@@ -810,6 +810,50 @@ out:
 	return ret;
 }
 
+static int create_global_root(struct btrfs_trans_handle *trans, u64 objectid,
+			      int root_id)
+{
+	struct btrfs_fs_info *fs_info = trans->fs_info;
+	struct btrfs_root *root;
+	struct btrfs_key key = {
+		.objectid = objectid,
+		.type = BTRFS_ROOT_ITEM_KEY,
+		.offset = root_id,
+	};
+	int ret = 0;
+
+	root = btrfs_create_tree(trans, fs_info, &key);
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
+		goto out;
+	}
+	ret = btrfs_global_root_insert(fs_info, root);
+out:
+	if (ret)
+		btrfs_abort_transaction(trans, ret);
+	return ret;
+}
+
+static int create_global_roots(struct btrfs_trans_handle *trans,
+			       int nr_global_roots)
+{
+	int ret, i;
+
+	for (i = 1; i < nr_global_roots; i++) {
+		ret = create_global_root(trans, BTRFS_EXTENT_TREE_OBJECTID, i);
+		if (ret)
+			return ret;
+		ret = create_global_root(trans, BTRFS_CSUM_TREE_OBJECTID, i);
+		if (ret)
+			return ret;
+		ret = create_global_root(trans, BTRFS_FREE_SPACE_TREE_OBJECTID, i);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
 static int insert_qgroup_items(struct btrfs_trans_handle *trans,
 			       struct btrfs_fs_info *fs_info,
 			       u64 qgroupid)

@@ -966,13 +1010,18 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 	struct btrfs_mkfs_config mkfs_cfg;
 	enum btrfs_csum_type csum_type = BTRFS_CSUM_TYPE_CRC32;
 	u64 system_group_size;
+	int nr_global_roots = sysconf(_SC_NPROCESSORS_ONLN);
 
 	crc32c_optimization_init();
 	btrfs_config_init();
 
 	while(1) {
 		int c;
-		enum { GETOPT_VAL_SHRINK = 257, GETOPT_VAL_CHECKSUM };
+		enum {
+			GETOPT_VAL_SHRINK = 257,
+			GETOPT_VAL_CHECKSUM,
+			GETOPT_VAL_GLOBAL_ROOTS,
+		};
 		static const struct option long_options[] = {
 			{ "byte-count", required_argument, NULL, 'b' },
 			{ "csum", required_argument, NULL,

@@ -996,6 +1045,9 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 			{ "quiet", 0, NULL, 'q' },
 			{ "verbose", 0, NULL, 'v' },
 			{ "shrink", no_argument, NULL, GETOPT_VAL_SHRINK },
+#if EXPERIMENTAL
+			{ "num-global-roots", required_argument, NULL, GETOPT_VAL_GLOBAL_ROOTS },
+#endif
 			{ "help", no_argument, NULL, GETOPT_VAL_HELP },
 			{ NULL, 0, NULL, 0}
 		};

@@ -1100,6 +1152,9 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 			case GETOPT_VAL_CHECKSUM:
 				csum_type = parse_csum_type(optarg);
 				break;
+			case GETOPT_VAL_GLOBAL_ROOTS:
+				nr_global_roots = (int)arg_strtou64(optarg);
+				break;
 			case GETOPT_VAL_HELP:
 			default:
 				print_usage(c != GETOPT_VAL_HELP);

@@ -1239,6 +1294,11 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 	if (features & BTRFS_FEATURE_INCOMPAT_EXTENT_TREE_V2) {
 		features |= BTRFS_FEATURE_INCOMPAT_NO_HOLES;
 		runtime_features |= BTRFS_RUNTIME_FEATURE_FREE_SPACE_TREE;
+
+		if (!nr_global_roots) {
+			error("you must set a non-zero num-global-roots value");
+			exit(1);
+		}
 	}
 
 	if (zoned) {

@@ -1466,6 +1526,14 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 		goto error;
 	}
 
+	if (features & BTRFS_FEATURE_INCOMPAT_EXTENT_TREE_V2) {
+		ret = create_global_roots(trans, nr_global_roots);
+		if (ret) {
+			error("failed to create global roots: %d", ret);
+			goto error;
+		}
+	}
+
 	ret = make_root_dir(trans, root);
 	if (ret) {
 		error("failed to setup the root directory: %d", ret);