Re: [PATCH] mm, oom: fix for hiding mm which is shared with kthread or global init

[Michal Hocko <mhocko@suse.cz>]

On Tue 19-07-16 19:36:40, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 19-07-16 08:40:48, Michal Hocko wrote:
> > > On Tue 19-07-16 06:30:42, Tetsuo Handa wrote:
> > > > Michal Hocko wrote:
> > > > > I really do not think that this unlikely case really has to be handled
> > > > > now. We are very likely going to move to a different model of oom victim
> > > > > detection soon. So let's do not add new hacks. exit_oom_victim from
> > > > > oom_kill_process just looks like sand in eyes.
> > > > 
> > > > Then, please revert "mm, oom: hide mm which is shared with kthread or global init"
> > > > ( http://lkml.kernel.org/r/1466426628-15074-11-git-send-email-mhocko@kernel.org ).
> > > > I don't like that patch because it is doing pointless find_lock_task_mm() test
> > > > and is telling a lie because it does not guarantee that we won't hit OOM livelock.
> > > 
> > > The above patch doesn't make the situation worse wrt livelock. I
> > > consider it an improvement. It adds find_lock_task_mm into
> > > oom_scan_process_thread but that can hardly be worse than just the
> > > task->signal->oom_victims check because we can catch MMF_OOM_REAPED. If
> > > we are mm loss, which is a less likely case, then we behave the same as
> > > with the previous implementation.
> > > 
> > > So I do not really see a reason to revert that patch for now.
> > 
> > And that being said. If you strongly disagree with the wording then what
> > about the following:
> > "
> >     In order to help a forward progress for the OOM killer, make sure that
> >     this really rare cases will not get into the way and hide the mm from the
> >     oom killer by setting MMF_OOM_REAPED flag for it.  oom_scan_process_thread
> >     will ignore any TIF_MEMDIE task if it has MMF_OOM_REAPED flag set to catch
> >     these oom victims.
> >     
> >     After this patch we should guarantee a forward progress for the OOM killer
> >     even when the selected victim is sharing memory with a kernel thread or
> >     global init as long as the victims mm is still alive.
> > "
> 
> No, I don't like "as long as the victims mm is still alive" exception.

Why? Because of the wording or in principle?

> If you don't like exit_oom_victim() from oom_kill_process(), what about
> alternative shown below?
> 
>  	if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims)) {
>  		struct task_struct *p = find_lock_task_mm(task);
>  		enum oom_scan_t ret = OOM_SCAN_ABORT;
>  
>  		if (p) {
>  			if (test_bit(MMF_OOM_REAPED, &p->mm->flags))
>  				ret = OOM_SCAN_CONTINUE;
>  			task_unlock(p);
> +#ifdef CONFIG_MMU
> +		} else {
> +			/*
> +			 * MMF_OOM_REAPED was set at oom_kill_process() without
> +			 * waking up the OOM reaper, but this thread group lost
> +			 * its mm. Therefore, pretend as if the OOM reaper lost
> +			 * its mm (i.e. select next OOM victim).
> +			 * But be sure to prevent CONFIG_MMU=n from acting
> +			 * as if exit_oom_victim() in exit_mm() has moved from
> +			 * after mmput() to before mmput().
> +			 */
> +			ret = OOM_SCAN_CONTINUE;
> +#endif
>  		}
>  		return ret;
>  	}
> 
> By using this alternative, we can really guarantee a forward progress for
> the OOM killer even when the selected victim is sharing memory with a kernel
> thread or global init. No "as long as the victims mm is still alive" exception.

I wouldn't complicate the pile which is waiting for the merge window and
risk introducing some last minute bugs.
 
> Also, this alternative (when combined with removal of MMF_OOM_NOT_REAPABLE) has
> a bonus that we no longer need to call exit_oom_victim() from the OOM reaper
> because the OOM killer can move on to next OOM victim after the OOM reaper
> set MMF_OOM_REAPED to that mm. That is, we can immediately disallow
> exit_oom_victim() on remote thread and apply oom_killer_disable() timeout
> patch and revert "oom, suspend: fix oom_reaper vs. oom_killer_disable race".
> 
> If we remember victim's mm via your "oom: keep mm of the killed task available"
> or my "mm,oom: Use list of mm_struct used by OOM victims.", we can force the
> OOM reaper to try to reap by intervening to regular __mmput() from mmput() from
> exit_mm() by purposely taking a reference on mm->mm_users. Then, we can always
> try to reclaim some memory using the OOM reaper before risking exit_aio() from
> __mmput() from mmput() from exit_mm() to stall, for we can keep the OOM killer
> waiting until MMF_OOM_REAPED is set using your or my patch.

Let's discuss these things later on after merge window along with anothe
changes.

-- 
Michal Hocko
SUSE Labs

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