[PATCH v2 09/10] btrfs: relocation: Refactor the finishing part of upper linkage into finish_upper_links()

Qu Wenruo
2020-03-02T09:46:22Z

After handle_one_tree_backref(), all newly added (not cached) edges and
nodes have the following features:

- Only backref_edge::list[LOWER] is linked.
This means, we can only iterate from botton to top, not the other
direction.

- Newly added nodes are not added to cache rb_tree yet

So to finish the backref cache, we still need to finish the links and
add all nodes into backref cache rb_tree.

This patch will refactor the existing code into finish_upper_links(),
add more comments of each branch, and why we need to do all these works.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
fs/btrfs/relocation.c | 163 +++++++++++++++++++++++++++---------------
1 file changed, 107 insertions(+), 56 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 26089694b3b5..b42c3d088be4 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -998,6 +998,106 @@ static int handle_one_tree_block(struct reloc_control *rc,
return ret;
}

+/*
+ * In handle_one_tree_backref(), we have only linked the lower node to the edge,
+ * but the upper node hasn't been linked to the edge.
+ * This means we can only iterate through backref_node::upper to reach parent
+ * edges, but not through backref_node::lower to reach children edges.
+ *
+ * This function will finish the backref_node::lower to related edges, so that
+ * backref cache can be bi-directionally iterated.
+ *
+ * Also, this will add the nodes to backref cache for next run.
+ */
+static int finish_upper_links(struct backref_cache *cache,
+ struct backref_node *start,
+ struct list_head *useless_node)
+{
+ struct backref_edge *edge;
+ LIST_HEAD(pending_edge);
+
+ /*
+ * Use breadth first search to iterate all related edges.
+ *
+ * The start point is all the edges of this node
+ */
+ list_for_each_entry(edge, &start->upper, list[LOWER])
+ list_add_tail(&edge->list[UPPER], &pending_edge);
+
+ while (!list_empty(&pending_edge)) {
+ struct backref_node *upper;
+ struct backref_node *lower;
+ struct rb_node *rb_node;
+
+ edge = list_entry(pending_edge.next, struct backref_edge,
+ list[UPPER]);
+ list_del_init(&edge->list[UPPER]);
+ upper = edge->node[UPPER];
+ lower = edge->node[LOWER];
+
+ /* Parent is detached, no need to keep any edges */
+ if (upper->detached) {
+ list_del(&edge->list[LOWER]);
+ free_backref_edge(cache, edge);
+
+ /* Lower node is orphan, queue for cleanup */
+ if (list_empty(&lower->upper))
+ list_add(&lower->list, useless_node);
+ continue;
+ }
+
+ /*
+ * All new nodes added in current build_backref_tree() haven't
+ * been linked to the cache rb tree.
+ * So if we have upper->rb_node populated, this means a cache
+ * hit. We only need to link the edge, as @upper and all its
+ * parent have already been linked.
+ */
+ if (!RB_EMPTY_NODE(&upper->rb_node)) {
+ if (upper->lowest) {
+ list_del_init(&upper->lower);
+ upper->lowest = 0;
+ }
+
+ list_add_tail(&edge->list[UPPER], &upper->lower);
+ continue;
+ }
+
+ /* Sanity check, we shouldn't have any unchecked nodes */
+ if (!upper->checked) {
+ ASSERT(0);
+ return -EUCLEAN;
+ }
+
+ /* Sanity check, cowonly node has non-cowonly parent */
+ if (start->cowonly != upper->cowonly) {
+ ASSERT(0);
+ return -EUCLEAN;
+ }
+
+ /* Only cache non-cowonly (subvolume trees) tree blocks */
+ if (!upper->cowonly) {
+ rb_node = tree_insert(&cache->rb_root, upper->bytenr,
+ &upper->rb_node);
+ if (rb_node) {
+ backref_tree_panic(rb_node, -EEXIST,
+ upper->bytenr);
+ return -EUCLEAN;
+ }
+ }
+
+ list_add_tail(&edge->list[UPPER], &upper->lower);
+
+ /*
+ * Also queue all the parent edges of this uncached node
+ * to finish the upper linkage
+ */
+ list_for_each_entry(edge, &upper->upper, list[LOWER])
+ list_add_tail(&edge->list[UPPER], &pending_edge);
+ }
+ return 0;
+}
+
/*
* build backref tree for a given tree block. root of the backref tree
* corresponds the tree block, leaves of the backref tree correspond
@@ -1028,7 +1128,6 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
struct rb_node *rb_node;
LIST_HEAD(list); /* Pending edge list, upper node needs to be checked */
LIST_HEAD(useless);
- int cowonly;
int ret;
int err = 0;

@@ -1074,8 +1173,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
* into the cache.
*/
ASSERT(node->checked);
- cowonly = node->cowonly;
- if (!cowonly) {
+ if (!node->cowonly) {
rb_node = tree_insert(&cache->rb_root, node->bytenr,
&node->rb_node);
if (rb_node)
@@ -1083,60 +1181,13 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
list_add_tail(&node->lower, &cache->leaves);
}

- list_for_each_entry(edge, &node->upper, list[LOWER])
- list_add_tail(&edge->list[UPPER], &list);
-
- while (!list_empty(&list)) {
- edge = list_entry(list.next, struct backref_edge, list[UPPER]);
- list_del_init(&edge->list[UPPER]);
- upper = edge->node[UPPER];
- if (upper->detached) {
- list_del(&edge->list[LOWER]);
- lower = edge->node[LOWER];
- free_backref_edge(cache, edge);
- if (list_empty(&lower->upper))
- list_add(&lower->list, &useless);
- continue;
- }
-
- if (!RB_EMPTY_NODE(&upper->rb_node)) {
- if (upper->lowest) {
- list_del_init(&upper->lower);
- upper->lowest = 0;
- }
-
- list_add_tail(&edge->list[UPPER], &upper->lower);
- continue;
- }
-
- if (!upper->checked) {
- /*
- * Still want to blow up for developers since this is a
- * logic bug.
- */
- ASSERT(0);
- err = -EINVAL;
- goto out;
- }
- if (cowonly != upper->cowonly) {
- ASSERT(0);
- err = -EINVAL;
- goto out;
- }
-
- if (!cowonly) {
- rb_node = tree_insert(&cache->rb_root, upper->bytenr,
- &upper->rb_node);
- if (rb_node)
- backref_tree_panic(rb_node, -EEXIST,
- upper->bytenr);
- }
-
- list_add_tail(&edge->list[UPPER], &upper->lower);
-
- list_for_each_entry(edge, &upper->upper, list[LOWER])
- list_add_tail(&edge->list[UPPER], &list);
+ /* Finish the upper linkage of newly added edges/nodes */
+ ret = finish_upper_links(cache, node, &useless);
+ if (ret < 0) {
+ err = ret;
+ goto out;
}
+
/*
* process useless backref nodes. backref nodes for tree leaves
* are deleted from the cache. backref nodes for upper level
--
2.25.1