Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers

[Ralph Campbell]

On 1/16/20 8:00 AM, Jason Gunthorpe wrote:
> On Wed, Jan 15, 2020 at 02:09:47PM -0800, Ralph Campbell wrote:
>
> > I don't understand the lifetime/membership issue. The driver is the only thing
> > that allocates, inserts, or removes struct mmu_interval_notifier and thus
> > completely controls the lifetime.
>
> If the returned value is on the defered list it could be freed at any
> moment. The existing locks do not prevent it.
>
> > > > +		ret = nouveau_svmm_interval_find(svmm, &range);
> > > > +		if (ret) {
> > > > +			up_read(&mm->mmap_sem);
> > > > +			return ret;
> > > > +		}
> > > > +		range.notifier_seq = mmu_interval_read_begin(range.notifier);
> > > >    		ret = hmm_range_fault(&range, 0);
> > > >    		up_read(&mm->mmap_sem);
> > > >    		if (ret <= 0) {
> > >
> > > I'm still not sure this is a better approach than what ODP does. It
> > > looks very expensive on the fault path..
> > >
> > > Jason
> >
> > ODP doesn't have this problem because users have to call ib_reg_mr()
> > before any I/O can happen to the process address space.
>
> ODP supports a single 'full VA' call at process startup, just like
> these cases.
>
> > That is when mmu_interval_notifier_insert() /
> > mmu_interval_notifier_remove() can be called and the driver doesn't
> > have to worry about the interval changing sizes or being removed
> > while I/O is happening.
>
> No, for the 'ODP full process VA' (aka implicit ODP) mode it
> dynamically maintains a list of intervals. ODP chooses the align the
> dynamic intervals to it's HW page table levels, and not to SW VMAs.
> This is much simpler to manage and faster to fault, at the cost of
> capturing more VA for invalidations which have to be probed against
> the HW shadow PTEs.
>
> > It isn't that expensive, there is an extra driver lock/unlock as
> > part of the lookup and possibly a find_vma() and kmalloc(GFP_ATOMIC)
> > for new intervals. Also, the deferred interval updates for munmap().
> > Compared to the cost of updating PTEs in the device and GPU fault
> > handling, this is minimal overhead.
>
> Well, compared to ODP which does a single xa lookup with no lock to
> find its interval, this looks very expensive and not parallel.
>
> I think if there is merit in having ranges cover the vmas and track
> changes then there is probably merit in having the core code provide
> much of that logic, not the driver.
>
> But it would be interesting to see some kind of analysis on the two
> methods to decide if the complexity is worthwhile.
>
> Jason

Can you point me to the latest ODP code? Seems like my understanding is
quite off.