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

Vasily Averin
2021-10-21T13:24:27Z

On 21.10.2021 14:49, Michal Hocko wrote:
> On Thu 21-10-21 11:03:43, Vasily Averin wrote:
>> On 18.10.2021 12:04, Michal Hocko wrote:
>>> On Mon 18-10-21 11:13:52, Vasily Averin wrote:
>>> [...]
>>>> How could this happen?
>>>>
>>>> User-space task inside the memcg-limited container generated a page fault,
>>>> its handler do_user_addr_fault() called handle_mm_fault which could not
>>>> allocate the page due to exceeding the memcg limit and returned VM_FAULT_OOM.
>>>> Then do_user_addr_fault() called pagefault_out_of_memory() which executed
>>>> out_of_memory() without set of memcg.
>>
>>> I will be honest that I am not really happy about pagefault_out_of_memory.
>>> I have tried to remove it in the past. Without much success back then,
>>> unfortunately[1].
>>>
>>> [1] I do not have msg-id so I cannot provide a lore link but google
>>> pointed me to https://www.mail-archive.com/linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org/msg1400402.html
>>
>> I re-read this discussion and in general I support your position.
>> As far as I understand your opponents cannot explain why "random kill" is mandatory here,
>> they are just afraid that it might be useful here and do not want to remove it completely.
>
> That aligns with my recollection.
>
>> Ok, let's allow him to do it. Moreover I'm ready to keep it as default behavior.
>>
>> However I would like to have some choice in this point.
>>
>> In general we can:
>> - continue to use "random kill" and rely on the wisdom of the ancestors.
>
> I do not follow. Does that mean to preserve existing oom killer from
> #PF?
>
>> - do nothing, repeat #PF and rely on fate: "nothing bad will happen if we do it again".
>> - add some (progressive) killable delay, rely on good will of (unkillable) neighbors and wait for them to release required memory.
>
> Again, not really sure what you mean
>
>> - mark the current task as cycled in #PF and somehow use this mark in allocator
>
> How?
>
>> - make sure that the current task is really cycled, have no progress, send him fatal signal to kill it and break the cycle.
>
> No! We cannot really kill the task if we could we would have done it by
> the oom killer already
>
>> - implement any better ideas,
>> - use any combination of previous points
>>
>> We can select required strategy for example via sysctl.
>
> Absolutely no! How can admin know any better than the kernel?
>
>> For me "random kill" is worst choice,
>> Why can't we just kill the looped process instead?
>
> See above.
>
>> It can be marked as oom-unkillable, so OOM-killer was unable to select it.
>> However I doubt it means "never kill it", for me it is something like "last possible victim" priority.
>
> It means never kill it because of OOM. If it is retrying because of OOM
> then it is effectively the same thing.
>
> The oom killer from the #PF doesn't really provide any clear advantage
> these days AFAIK. On the other hand it allows for a very disruptive
> behavior. In a worst case it can lead to a system panic if the
> VM_FAULT_OOM is not really caused by a memory shortage but rather a
> wrong error handling. If a task is looping there without any progress
> then it is still kilallable which is a much saner behavior IMHO.

Let's continue this discussion in "Re: [PATCH memcg 3/3] memcg: handle memcg oom failures" thread.
.