xfs: plug directory buffer readahead

Similar to bulkstat inode chunk readahead, we need to plug directory
data buffer readahead during getdents to ensure that we can merge
adjacent readahead requests and sort out of order requests optimally
before they are dispatched. This improves the readahead efficiency
and reduces the IO load it generates as the IO patterns are
significantly better for both contiguous and fragmented directories.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_dir2_leaf.c

@@ -1108,6 +1108,7 @@ xfs_dir2_leaf_readbuf(
 	struct xfs_mount	*mp = dp->i_mount;
 	struct xfs_buf		*bp = *bpp;
 	struct xfs_bmbt_irec	*map = mip->map;
+	struct blk_plug		plug;
 	int			error = 0;
 	int			length;
 	int			i;
@@ -1236,6 +1237,7 @@ xfs_dir2_leaf_readbuf(
 	/*
 	 * Do we need more readahead?
 	 */
+	blk_start_plug(&plug);
 	for (mip->ra_index = mip->ra_offset = i = 0;
 	     mip->ra_want > mip->ra_current && i < mip->map_blocks;
 	     i += mp->m_dirblkfsbs) {
@@ -1287,6 +1289,7 @@ xfs_dir2_leaf_readbuf(
 			}
 		}
 	}
+	blk_finish_plug(&plug);
 
 out:
 	*bpp = bp;