Re: WARNING in try_charge

Tetsuo Handa
2018-08-06T21:50:20Z

On 2018/08/07 5:55, Michal Hocko wrote:
> On Tue 07-08-18 05:46:04, Tetsuo Handa wrote:
>> On 2018/08/07 5:34, Michal Hocko wrote:
>>> On Tue 07-08-18 05:26:23, Tetsuo Handa wrote:
>>>> On 2018/08/07 2:56, Michal Hocko wrote:
>>>>> So the oom victim indeed passed the above force path after the oom
>>>>> invocation. But later on hit the page fault path and that behaved
>>>>> differently and for some reason the force path hasn't triggered. I am
>>>>> wondering how could we hit the page fault path in the first place. The
>>>>> task is already killed! So what the hell is going on here.
>>>>>
>>>>> I must be missing something obvious here.
>>>>>
>>>> YOU ARE OBVIOUSLY MISSING MY MAIL!
>>>>
>>>> I already said this is "mm, oom: task_will_free_mem(current) should ignore MMF_OOM_SKIP for once."
>>>> problem which you are refusing at https://www.spinics.net/lists/linux-mm/msg133774.html .
>>>> And you again ignored my mail. Very sad...
>>>
>>> Your suggestion simply didn't make much sense. There is nothing like
>>> first check is different from the rest.
>>>
>>
>> I don't think your patch is appropriate. It avoids hitting WARN(1) but does not avoid
>> unnecessary killing of OOM victims.
>>
>> If you look at https://syzkaller.appspot.com/text?tag=CrashLog&x=15a1c770400000 , you will
>> notice that both 23766 and 23767 are killed due to task_will_free_mem(current) == false.
>> This is "unnecessary killing of additional processes".
>
> Have you noticed the mere detail that the memcg has to kill any task
> attempting the charge because the hard limit is 0? There is simply no
> other way around. You cannot charge. There is no unnecessary killing.
> Full stop. We do allow temporary breach of the hard limit just to let
> the task die and uncharge on the way out.
>

select_bad_process() is called just because
task_will_free_mem("already killed current thread which has not completed __mmput()") == false
is a bug. I'm saying that the OOM killer should not give up as soon as MMF_OOM_SKIP is set.

static bool oom_has_pending_victims(struct oom_control *oc)
{
struct task_struct *p, *tmp;
bool ret = false;
bool gaveup = false;

if (is_sysrq_oom(oc))
return false;
/*
* Wait for pending victims until __mmput() completes or stalled
* too long.
*/
list_for_each_entry_safe(p, tmp, &oom_victim_list, oom_victim_list) {
struct mm_struct *mm = p->signal->oom_mm;

if (oom_unkillable_task(p, oc->memcg, oc->nodemask))
continue;
ret = true;
+ /*
+ * Since memcg OOM allows forced charge, we can safely wait
+ * until __mmput() completes.
+ */
+ if (is_memcg_oom(oc))
+ return true;
#ifdef CONFIG_MMU
/*
* Since the OOM reaper exists, we can safely wait until
* MMF_OOM_SKIP is set.
*/
if (!test_bit(MMF_OOM_SKIP, &mm->flags)) {
if (!oom_reap_target) {
get_task_struct(p);
oom_reap_target = p;
trace_wake_reaper(p->pid);
wake_up(&oom_reaper_wait);
}
#endif
continue;
}
#endif
/* We can wait as long as OOM score is decreasing over time. */
if (!victim_mm_stalling(p, mm))
continue;
gaveup = true;
list_del(&p->oom_victim_list);
/* Drop a reference taken by mark_oom_victim(). */
put_task_struct(p);
}
if (gaveup)
debug_show_all_locks();

return ret;
}