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>

diff --git a/fs/xfs/libxfs/xfs_iext_tree.c b/fs/xfs/libxfs/xfs_iext_tree.c
index 3974989..81e0480 100644 (file)
--- a/fs/xfs/libxfs/xfs_iext_tree.c
+++ b/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];