drm/i915: Pass around sg_table to get_pages/put_pages backend

The plan is to move obj->pages out from under the struct_mutex into its
own per-object lock. We need to prune any assumption of the struct_mutex
from the get_pages/put_pages backends, and to make it easier we pass
around the sg_table to operate on rather than indirectly via the obj.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161028125858.23563-13-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
index 124f69a..f95061f 100644
--- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
+++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
@@ -91,6 +91,13 @@ static bool can_release_pages(struct drm_i915_gem_object *obj)
 	return swap_available() || obj->mm.madv == I915_MADV_DONTNEED;
 }
 
+static bool unsafe_drop_pages(struct drm_i915_gem_object *obj)
+{
+	if (i915_gem_object_unbind(obj) == 0)
+		__i915_gem_object_put_pages(obj);
+	return !obj->mm.pages;
+}
+
 /**
  * i915_gem_shrink - Shrink buffer object caches
  * @dev_priv: i915 device
@@ -192,9 +199,7 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
 
 			i915_gem_object_get(obj);
 
-			/* For the unbound phase, this should be a no-op! */
-			i915_gem_object_unbind(obj);
-			if (__i915_gem_object_put_pages(obj) == 0)
+			if (unsafe_drop_pages(obj))
 				count += obj->base.size >> PAGE_SHIFT;
 
 			i915_gem_object_put(obj);