Nouveau - Nov 2019 - [PATCH v2 09/15] xen/gntdev: use mmu_range_notifier_insert

On Mon, Nov 04, 2019 at 05:03:31PM -0500, Boris Ostrovsky
wrote:
> On 10/28/19 4:10 PM, Jason Gunthorpe wrote:
> > @@ -445,17 +438,9 @@ static void gntdev_vma_close(struct
vm_area_struct *vma)
> >  	struct gntdev_priv *priv = file->private_data;
> >  
> >  	pr_debug("gntdev_vma_close %p\n", vma);
> > -	if (use_ptemod) {
> > -		/* It is possible that an mmu notifier could be running
> > -		 * concurrently, so take priv->lock to ensure that the vma
won't
> > -		 * vanishing during the unmap_grant_pages call, since we will
> > -		 * spin here until that completes. Such a concurrent call will
> > -		 * not do any unmapping, since that has been done prior to
> > -		 * closing the vma, but it may still iterate the unmap_ops list.
> > -		 */
> > -		mutex_lock(&priv->lock);
> > +	if (use_ptemod && map->vma == vma) {
> 
> 
> Is it possible for map->vma not to be equal to vma?
It could be NULL at least if use_ptemod is not set.

Otherwise, I'm not sure, the confusing bit is that the map comes from
here:

        map = gntdev_find_map_index(priv, index, count);

It looks like the intent is that the map->vma is always set to the
only vma that has the map as private_data.

So, I suppose it can be relaxed to a null test and a WARN_ON that it
hasn't changed?

Jason

On Tue, Nov 05, 2019 at 10:16:46AM -0500, Boris Ostrovsky wrote:
> > So, I suppose it can be relaxed to a null test and a WARN_ON that it
> > hasn't changed?
> 
> You mean
> 
> if (use_ptemod) {
> ??????? WARN_ON(map->vma != vma);
> ??????? ...
> 
> 
> Yes, that sounds good.
I amended my copy of the patch with the above, has this rework shown
signs of working?

@@ -436,7 +436,8 @@ static void gntdev_vma_close(struct vm_area_struct *vma)
        struct gntdev_priv *priv = file->private_data;
 
        pr_debug("gntdev_vma_close %p\n", vma);
-       if (use_ptemod && map->vma == vma) {
+       if (use_ptemod) {
+               WARN_ON(map->vma != vma);
                mmu_range_notifier_remove(&map->notifier);
                map->vma = NULL;
        }

Jason