Re: [v3 1/6] mm, oom: use oom_victims counter to synchronize oom victim selection

[Tetsuo Handa <penguin-kernel-JPay3/Yim36HaxMnTkn67Xf5DAMn2ifp@public.gmane.org>]

Tetsuo Handa wrote:
> Roman Gushchin wrote:
> > On Thu, Jun 22, 2017 at 09:40:28AM +0900, Tetsuo Handa wrote:
> > > Roman Gushchin wrote:
> > > > --- a/mm/oom_kill.c
> > > > +++ b/mm/oom_kill.c
> > > > @@ -992,6 +992,13 @@ bool out_of_memory(struct oom_control *oc)
> > > >  	if (oom_killer_disabled)
> > > >  		return false;
> > > >  
> > > > +	/*
> > > > +	 * If there are oom victims in flight, we don't need to select
> > > > +	 * a new victim.
> > > > +	 */
> > > > +	if (atomic_read(&oom_victims) > 0)
> > > > +		return true;
> > > > +
> > > >  	if (!is_memcg_oom(oc)) {
> > > >  		blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
> > > >  		if (freed > 0)
> > > 
> > > The OOM reaper is not available for CONFIG_MMU=n kernels, and timeout based
> > > giveup is not permitted, but a multithreaded process might be selected as
> > > an OOM victim. Not setting TIF_MEMDIE to all threads sharing an OOM victim's
> > > mm increases possibility of preventing some OOM victim thread from terminating
> > > (e.g. one of them cannot leave __alloc_pages_slowpath() with mmap_sem held for
> > > write due to waiting for the TIF_MEMDIE thread to call exit_oom_victim() when
> > > the TIF_MEMDIE thread is waiting for the thread with mmap_sem held for write).
> > 
> > I agree, that CONFIG_MMU=n is a special case, and the proposed approach can't
> > be used directly. But can you, please, why do you find the first  chunk wrong?
> 
> Since you are checking oom_victims before checking task_will_free_mem(current),
> only one thread can get TIF_MEMDIE. This is where a multithreaded OOM victim without
> the OOM reaper can get stuck forever.

Oops, I misinterpreted. This is where a multithreaded OOM victim with or without
the OOM reaper can get stuck forever. Think about a process with two threads is
selected by the OOM killer and only one of these two threads can get TIF_MEMDIE.

  Thread-1                 Thread-2                 The OOM killer           The OOM reaper

                           Calls down_write(&current->mm->mmap_sem).
  Enters __alloc_pages_slowpath().
                           Enters __alloc_pages_slowpath().
  Takes oom_lock.
  Calls out_of_memory().
                                                    Selects Thread-1 as an OOM victim.
  Gets SIGKILL.            Gets SIGKILL.
  Gets TIF_MEMDIE.
  Releases oom_lock.
  Leaves __alloc_pages_slowpath() because Thread-1 has TIF_MEMDIE.
                                                                             Takes oom_lock.
                                                                             Will do nothing because down_read_trylock() fails.
                                                                             Releases oom_lock.
                                                                             Gives up and sets MMF_OOM_SKIP after one second.
                           Takes oom_lock.
                           Calls out_of_memory().
                           Will not check MMF_OOM_SKIP because Thread-1 still has TIF_MEMDIE. // <= get stuck waiting for Thread-1.
                           Releases oom_lock.
                           Will not leave __alloc_pages_slowpath() because Thread-2 does not have TIF_MEMDIE.
                           Will not call up_write(&current->mm->mmap_sem).
  Reaches do_exit().
  Calls down_read(&current->mm->mmap_sem) in exit_mm() in do_exit(). // <= get stuck waiting for Thread-2.
  Will not call up_read(&current->mm->mmap_sem) in exit_mm() in do_exit().
  Will not clear TIF_MEMDIE in exit_oom_victim() in exit_mm() in do_exit().