[GFS2] Reduce truncate IO traffic

Current GFS2 setattr call unconditionally invokes do_shrink even the
requested size and actual file size are equal. This has generated large
amount of extra IOs found during NFS benchmark runs. This patch moves
the relevant logic out of shrink code path. Since setattr is a system
call, the time stamps update is still required.
Signed-off-by: NS. Wendy Cheng <wcheng@redhat.com>
Signed-off-by: NSteven Whitehouse <swhiteho@redhat.com>

fs/gfs2/bmap.c

@@ -1085,6 +1085,33 @@ static int do_shrink(struct gfs2_inode *ip, u64 size)
 	return error;
 }
 
+static int do_touch(struct gfs2_inode *ip, u64 size)
+{
+	struct gfs2_sbd *sdp = GFS2_SB(&ip->i_inode);
+	struct buffer_head *dibh;
+	int error;
+
+	error = gfs2_trans_begin(sdp, RES_DINODE, 0);
+	if (error)
+		return error;
+
+	down_write(&ip->i_rw_mutex);
+
+	error = gfs2_meta_inode_buffer(ip, &dibh);
+	if (error)
+		goto do_touch_out;
+
+	ip->i_inode.i_mtime = ip->i_inode.i_ctime = CURRENT_TIME;
+	gfs2_trans_add_bh(ip->i_gl, dibh, 1);
+	gfs2_dinode_out(ip, dibh->b_data);
+	brelse(dibh);
+
+do_touch_out:
+	up_write(&ip->i_rw_mutex);
+	gfs2_trans_end(sdp);
+	return error;
+}
+
 /**
  * gfs2_truncatei - make a file a given size
  * @ip: the inode
@@ -1105,8 +1132,11 @@ int gfs2_truncatei(struct gfs2_inode *ip, u64 size)
 
 	if (size > ip->i_di.di_size)
 		error = do_grow(ip, size);
-	else
+	else if (size < ip->i_di.di_size)
 		error = do_shrink(ip, size);
+	else
+		/* update time stamps */
+		error = do_touch(ip, size);
 
 	return error;
 }