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

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

On Sun, Dec 28, 2014 at 08:46:42AM +0000, Haggai Eran wrote:
> 
> On Dec 26, 2014 9:20 AM, Jerome Glisse <j.glisse@gmail.com> wrote:
> >
> > 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.
> 
> I thought that the fact that mm_take_all_locks locked the i_mmap_mutex of
> every file would prevent this from happening, because the notifier is added
> when the mutex is locked, and the truncate operation also locks it. Am I
> missing something?

No you right again, i was convince in my mind that mmu_notifier register was
only taking the mmap semaphore in write mode for some reasons while it is
in fact also calling mm_take_all_locks(). So yes this protect registration
from all concurrent invalidation.

> 
> >
> > >
> > > >      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 ?
> 
> Sure, go ahead.
> 
> Regards,
> Haggai