drm/i915: use shmem_truncate_range

The interface to ->truncate_range is changing very slightly: once "tmpfs:
take control of its truncate_range" has been applied, this can be applied.
 For now there is only a slight inefficiency while this remains unapplied,
but it will soon become essential for managing shmem's use of swap.

Change i915_gem_object_truncate() to use shmem_truncate_range() directly:
which should also spare i915 later change if we switch from
inode_operations->truncate_range to file_operations->fallocate.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Keith Packard <keithp@keithp.com>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/i915_gem.c

@@ -1696,13 +1696,10 @@ i915_gem_object_truncate(struct drm_i915_gem_object *obj)
 	/* 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.
+	 * backing pages, *now*.
 	 */
 	inode = obj->base.filp->f_path.dentry->d_inode;
-	truncate_inode_pages(inode->i_mapping, 0);
-	if (inode->i_op->truncate_range)
-		inode->i_op->truncate_range(inode, 0, (loff_t)-1);
+	shmem_truncate_range(inode, 0, (loff_t)-1);
 
 	obj->madv = __I915_MADV_PURGED;
 }