Re: [patch v2] mm, oom: fix concurrent munlock and oom reaperunmap

David Rientjes
2018-04-24T21:07:55Z

On Tue, 24 Apr 2018, Michal Hocko wrote:

> > > > My patch has passed intensive testing on both x86 and powerpc, so I'll ask
> > > > that it's pushed for 4.17-rc3. Many thanks to Tetsuo for the suggestion
> > > > on calling __oom_reap_task_mm() from exit_mmap().
> > >
> > > Yeah, but your patch does have a problem with blockable mmu notifiers
> > > IIUC.
> >
> > What on earth are you talking about? exit_mmap() does
> > mmu_notifier_release(). There are no blockable mmu notifiers.
>
> MMF_OOM_SKIP - remember? The thing that guarantees a forward progress.
> So we cannot really depend on setting MMF_OOM_SKIP if a
> mmu_notifier_release blocks for an excessive/unbounded amount of time.
>

If the thread is blocked in exit_mmap() because of mmu_notifier_release()
then the oom reaper will eventually grab mm->mmap_sem (nothing holding it
in exit_mmap()), return true, and oom_reap_task() will set MMF_OOM_SKIP.
This is unchanged with the patch and is a completely separate issue.

> Look I am not really interested in disussing this to death but it would
> be really _nice_ if you could calm down a bit, stop fighting for the solution
> you have proposed and ignore the feedback you are getting.
>

I assume we should spend more time considering the two untested patches
you have sent, one of which killed 17 processes while a 8GB memory hog was
exiting because the oom reaper couldn't grab mm->mmap_sem and set
MMF_OOM_SKIP.

> There are two things to care about here. Stop the race that can blow up
> and do not regress MMF_OOM_SKIP guarantee. Can we please do that.

My patch does both.

Thanks.