[Intel-gfx] [PATCH] drm/i915/evict: watch out for unevictable nodes

[Chris Wilson]

Quoting Chris Wilson (2020-04-08 20:44:43)
> Quoting Matthew Auld (2020-04-08 18:04:56)
> > In an address space there can be sprinkling of I915_COLOR_UNEVICTABLE
> > nodes, which lack a parent vma. For platforms with cache coloring we
> > might be very unlucky and abut with such a node thinking we can simply
> > unbind the vma.
> 
> I did notice this myself recently (from observation); it's highly
> unlikely to ever matter.
> 
> > Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_gem_evict.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem_evict.c b/drivers/gpu/drm/i915/i915_gem_evict.c
> > index 4518b9b35c3d..9e462c6a4c6a 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_evict.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_evict.c
> > @@ -227,6 +227,10 @@ i915_gem_evict_something(struct i915_address_space *vm,
> >         }
> >  
> >         while (ret == 0 && (node = drm_mm_scan_color_evict(&scan))) {
> > +               /* If we find any non-objects (!vma), we cannot evict them */
> > +               if (node->color == I915_COLOR_UNEVICTABLE)
> > +                       return -ENOSPC;
> 
> Returning error immediately looks ok, I was worried as around here we
> usually have lists to clean up, but this is after those. However, I
> would suggest that setting ret = -ENOSPC would be more consistent with
> the flow.

Actually, while this prevents an explosion [good], it now returns an
unexpected error [bad]. No explosion is the lesser evil.
-Chris