xfs: add comments documenting the rebalance algorithm

Reported-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/libxfs/xfs_iext_tree.c

@@ -672,6 +672,11 @@ xfs_iext_rebalance_node(
 	struct xfs_iext_node	*node,
 	int			nr_entries)
 {
+	/*
+	 * If the neighbouring nodes are completely full, or have different
+	 * parents, we might never be able to merge our node, and will only
+	 * delete it once the number of entries hits zero.
+	 */
 	if (nr_entries == 0)
 		return node;
 
@@ -693,6 +698,11 @@ xfs_iext_rebalance_node(
 		int nr_next = xfs_iext_node_nr_entries(next, 0), i;
 
 		if (nr_entries + nr_next <= KEYS_PER_NODE) {
+			/*
+			 * Merge the next node into this node so that we don't
+			 * have to do an additional update of the keys in the
+			 * higher levels.
+			 */
 			for (i = 0; i < nr_next; i++) {
 				node->keys[nr_entries + i] = next->keys[i];
 				node->ptrs[nr_entries + i] = next->ptrs[i];
@@ -741,6 +751,11 @@ again:
 		return;
 
 	if (level < ifp->if_height) {
+		/*
+		 * If we aren't at the root yet try to find a neighbour node to
+		 * merge with (or delete the node if it is empty), and then
+		 * recurse up to the next level.
+		 */
 		level++;
 		parent = xfs_iext_find_level(ifp, offset, level);
 		pos = xfs_iext_node_pos(parent, offset);
@@ -755,6 +770,10 @@ again:
 			goto again;
 		}
 	} else if (nr_entries == 1) {
+		/*
+		 * If we are at the root and only one entry is left we can just
+		 * free this node and update the root pointer.
+		 */
 		ASSERT(node == ifp->if_u1.if_root);
 		ifp->if_u1.if_root = node->ptrs[0];
 		ifp->if_height--;
@@ -789,6 +808,11 @@ xfs_iext_rebalance_leaf(
 		int nr_next = xfs_iext_leaf_nr_entries(ifp, leaf->next, 0), i;
 
 		if (fill + nr_next <= RECS_PER_LEAF) {
+			/*
+			 * Merge the next node into this node so that we don't
+			 * have to do an additional update of the keys in the
+			 * higher levels.
+			 */
 			for (i = 0; i < nr_next; i++)
 				leaf->recs[fill + i] = leaf->next->recs[i];