);
-	if (next_mz)
+	if (next_mz) {
+		spin_lock_irq(&mctz->lock);
+		excess = soft_limit_excess(next_mz->memcg);
+		__mem_cgroup_insert_exceeded(next_mz, mctz, excess);
+		spin_unlock_irq(&mctz->lock);
 		css_put(&next_mz->memcg->css);
+	}
 	return nr_reclaimed;
 }
 
-- 
2.33.1