Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer.

[Michal Hocko]

On Wed 20-06-18 21:21:21, Tetsuo Handa wrote:
> On 2018/06/20 20:55, Michal Hocko wrote:
> > On Wed 20-06-18 20:20:38, Tetsuo Handa wrote:
> >> Sleeping with oom_lock held can cause AB-BA lockup bug because
> >> __alloc_pages_may_oom() does not wait for oom_lock. Since
> >> blocking_notifier_call_chain() in out_of_memory() might sleep, sleeping
> >> with oom_lock held is currently an unavoidable problem.
> > 
> > Could you be more specific about the potential deadlock? Sleeping while
> > holding oom lock is certainly not nice but I do not see how that would
> > result in a deadlock assuming that the sleeping context doesn't sleep on
> > the memory allocation obviously.
> 
> "A" is "owns oom_lock" and "B" is "owns CPU resources". It was demonstrated
> at "mm,oom: Don't call schedule_timeout_killable() with oom_lock held." proposal.

This is not a deadlock but merely a resource starvation AFAIU.

> But since you don't accept preserving the short sleep which is a heuristic for
> reducing the possibility of AB-BA lockup, the only way we would accept will be
> wait for the owner of oom_lock (e.g. by s/mutex_trylock/mutex_lock/ or whatever)
> which is free of heuristic and free of AB-BA lockup.
> 
> > 
> >> As a preparation for not to sleep with oom_lock held, this patch brings
> >> OOM notifier callbacks to outside of OOM killer, with two small behavior
> >> changes explained below.
> > 
> > Can we just eliminate this ugliness and remove it altogether? We do not
> > have that many notifiers. Is there anything fundamental that would
> > prevent us from moving them to shrinkers instead?
> > 
> 
> For long term, it would be possible. But not within this patch. For example,
> I think that virtio_balloon wants to release memory only when we have no
> choice but OOM kill. If virtio_balloon trivially releases memory, it will
> increase the risk of killing the entire guest by OOM-killer from the host
> side.

I would _prefer_ to think long term here. The sleep inside the oom lock is
not something real workload are seeing out there AFAICS. Adding quite
some code to address such a case doesn't justify the inclusion IMHO.

-- 
Michal Hocko
SUSE Labs