Re: [PATCH memcg 3/3] memcg: handle memcg oom failures

[Michal Hocko <mhocko@suse.com>]

On Thu 21-10-21 18:05:28, Vasily Averin wrote:
> On 21.10.2021 14:49, Michal Hocko wrote:
> > I do understand that handling a very specific case sounds easier but it
> > would be better to have a robust fix even if that requires some more
> > head scratching. So far we have collected several reasons why the it is
> > bad to trigger oom killer from the #PF path. There is no single argument
> > to keep it so it sounds like a viable path to pursue. Maybe there are
> > some very well hidden reasons but those should be documented and this is
> > a great opportunity to do either of the step.
> > 
> > Moreover if it turns out that there is a regression then this can be
> > easily reverted and a different, maybe memcg specific, solution can be
> > implemented.
> 
> Now I'm agree,
> however I still have a few open questions.
> 
> 1) VM_FAULT_OOM may be triggered w/o execution of out_of_memory()
> for exampel it can be caused by incorrect vm fault operations, 
> (a) which can return this error without calling allocator at all.

I would argue this to be a bug. How can that particular code tell
whether the system is OOM and the oom killer is the a reasonable measure
to take?

> (b) or which can provide incorrect gfp flags and allocator can fail without execution of out_of_memory.

I am not sure I can see any sensible scenario where pagefault oom killer
would be an appropriate fix for that.

> (c) This may happen on stable/LTS kernels when successful allocation was failed by hit into limit of legacy memcg-kmem contoller.
> We'll drop it in upstream kernels, however how to handle it in old kenrels?

Triggering the global oom killer for legacy kmem charge failure is
clearly wrong. Removing oom killer from #PF would fix that problem.

> We can make sure that out_of_memory or alocator was called by set of some per-task flags.

I am not sure I see how that would be useful other than reporting a
dubious VM_FAULT_OOM usage. I am also not sure how that would be
implemented as allocator can be called several times not to mention that
the allocation itself could have been done from a different context -
e.g. WQ.

> Can pagefault_out_of_memory() send itself a SIGKILL in all these cases?

In principle it can as sending signal is not prohibited. I would argue
it should not though because it is just wrong thing to do in all those
cases.

> If not -- task will be looped. 

Yes, but it will be killable from userspace. So this is not an
unrecoverable situation.
> It is much better than execution of global OOM, however it would be even better to avoid it somehow.

How?

> You said: "We cannot really kill the task if we could we would have done it by the oom killer already".
> However what to do if we even not tried to use oom-killer? (see (b) and (c)) 
> or if we did not used the allocator at all (see (a))

See above

> 2) in your patch we just exit from pagefault_out_of_memory(). and restart new #PF.
> We can call schedule_timeout() and wait some time before a new #PF restart.
> Additionally we can increase this delay in each new cycle. 
> It helps to save CPU time for other tasks.
> What do you think about?

I do not have a strong opinion on this. A short sleep makes sense. I am
not sure a more complex implementation is really needed.
-- 
Michal Hocko
SUSE Labs