drm/i915/evict: watch out for unevictable nodes

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.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408170456.399604-1-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_gem_evict.c

@@ -228,7 +228,12 @@ found:
 
 	while (ret == 0 && (node = drm_mm_scan_color_evict(&scan))) {
 		vma = container_of(node, struct i915_vma, node);
-		ret = __i915_vma_unbind(vma);
+
+		/* If we find any non-objects (!vma), we cannot evict them */
+		if (vma->node.color != I915_COLOR_UNEVICTABLE)
+			ret = __i915_vma_unbind(vma);
+		else
+			ret = -ENOSPC; /* XXX search failed, try again? */
 	}
 
 	return ret;