diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
index 38d16fe1f6d8..47322adb7690 100644
--- a/fs/xfs/libxfs/xfs_sb.c
+++ b/fs/xfs/libxfs/xfs_sb.c
@@ -1347,6 +1347,9 @@ xfs_log_sb(
 	 * feature was introduced.  This counter can go negative due to the way
 	 * we handle nearly-lockless reservations, so we must use the _positive
 	 * variant here to avoid writing out nonsense frextents.
+	 *
+	 * RT groups are only supported on v5 file systems, which always
+	 * have lazy SB counters.
 	 */
 	if (xfs_has_rtgroups(mp) && !xfs_has_zoned(mp)) {
 		mp->m_sb.sb_frextents =