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xfs: add comments documenting the rebalance algorithm

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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>
This commit is contained in:
Christoph Hellwig 2017-11-09 09:11:43 -08:00 • committed by Darrick J. Wong
parent b9aee1d5fe
commit 3e27c418a7
1 changed files with 24 additions and 0 deletions
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24
fs/xfs/libxfs/xfs_iext_tree.c
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@ -672,6 +672,11 @@ xfs_iext_rebalance_node(
struct xfs_iext_node *node, struct xfs_iext_node *node,
int nr_entries) 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) if (nr_entries == 0)
return node; return node;
@ -693,6 +698,11 @@ xfs_iext_rebalance_node(
int nr_next = xfs_iext_node_nr_entries(next, 0), i; int nr_next = xfs_iext_node_nr_entries(next, 0), i;
if (nr_entries + nr_next <= KEYS_PER_NODE) { 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++) { for (i = 0; i < nr_next; i++) {
node->keys[nr_entries + i] = next->keys[i]; node->keys[nr_entries + i] = next->keys[i];
node->ptrs[nr_entries + i] = next->ptrs[i]; node->ptrs[nr_entries + i] = next->ptrs[i];
@ -741,6 +751,11 @@ again:
return; return;
if (level < ifp->if_height) { 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++; level++;
parent = xfs_iext_find_level(ifp, offset, level); parent = xfs_iext_find_level(ifp, offset, level);
pos = xfs_iext_node_pos(parent, offset); pos = xfs_iext_node_pos(parent, offset);
@ -755,6 +770,10 @@ again:
goto again; goto again;
} }
} else if (nr_entries == 1) { } 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); ASSERT(node == ifp->if_u1.if_root);
ifp->if_u1.if_root = node->ptrs[0]; ifp->if_u1.if_root = node->ptrs[0];
ifp->if_height--; ifp->if_height--;
@ -789,6 +808,11 @@ xfs_iext_rebalance_leaf(
int nr_next = xfs_iext_leaf_nr_entries(ifp, leaf->next, 0), i; int nr_next = xfs_iext_leaf_nr_entries(ifp, leaf->next, 0), i;
if (fill + nr_next <= RECS_PER_LEAF) { 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++) for (i = 0; i < nr_next; i++)
leaf->recs[fill + i] = leaf->next->recs[i]; leaf->recs[fill + i] = leaf->next->recs[i];