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

[Michal Hocko]

On Tue 24-04-18 17:08:15, Michal Hocko wrote:
> On Tue 24-04-18 14:07:52, David Rientjes wrote:
> > 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.
> 
> I must be missing something or we are talking past each other.

Ohh. Ok, so _I_ was missing that mm_has_notifiers is false after
mmu_notifier_release. So we cannot block at that time. Then we are good.
Sorry about the confusion!
-- 
Michal Hocko
SUSE Labs