[PATCH v2] mm: memcontrol: Don't flood OOM messages with no eligible task.

[Tetsuo Handa]

syzbot is hitting RCU stall at shmem_fault() [1].
This is because memcg-OOM events with no eligible task (current thread
is marked as OOM-unkillable) continued calling dump_header() from
out_of_memory() enabled by commit 3100dab2aa09dc6e ("mm: memcontrol:
print proper OOM header when no eligible victim left.").

Let's make sure that next dump_header() waits for at least 60 seconds from
previous "Out of memory and no killable processes..." message, if requested
memcg already reported it. This change allows threads in requested memcg to
complete memory allocation requests for doing recovery operation, and also
allows us to know whether threads in requested memcg is doing recovery
operation.

[1] https://syzkaller.appspot.com/bug?id=4ae3fff7fcf4c33a47c1192d2d62d2e03efffa64

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzbot+77e6b28a7a7106ad0def@xxxxxxxxxxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
---
 include/linux/memcontrol.h |  3 +++
 mm/oom_kill.c              | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 652f602..8da2340 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -230,6 +230,9 @@ struct mem_cgroup {
 	 */
 	bool oom_group;
 
+	/* Previous OOM-kill request failed due to no eligible task? */
+	bool oom_no_eligible_warned;
+
 	/* protected by memcg_oom_lock */
 	bool		oom_lock;
 	int		under_oom;
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f10aa53..52a8319 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1106,6 +1106,21 @@ bool out_of_memory(struct oom_control *oc)
 	select_bad_process(oc);
 	/* Found nothing?!?! */
 	if (!oc->chosen) {
+#ifdef CONFIG_MEMCG
+		/*
+		 * Don't flood with dump_header() if already reported, in case
+		 * current->signal->oom_score_adj == OOM_SCORE_ADJ_MIN. Maybe
+		 * this variable should be per "struct mem_cgroup". But since
+		 * we can't prove that multiple concurrent memcg OOM without
+		 * eligible task won't cause flooding, choose global variable
+		 * for safety.
+		 */
+		static u64 last_warned;
+
+		if (is_memcg_oom(oc) && oc->memcg->oom_no_eligible_warned &&
+		    time_before64(get_jiffies_64(), last_warned + 60 * HZ))
+			return false;
+#endif
 		dump_header(oc, NULL);
 		pr_warn("Out of memory and no killable processes...\n");
 		/*
@@ -1115,6 +1130,14 @@ bool out_of_memory(struct oom_control *oc)
 		 */
 		if (!is_sysrq_oom(oc) && !is_memcg_oom(oc))
 			panic("System is deadlocked on memory\n");
+#ifdef CONFIG_MEMCG
+		if (is_memcg_oom(oc)) {
+			oc->memcg->oom_no_eligible_warned = true;
+			last_warned = get_jiffies_64();
+		}
+	} else if (is_memcg_oom(oc)) {
+		oc->memcg->oom_no_eligible_warned = false;
+#endif
 	}
 	if (oc->chosen && oc->chosen != (void *)-1UL)
 		oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" :
-- 
1.8.3.1