[Intel-gfx] [PATCH 3/3] drm/i915/userptr: Probe vma range before gup

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-01-15 10:19:11)
> 
> On 14/01/2019 21:17, Chris Wilson wrote:
> > @@ -620,40 +682,24 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
> >                       return -EAGAIN;
> >       }
> >   
> > -     pvec = NULL;
> > -     pinned = 0;
> > +     if (mm == current->mm && try_fast_gup(obj) == 0)
> > +             return 0;
> 
> Is try_fast_gup guaranteed to fail on ranges probe_range would have 
> rejected?

If a page isn't present, it will fail. For our purposes that means
fast_gup fails for GGTT mmaps, as well as other ranges.

> > -     if (mm == current->mm) {
> > -             pvec = kvmalloc_array(num_pages, sizeof(struct page *),
> > -                                   GFP_KERNEL |
> > -                                   __GFP_NORETRY |
> > -                                   __GFP_NOWARN);
> > -             if (pvec) /* defer to worker if malloc fails */
> > -                     pinned = __get_user_pages_fast(obj->userptr.ptr,
> > -                                                    num_pages,
> > -                                                    !i915_gem_object_is_readonly(obj),
> > -                                                    pvec);
> > -     }
> > +     /* lockdep doesn't yet automatically allow nesting of readers */
> > +     down_read_nested(&mm->mmap_sem, SINGLE_DEPTH_NESTING);
> 
> Why is nesting needed - comment needs to explain.

We get called with mm->mmap_sem held for reading on a fault path, and
without mm->mmap_sem held otherwise. lockdep doesn't allow nesting of
readers, hence the comment.
-Chris