[PATCH] mm, memcg: avoid oom if cgroup is not populated

[Yafang Shao <laoar.shao@gmail.com>]

There's one case that the processes in a memcg are all exit (due to OOM
group or some other reasons), but the file page caches are still exist.
These file page caches may be protected by memory.min so can't be
reclaimed. If we can't success to restart the processes in this memcg or
don't want to make this memcg offline, then we want to drop the file page
caches.
The advantage of droping this file caches is it can avoid the reclaimer
(either kswapd or direct) scanning and reclaiming pages from all memcgs
exist in this system, because currently the reclaimer will fairly reclaim
pages from all memcgs if the system is under memory pressure.
The possible method to drop these file page caches is setting the
hard limit of this memcg to 0. Unfortunately this may invoke the OOM killer
and generates lots of misleading outputs, that should not happen.
One misleading output is "Out of memory and no killable processes...",
while really there is no tasks rather than no killable tasks. Furthermore,
the OOM output is not expected by the admin if he or she only wants to drop
the cahes and knows there're no processes running in this memcg.

If memcg is not populated, we should not invoke the OOM killer.

Fixes: b6e6edcf ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
---
 mm/memcontrol.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1c4c08b..4e08905 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6139,9 +6139,13 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 			continue;
 		}
 
-		memcg_memory_event(memcg, MEMCG_OOM);
-		if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
+		if (cgroup_is_populated(memcg->css.cgroup)) {
+			memcg_memory_event(memcg, MEMCG_OOM);
+			if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
+				break;
+		} else  {
 			break;
+		}
 	}
 
 	memcg_wb_domain_size_changed(memcg);
-- 
1.8.3.1