btrfs: reloc: refactor finishing part of upper linkage into finish_upper_links()

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 the work.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -1080,6 +1080,118 @@ static int handle_one_tree_block(struct backref_cache *cache,
 	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 the next run.
+ */
+static int finish_upper_links(struct backref_cache *cache,
+			      struct backref_node *start)
+{
+	struct list_head *useless_node = &cache->useless_node;
+	struct backref_edge *edge;
+	struct rb_node *rb_node;
+	LIST_HEAD(pending_edge);
+
+	ASSERT(start->checked);
+
+	/* Insert this node to cache if it's not COW-only */
+	if (!start->cowonly) {
+		rb_node = tree_insert(&cache->rb_root, start->bytenr,
+				      &start->rb_node);
+		if (rb_node)
+			backref_tree_panic(rb_node, -EEXIST, start->bytenr);
+		list_add_tail(&start->lower, &cache->leaves);
+	}
+
+	/*
+	 * Use breadth first search to iterate all related edges.
+	 *
+	 * The starting points are 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_first_entry(&pending_edge, 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, COW-only node has non-COW-only parent */
+		if (start->cowonly != upper->cowonly) {
+			ASSERT(0);
+			return -EUCLEAN;
+		}
+
+		/* Only cache non-COW-only (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 roots of
@@ -1107,8 +1219,6 @@ static noinline_for_stack struct backref_node *build_backref_tree(
 	struct backref_node *lower;
 	struct backref_node *node = NULL;
 	struct backref_edge *edge;
-	struct rb_node *rb_node;
-	int cowonly;
 	int ret;
 	int err = 0;
 
@@ -1149,75 +1259,13 @@ static noinline_for_stack struct backref_node *build_backref_tree(
 		}
 	} while (edge);
 
-	/*
-	 * everything goes well, connect backref nodes and insert backref nodes
-	 * into the cache.
-	 */
-	ASSERT(node->checked);
-	cowonly = node->cowonly;
-	if (!cowonly) {
-		rb_node = tree_insert(&cache->rb_root, node->bytenr,
-				      &node->rb_node);
-		if (rb_node)
-			backref_tree_panic(rb_node, -EEXIST, node->bytenr);
-		list_add_tail(&node->lower, &cache->leaves);
+	/* Finish the upper linkage of newly added edges/nodes */
+	ret = finish_upper_links(cache, node);
+	if (ret < 0) {
+		err = ret;
+		goto out;
 	}
 
-	list_for_each_entry(edge, &node->upper, list[LOWER])
-		list_add_tail(&edge->list[UPPER], &cache->pending_edge);
-
-	while (!list_empty(&cache->pending_edge)) {
-		edge = list_first_entry(&cache->pending_edge,
-				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, &cache->useless_node);
-			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], &cache->pending_edge);
-	}
 	/*
 	 * process useless backref nodes. backref nodes for tree leaves
 	 * are deleted from the cache. backref nodes for upper level