OOM notification for cgroupsv1 broken in 4.19

Hi,

This patch: 29ef680ae7c21110af8e6416d84d8a72fc147b14 [PATCH] memcg, oom: move out_of_memory back to the charge path

has broken the eventfd notification for cgroups-v1. This is because mem_cgroup_oom_notify() is called only in mem_cgroup_oom_synchronize and not with the new, additional call to mem_cgroup_out_of_memory in the charge path.

- B

Yes, you are right and this is a clear regression. Does the following patch fixes the issue for you? I am not super happy about the code duplication but I wasn't able to separate this out from mem_cgroup_oom_synchronize because that one has to handle the oom_killer disabled case which is not the case in the charge path because we simply back off and hand over to mem_cgroup_oom_synchronize in that case.
---

From: Michal Hocko <[email protected]>
Date: Fri, 21 Dec 2018 16:28:29 +0100
Subject: [PATCH] memcg, oom: notify on oom killer invocation from the charge
path

Burt Holzman has noticed that memcg v1 doesn't notify about OOM events via eventfd anymore. The reason is that 29ef680ae7c2 ("memcg, oom: move out_of_memory back to the charge path") has moved the oom handling back to the charge path. While doing so the notification was left behind in mem_cgroup_oom_synchronize.

Fix the issue by replicating the oom hierarchy locking and the notification.

---
 mm/memcontrol.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

index 6e1469b80cb7..7e6bf74ddb1e 100644
@@ -1666,6 +1666,9 @@ enum oom_status {
 
 static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
 {
+	enum oom_status ret;
+	bool locked;
+
 	if (order > PAGE_ALLOC_COSTLY_ORDER)
 		return OOM_SKIPPED;
 

@@ -1700,10 +1703,23 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
 		return OOM_ASYNC;
 	}
 
+	mem_cgroup_mark_under_oom(memcg);
+
+	locked = mem_cgroup_oom_trylock(memcg);
+
+	if (locked)
+		mem_cgroup_oom_notify(memcg);
+
+	mem_cgroup_unmark_under_oom(memcg);
 	if (mem_cgroup_out_of_memory(memcg, mask, order))
-		return OOM_SUCCESS;
+		ret = OOM_SUCCESS;
+	else
+		ret = OOM_FAILED;
 
-	return OOM_FAILED;
+	if (locked)
+		mem_cgroup_oom_unlock(memcg);
+
+	return ret;
 }
 
 /**

2.19.2

-- 
Michal Hocko
SUSE Labs

Hi Michal,

Thanks for the quick response & patch. I can confirm that with this patch the notification is working for my sample test case.

- B

Looks good to me. The async side really does too much other stuff to cleanly share code between them, so I don't mind separate code even if it means they both have to do the mark, lock, notify dance.

From: Michal Hocko <[email protected]>

Burt Holzman has noticed that memcg v1 doesn't notify about OOM events via eventfd anymore. The reason is that 29ef680ae7c2 ("memcg, oom: move out_of_memory back to the charge path") has moved the oom handling back to the charge path. While doing so the notification was left behind in mem_cgroup_oom_synchronize.

Fix the issue by replicating the oom hierarchy locking and the notification.

---
Hi Andrew,
I forgot to CC you on the patch sent as a reply to the original bug report [1] so I am reposting with Ack from Johannes. Burt has confirmed this is resolving the regression for him [2]. 4.20 is out but I have marked the patch for stable so it should hit both 4.19 and 4.20.

[1] http://lkml.kernel.org/r/[email protected]
[2] http://lkml.kernel.org/r/[email protected]

 mm/memcontrol.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

index 6e1469b80cb7..7e6bf74ddb1e 100644
@@ -1666,6 +1666,9 @@ enum oom_status {
 
 static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
 {
+	enum oom_status ret;
+	bool locked;
+
 	if (order > PAGE_ALLOC_COSTLY_ORDER)
 		return OOM_SKIPPED;
 

@@ -1700,10 +1703,23 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
 		return OOM_ASYNC;
 	}
 
+	mem_cgroup_mark_under_oom(memcg);
+
+	locked = mem_cgroup_oom_trylock(memcg);
+
+	if (locked)
+		mem_cgroup_oom_notify(memcg);
+
+	mem_cgroup_unmark_under_oom(memcg);
 	if (mem_cgroup_out_of_memory(memcg, mask, order))
-		return OOM_SUCCESS;
+		ret = OOM_SUCCESS;
+	else
+		ret = OOM_FAILED;
 
-	return OOM_FAILED;
+	if (locked)
+		mem_cgroup_oom_unlock(memcg);
+
+	return ret;
 }
 
 /**

2.19.2