diff --git a/fs/xfs/libxfs/xfs_trans_space.c b/fs/xfs/libxfs/xfs_trans_space.c
index 9b8f495c9049..c4cd547033e5 100644
--- a/fs/xfs/libxfs/xfs_trans_space.c
+++ b/fs/xfs/libxfs/xfs_trans_space.c
@@ -22,8 +22,23 @@ xfs_parent_calc_space_res(
 	unsigned int		namelen)
 {
 	/*
-	 * Parent pointers are always the first attr in an attr tree, and never
-	 * larger than a block
+	 * A parent pointer is recorded per dirent, so an inode with N links
+	 * carries N of them and the attr fork can already be in leaf or node
+	 * format when one is added.  That does not affect the reservation:
+	 * XFS_DAENTER_SPACE_RES covers a split at every level of a
+	 * maximum-depth attr dabtree, whatever format the fork is in now.
+	 *
+	 * The name is a dirent name and the value is a struct xfs_parent_rec,
+	 * so the leaf entry is always local and never exceeds 272 bytes.
+	 * Parent pointers require V5, hence a 1k minimum block size, so the
+	 * entry always stays under half a block and this needs none of the
+	 * double split allowance that xfs_attr_calc_size() makes.
+	 *
+	 * The second term hands a byte count to a macro whose parameter counts
+	 * mappings, so it asks for more extent-add allowance than the single
+	 * mapping a parent pointer adds - how much more depends on the block
+	 * size.  It over-reserves either way, which is why it is left alone:
+	 * correcting the unit would shrink a reservation that is only generous.
 	 */
 	return XFS_DAENTER_SPACE_RES(mp, XFS_ATTR_FORK) +
 	       XFS_NEXTENTADD_SPACE_RES(mp, namelen, XFS_ATTR_FORK);