Tetsuo Handa wrote:

If we can tolerate lack of process name and its pid when reporting
success/failure (or we pass them via mm_struct or walk the process list or
whatever else), I think we can do something like below patch (most revert of
"oom: clear TIF_MEMDIE after oom_reaper managed to unmap the address space").

 	if (attempts > MAX_OOM_REAP_RETRIES) {
-		pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
-				task_pid_nr(tsk), tsk->comm);
+		pr_info("oom_reaper: unable to reap memory\n");
 		debug_show_all_locks();
 	}

Since possible cause of unable to reap memory for oom_reap_vmas() is limited to

  Somebody was waiting at down_write(&mm->mmap_sem)
  (where converting to down_write_killable(&mm->mmap_sem) helps).

or

  Somebody was waiting on unkillable lock between
  down_write(&mm->mmap_sem) and up_write(&mm->mmap_sem)
  (where we will need to convert such locks killable).

or

  Somebody was doing !__GFP_FS && !__GFP_NOFAIL allocation between
  down_write(&mm->mmap_sem) and up_write(&mm->mmap_sem), and unable
  to call out_of_memory() in order to acquire TIF_MEMDIE (where setting
  TIF_MEMDIE to all threads using that mm by oom_kill_process() helps).

or

  Somebody was doing __GFP_FS || __GFP_NOFAIL allocation between
  down_write(&mm->mmap_sem) and up_write(&mm->mmap_sem), but unable
  to call out_of_memory() for more than one second due to oom_lock
  contention and/or scheduling priority (where setting TIF_MEMDIE
  to all threads using that mm by oom_kill_process() helps).

, we want to check traces of threads using that mm rather than locks
held by all threads. In addition to that, CONFIG_PROVE_LOCKING is not
enabled in most production systems.

I think below patch is more helpful than debug_show_all_locks().
(Though kmallocwd patch will report "unable to reap mm" case and
"unable to leave too_many_isolated() loop" case and any other
not-yet-identified cases which stall memory allocation.)

----------

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 2199c71..affbb79 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c

@@ -502,8 +502,26 @@ static void oom_reap_vmas(struct mm_struct *mm)
 		schedule_timeout_idle(HZ/10);
 
 	if (attempts > MAX_OOM_REAP_RETRIES) {
+		struct task_struct *p;
+		struct task_struct *t;
+
 		pr_info("oom_reaper: unable to reap memory\n");
-		debug_show_all_locks();
+		rcu_read_lock();
+		for_each_process_thread(p, t) {
+			if (likely(t->mm != mm))
+				continue;
+			pr_info("oom_reaper: %s(%u) flags=0x%x%s%s%s%s\n",
+				t->comm, t->pid, t->flags,
+				(t->state & TASK_UNINTERRUPTIBLE) ?
+				" uninterruptible" : "",
+				(t->flags & PF_EXITING) ? " exiting" : "",
+				fatal_signal_pending(t) ? " dying" : "",
+				test_tsk_thread_flag(t, TIF_MEMDIE) ?
+				" victim" : "");
+			sched_show_task(t);
+			debug_show_held_locks(t);
+		}
+		rcu_read_unlock();
 	}
 
 	/* Drop a reference taken by wake_oom_reaper */
----------

Well, I think we can define CONFIG_OOM_REAPER which defaults to y
and depends on CONFIG_MMU, rather than scatter around CONFIG_MMU.
That will help catching build failure on CONFIG_MMU=n case...

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