Re: [PATCH 2/7] mmu_notifier: keep track of active invalidation ranges v2

[Jerome Glisse <j.glisse@gmail.com>]

On Thu, Dec 25, 2014 at 10:29:44AM +0200, Haggai Eran wrote:
> On 22/12/2014 18:48, j.glisse@gmail.com wrote:
> >  static inline void mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> > -						       unsigned long start,
> > -						       unsigned long end,
> > -						       enum mmu_event event)
> > +						       struct mmu_notifier_range *range)
> >  {
> > +	/*
> > +	 * Initialize list no matter what in case a mmu_notifier register after
> > +	 * a range_start but before matching range_end.
> > +	 */
> > +	INIT_LIST_HEAD(&range->list);
> 
> I don't see how can an mmu_notifier register after a range_start but
> before a matching range_end. The mmu_notifier registration locks all mm
> locks, and that should prevent any invalidation from running, right?

File invalidation (like truncation) can lead to this case.

> 
> >  	if (mm_has_notifiers(mm))
> > -		__mmu_notifier_invalidate_range_start(mm, start, end, event);
> > +		__mmu_notifier_invalidate_range_start(mm, range);
> >  }
> 
> ...
> 
> >  void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> > -					   unsigned long start,
> > -					   unsigned long end,
> > -					   enum mmu_event event)
> > +					   struct mmu_notifier_range *range)
> >  
> >  {
> >  	struct mmu_notifier *mn;
> > @@ -185,21 +183,36 @@ void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> >  	id = srcu_read_lock(&srcu);
> >  	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
> >  		if (mn->ops->invalidate_range_start)
> > -			mn->ops->invalidate_range_start(mn, mm, start,
> > -							end, event);
> > +			mn->ops->invalidate_range_start(mn, mm, range);
> >  	}
> >  	srcu_read_unlock(&srcu, id);
> > +
> > +	/*
> > +	 * This must happen after the callback so that subsystem can block on
> > +	 * new invalidation range to synchronize itself.
> > +	 */
> > +	spin_lock(&mm->mmu_notifier_mm->lock);
> > +	list_add_tail(&range->list, &mm->mmu_notifier_mm->ranges);
> > +	mm->mmu_notifier_mm->nranges++;
> > +	spin_unlock(&mm->mmu_notifier_mm->lock);
> >  }
> >  EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_start);
> 
> Don't you have a race here because you add the range struct after the
> callback?
> 
> -------------------------------------------------------------------------
> Thread A                    | Thread B
> -------------------------------------------------------------------------
> call mmu notifier callback  |
>   clear SPTE                |
>                             | device page fault
>                             |   mmu_notifier_range_is_valid returns true
>                             |   install new SPTE
> add event struct to list    |
> mm clears/modifies the PTE  |
> -------------------------------------------------------------------------
> 
> So we are left with different entries in the host page table and the
> secondary page table.
> 
> I would think you'd want the event struct to be added to the list before
> the callback is run.
> 

Yes you right, but the comment i left trigger memory that i did that on
purpose a one point probably with a different synch mecanism inside hmm.
I will try to medidate a bit see if i can bring back memory why i did it
that way in respect to previous design.

In all case i will respin with that order modified. Can i add you review
by after doing so ?

Cheers,
Jerome

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