drm/i915: Truncate the shmem backing pages on purge

shmfs doesn't actually implement i_ops->truncate() so we were not
immedatiately releasing the backing pages when shrinking the gfx cache
under OOM. Instead use a combination of truncate_inode_pages() and
i_ops->truncate_range() as is used by shmem_delete_inode().
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_gem.c

@@ -1505,9 +1505,16 @@ i915_gem_object_truncate(struct drm_gem_object *obj)
 	struct drm_i915_gem_object *obj_priv = to_intel_bo(obj);
 	struct inode *inode;
 
+	/* Our goal here is to return as much of the memory as
+	 * is possible back to the system as we are called from OOM.
+	 * To do this we must instruct the shmfs to drop all of its
+	 * backing pages, *now*. Here we mirror the actions taken
+	 * when by shmem_delete_inode() to release the backing store.
+	 */
 	inode = obj->filp->f_path.dentry->d_inode;
-	if (inode->i_op->truncate)
-		inode->i_op->truncate (inode);
+	truncate_inode_pages(inode->i_mapping, 0);
+	if (inode->i_op->truncate_range)
+		inode->i_op->truncate_range(inode, 0, (loff_t)-1);
 
 	obj_priv->madv = __I915_MADV_PURGED;
 }