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[RFC PATCH] memcg, oom: throttle dump_header for memcg ooms without eligible tasks




From: Michal Hocko <mhocko@xxxxxxxx>

syzbot has noticed that it can trigger RCU stalls from the memcg oom
path:
RIP: 0010:dump_stack+0x358/0x3ab lib/dump_stack.c:118
Code: 74 0c 48 c7 c7 f0 f5 31 89 e8 9f 0e 0e fa 48 83 3d 07 15 7d 01 00 0f
84 63 fe ff ff e8 1c 89 c9 f9 48 8b bd 70 ff ff ff 57 9d <0f> 1f 44 00 00
e8 09 89 c9 f9 48 8b 8d 68 ff ff ff b8 ff ff 37 00
RSP: 0018:ffff88017d3a5c70 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
RAX: 0000000000040000 RBX: 1ffffffff1263ebe RCX: ffffc90001e5a000
RDX: 0000000000040000 RSI: ffffffff87b4e0f4 RDI: 0000000000000246
RBP: ffff88017d3a5d18 R08: ffff8801d7e02480 R09: fffffbfff13da030
R10: fffffbfff13da030 R11: 0000000000000003 R12: 1ffff1002fa74b96
R13: 00000000ffffffff R14: 0000000000000200 R15: 0000000000000000
  dump_header+0x27b/0xf72 mm/oom_kill.c:441
  out_of_memory.cold.30+0xf/0x184 mm/oom_kill.c:1109
  mem_cgroup_out_of_memory+0x15e/0x210 mm/memcontrol.c:1386
  mem_cgroup_oom mm/memcontrol.c:1701 [inline]
  try_charge+0xb7c/0x1710 mm/memcontrol.c:2260
  mem_cgroup_try_charge+0x627/0xe20 mm/memcontrol.c:5892
  mem_cgroup_try_charge_delay+0x1d/0xa0 mm/memcontrol.c:5907
  shmem_getpage_gfp+0x186b/0x4840 mm/shmem.c:1784
  shmem_fault+0x25f/0x960 mm/shmem.c:1982
  __do_fault+0x100/0x6b0 mm/memory.c:2996
  do_read_fault mm/memory.c:3408 [inline]
  do_fault mm/memory.c:3531 [inline]

The primary reason of the stall lies in an expensive printk handling
of oom report flood because a misconfiguration on the syzbot side
caused that there is simply no eligible task because they have
OOM_SCORE_ADJ_MIN set. This generates the oom report for each allocation
from the memcg context.

While normal workloads should be much more careful about potential heavy
memory consumers that are OOM disabled it makes some sense to rate limit
a potentially expensive oom reports for cases when there is no eligible
victim found. Do that by moving the rate limit logic inside dump_header.
We no longer rely on the caller to do that. It was only oom_kill_process
which has been throttling. Other two call sites simply didn't have to
care because one just paniced on the OOM when configured that way and
no eligible task would panic for the global case as well. Memcg changed
the picture because we do not panic and we might have multiple sources
of the same event.

Once we are here, make sure that the reason to trigger the OOM is
printed without ratelimiting because this is really valuable to
debug what happened.

Reported-by: syzbot+77e6b28a7a7106ad0def@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: guro@xxxxxx
Cc: hannes@xxxxxxxxxxx
Cc: kirill.shutemov@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: penguin-kernel@xxxxxxxxxxxxxxxxxxx
Cc: rientjes@xxxxxxxxxx
Cc: yang.s@xxxxxxxxxxxxxxx
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/oom_kill.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f10aa5360616..4ee393c85e27 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -430,6 +430,9 @@ static void dump_tasks(struct mem_cgroup *memcg, const nodemask_t *nodemask)
 
 static void dump_header(struct oom_control *oc, struct task_struct *p)
 {
+	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
+					      DEFAULT_RATELIMIT_BURST);
+
 	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), nodemask=%*pbl, order=%d, oom_score_adj=%hd\n",
 		current->comm, oc->gfp_mask, &oc->gfp_mask,
 		nodemask_pr_args(oc->nodemask), oc->order,
@@ -437,6 +440,9 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
 	if (!IS_ENABLED(CONFIG_COMPACTION) && oc->order)
 		pr_warn("COMPACTION is disabled!!!\n");
 
+	if (!__ratelimit(&oom_rs))
+		return;
+
 	cpuset_print_current_mems_allowed();
 	dump_stack();
 	if (is_memcg_oom(oc))
@@ -931,8 +937,6 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	struct task_struct *t;
 	struct mem_cgroup *oom_group;
 	unsigned int victim_points = 0;
-	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
-					      DEFAULT_RATELIMIT_BURST);
 
 	/*
 	 * If the task is already exiting, don't alarm the sysadmin or kill
@@ -949,8 +953,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	}
 	task_unlock(p);
 
-	if (__ratelimit(&oom_rs))
-		dump_header(oc, p);
+	dump_header(oc, p);
 
 	pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
 		message, task_pid_nr(p), p->comm, points);
-- 
2.19.0