[ofa-general] Re: [PATCH 08 of 11] anon-vma-rwsem

[Peter Zijlstra]

On Fri, 2008-05-09 at 20:55 +0200, Andrea Arcangeli wrote:
> On Fri, May 09, 2008 at 08:37:29PM +0200, Peter Zijlstra wrote:
> > Another possibility, would something like this work?
> > 
> >  
> >  /*
> >   * null out the begin function, no new begin calls can be made
> >   */
> >  rcu_assing_pointer(my_notifier.invalidate_start_begin, NULL); 
> > 
> >  /*
> >   * lock/unlock all rmap locks in any order - this ensures that any
> >   * pending start() will have its end() function called.
> >   */
> >  mm_barrier(mm);
> > 
> >  /*
> >   * now that no new start() call can be made and all start()/end() pairs
> >   * are complete we can remove the notifier.
> >   */
> >  mmu_notifier_remove(mm, my_notifier);
> > 
> > 
> > This requires a mmu_notifier instance per attached mm and that
> > __mmu_notifier_invalidate_range_start() uses rcu_dereference() to obtain
> > the function.
> > 
> > But I think its enough to ensure that:
> > 
> >   for each start an end will be called
> 
> We don't need that, it's perfectly ok if start is called but end is
> not, it's ok to unregister in the middle as I guarantee ->release is
> called before mmu_notifier_unregister returns (if ->release is needed
> at all, not the case for KVM/GRU).
> 
> Unregister is already solved with srcu/rcu without any additional
> complication as we don't need the guarantee that for each start an end
> will be called.
> 
> > It can however happen that end is called without start - but we could
> > handle that I think.
> 
> The only reason mm_lock() was introduced is to solve "register", to
> guarantee that for each end there was a start. We can't handle end
> called without start in the driver.
> 
> The reason the driver must be prevented to register in the middle of
> start/end, if that if it ever happens the driver has no way to know it
> must stop the secondary mmu page faults to call get_user_pages and
> instantiate sptes/secondarytlbs on pages that will be freed as soon as
> zap_page_range starts.

Right - then I got it backwards. Never mind me then..