btrfs: extract root promotion logic into promote_child_to_root()

The balance_level() function is overly long and contains a cold code path
that handles promoting a child node to root when the root has only one item.
This code has distinct logic that is clearer and more maintainable when
isolated in its own function.

No functional change.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: SaltyKitkat

fs/btrfs/ctree.c

@@ -861,6 +861,76 @@ struct extent_buffer *btrfs_read_node_slot(struct extent_buffer *parent,
 	return eb;
 }
 
+/*
+ * Promote a child node to become the new tree root.
+ *
+ * This helper is called during rebalancing when the root node contains only
+ * a single item (nritems == 1).  We can reduce the tree height by promoting
+ * that child to become the new root and freeing the old root node.  The path
+ * locks and references are updated accordingly.
+ *
+ * @trans: Transaction handle
+ * @root: Tree root structure to update
+ * @path: Path holding nodes and locks
+ * @level: Level of the parent (old root)
+ * @parent: The parent (old root) with exactly one item
+ *
+ * Return: 0 on success, negative errno on failure.  The transaction is aborted
+ * on critical errors.
+ */
+static noinline int promote_child_to_root(struct btrfs_trans_handle *trans,
+				    struct btrfs_root *root,
+				    struct btrfs_path *path,
+				    int level,
+				    struct extent_buffer *parent)
+{
+	struct extent_buffer *child;
+	int ret = 0;
+
+	ASSERT(btrfs_header_nritems(parent) == 1);
+
+	child = btrfs_read_node_slot(parent, 0);
+	if (IS_ERR(child))
+		return PTR_ERR(child);
+
+	btrfs_tree_lock(child);
+	ret = btrfs_cow_block(trans, root, child, parent, 0, &child,
+			      BTRFS_NESTING_COW);
+	if (ret) {
+		btrfs_tree_unlock(child);
+		free_extent_buffer(child);
+		return ret;
+	}
+
+	ret = btrfs_tree_mod_log_insert_root(root->node, child, true);
+	if (unlikely(ret < 0)) {
+		btrfs_tree_unlock(child);
+		free_extent_buffer(child);
+		btrfs_abort_transaction(trans, ret);
+		return ret;
+	}
+	rcu_assign_pointer(root->node, child);
+
+	add_root_to_dirty_list(root);
+	btrfs_tree_unlock(child);
+
+	path->locks[level] = 0;
+	path->nodes[level] = NULL;
+	btrfs_clear_buffer_dirty(trans, parent);
+	btrfs_tree_unlock(parent);
+	/* once for the path */
+	free_extent_buffer(parent);
+
+	root_sub_used_bytes(root);
+	ret = btrfs_free_tree_block(trans, btrfs_root_id(root), parent, 0, 1);
+	/* once for the root ptr */
+	free_extent_buffer_stale(parent);
+	if (unlikely(ret < 0)) {
+		btrfs_abort_transaction(trans, ret);
+		return ret;
+	}
+	return 0;
+}
 /*
  * node level balancing, used to make sure nodes are in proper order for
  * item deletion.  We balance from the top down, so we have to make sure
@@ -900,55 +970,11 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 	 * by promoting the node below to a root
 	 */
 	if (!parent) {
-		struct extent_buffer *child;
-
 		if (btrfs_header_nritems(mid) != 1)
 			return 0;
 
 		/* promote the child to a root */
-		child = btrfs_read_node_slot(mid, 0);
-		if (IS_ERR(child)) {
-			ret = PTR_ERR(child);
-			goto out;
-		}
-
-		btrfs_tree_lock(child);
-		ret = btrfs_cow_block(trans, root, child, mid, 0, &child,
-				      BTRFS_NESTING_COW);
-		if (ret) {
-			btrfs_tree_unlock(child);
-			free_extent_buffer(child);
-			goto out;
-		}
-
-		ret = btrfs_tree_mod_log_insert_root(root->node, child, true);
-		if (unlikely(ret < 0)) {
-			btrfs_tree_unlock(child);
-			free_extent_buffer(child);
-			btrfs_abort_transaction(trans, ret);
-			goto out;
-		}
-		rcu_assign_pointer(root->node, child);
-
-		add_root_to_dirty_list(root);
-		btrfs_tree_unlock(child);
-
-		path->locks[level] = 0;
-		path->nodes[level] = NULL;
-		btrfs_clear_buffer_dirty(trans, mid);
-		btrfs_tree_unlock(mid);
-		/* once for the path */
-		free_extent_buffer(mid);
-
-		root_sub_used_bytes(root);
-		ret = btrfs_free_tree_block(trans, btrfs_root_id(root), mid, 0, 1);
-		/* once for the root ptr */
-		free_extent_buffer_stale(mid);
-		if (unlikely(ret < 0)) {
-			btrfs_abort_transaction(trans, ret);
-			goto out;
-		}
-		return 0;
+		return promote_child_to_root(trans, root, path, level, mid);
 	}
 	if (btrfs_header_nritems(mid) >
 	    BTRFS_NODEPTRS_PER_BLOCK(fs_info) / 4)