Re: [PATCH memcg 0/1] false global OOM triggered by memcg-limited task

[Michal Hocko <mhocko@suse.com>]

On Tue 19-10-21 16:26:50, Vasily Averin wrote:
> On 19.10.2021 15:04, Michal Hocko wrote:
> > On Tue 19-10-21 13:54:42, Michal Hocko wrote:
> >> On Tue 19-10-21 13:30:06, Vasily Averin wrote:
> >>> On 19.10.2021 11:49, Michal Hocko wrote:
> >>>> On Tue 19-10-21 09:30:18, Vasily Averin wrote:
> >>>> [...]
> >>>>> With my patch ("memcg: prohibit unconditional exceeding the limit of dying tasks") try_charge_memcg() can fail:
> >>>>> a) due to fatal signal
> >>>>> b) when mem_cgroup_oom -> mem_cgroup_out_of_memory -> out_of_memory() returns false (when select_bad_process() found nothing)
> >>>>>
> >>>>> To handle a) we can follow to your suggestion and skip excution of out_of_memory() in pagefault_out_of memory()
> >>>>> To handle b) we can go to retry: if mem_cgroup_oom() return OOM_FAILED.
> >>>
> >>>> How is b) possible without current being killed? Do we allow remote
> >>>> charging?
> >>>
> >>> out_of_memory for memcg_oom
> >>>  select_bad_process
> >>>   mem_cgroup_scan_tasks
> >>>    oom_evaluate_task
> >>>     oom_badness
> >>>
> >>>         /*
> >>>          * Do not even consider tasks which are explicitly marked oom
> >>>          * unkillable or have been already oom reaped or the are in
> >>>          * the middle of vfork
> >>>          */
> >>>         adj = (long)p->signal->oom_score_adj;
> >>>         if (adj == OOM_SCORE_ADJ_MIN ||
> >>>                         test_bit(MMF_OOM_SKIP, &p->mm->flags) ||
> >>>                         in_vfork(p)) {
> >>>                 task_unlock(p);
> >>>                 return LONG_MIN;
> >>>         }
> >>>
> >>> This time we handle userspace page fault, so we cannot be kenrel thread,
> >>> and cannot be in_vfork().
> >>> However task can be marked as oom unkillable, 
> >>> i.e. have p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN
> >>
> >> You are right. I am not sure there is a way out of this though. The task
> >> can only retry for ever in this case. There is nothing actionable here.
> >> We cannot kill the task and there is no other way to release the memory.
> > 
> > Btw. don't we force the charge in that case?
> 
> We should force charge for allocation from inside page fault handler,
> to prevent endless cycle in retried page faults.
> However we should not do it for allocations from task context,
> to prevent memcg-limited vmalloc-eaters from to consume all host memory.

I don't see a big difference between those two. Because the #PF could
result into the very same situation depleting all the memory by
overcharging. A different behavior just leads to a confusion and
unexpected behavior. E.g. in the past we only triggered memcg OOM killer
from the #PF path and failed the charge otherwise. That is something
different but it shows problems we haven't anticipated and had user
visible problems. See 29ef680ae7c2 ("memcg, oom: move out_of_memory back
to the charge path").

> Also I would like to return to the following hunk.
> @@ -1575,7 +1575,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	 * A few threads which were not waiting at mutex_lock_killable() can
>  	 * fail to bail out. Therefore, check again after holding oom_lock.
>  	 */
> -	ret = should_force_charge() || out_of_memory(&oc);
> +	ret = task_is_dying() || out_of_memory(&oc);
>  
>  unlock:
>  	mutex_unlock(&oom_lock);
> 
> Now I think it's better to keep task_is_dying() check here.
> if task is dying, it is not necessary to push other task to free the memory.
> We broke vmalloc cycle already, so it looks like nothing should prevent us
> from returning to userspace, handle fatal signal, exit and free the memory.

That patch has to be discuss in its full length. There were other
details I have brought up AFAIU.
-- 
Michal Hocko
SUSE Labs