David Rientjes wrote:

On Thu, 14 Jan 2016, Tetsuo Handa wrote:

quoted

quoted

quoted

quoted

quoted

@@ -171,7 +195,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 	if (oom_unkillable_task(p, memcg, nodemask))
 		return 0;

-	p = find_lock_task_mm(p);
+	p = find_lock_non_victim_task_mm(p);
 	if (!p)
 		return 0;

I understand how this may make your test case pass, but I simply don't 
understand how this could possibly be the correct thing to do.  This would 
cause oom_badness() to return 0 for any process where a thread has 
TIF_MEMDIE set.  If the oom killer is called from the page allocator, 
kills a thread, and it is recalled before that thread may exit, then this 
will panic the system if there are no other eligible processes to kill.

Why? oom_badness() is called after oom_scan_process_thread() returned OOM_SCAN_OK.
oom_scan_process_thread() returns OOM_SCAN_ABORT if a thread has TIF_MEMDIE set.

oom_scan_process_thread() checks for TIF_MEMDIE on p, not on p's threads.
If one of p's threads has TIF_MEMDIE set and p does not, we actually want 
to set TIF_MEMDIE for p.  That's the current behavior since it will lead 
to p->mm memory freeing.  Your patch is excluding such processes entirely 
and selecting another process to kill unnecessarily.

I think p's threads are checked by oom_scan_process_thread() for TIF_MEMDIE
even if p does not have TIF_MEMDIE. What am I misunderstanding about what
for_each_process_thread(g, p) is doing?

  #define for_each_process_thread(p, t) for_each_process(p) for_each_thread(p, t)

  select_bad_process() {
    for_each_process_thread(g, p) {
      oom_scan_process_thread(oc, p, totalpages));
      oom_badness(p);
    }
  }

Yes, select_bad_process() iterates over threads, that is obvious.  The 
point is that today it can select a thread independent of whether any of 
its other threads have TIF_MEMDIE set, which is the desired behavior per 
the above.  With your change, that is no longer possible because we 
disregard _all_ threads if one of them has TIF_MEMDIE set.

I still cannot understand. Today select_bad_process() can select a thread
independent of whether any of its other threads have TIF_MEMDIE set. But
select_bad_process() after all ignores that thread selected by oom_badness()
logic and aborts the iterate loop as soon as oom_scan_process_thread() finds
a TIF_MEMDIE thread from all threads. Changing oom_badness() logic to skip
processes with TIF_MEMDIE threads does not change the task select_bad_process()
logic will finally return.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>