From 3e27c418a7a13b8dbf33f6eb49b0e461f011bdcd Mon Sep 17 00:00:00 2001 From: Christoph Hellwig Date: Thu, 9 Nov 2017 09:11:43 -0800 Subject: [PATCH] xfs: add comments documenting the rebalance algorithm Reported-by: Brian Foster Signed-off-by: Christoph Hellwig Reviewed-by: Brian Foster Reviewed-by: Darrick J. Wong Signed-off-by: Darrick J. Wong --- fs/xfs/libxfs/xfs_iext_tree.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/fs/xfs/libxfs/xfs_iext_tree.c b/fs/xfs/libxfs/xfs_iext_tree.c index 3974989b0929..81e0480822d8 100644 --- 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];