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

[Michal Hocko <mhocko@kernel.org>]

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. So let me
be explicit. What does prevent the following

oom_reaper				exit_mmap
					  mutex_lock(oom_lock)
  mutex_lock(oom_lock)			    __oom_reap_task_mm
  					      mmu_notifier_invalidate_range_start
					        # blockable mmu_notifier
						# which takes ages to
						# finish or depends on
						# an allocation (in)directly
-- 
Michal Hocko
SUSE Labs