How to setup configfs for a custom Linux kernel module

Question

I would like to use configfs for some personal out-of-tree kernel module of mine, but I'm having trouble understanding how to set it up.

I'm running Ubuntu Linux with the kernel v6.8.0-rc6.

Below is what I understood to be a minimally viable module with configfs working

#include <linux/configfs.h>
#include <linux/module.h>
#include <linux/init.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("me");
MODULE_DESCRIPTION("");

static struct configfs_subsystem subsys = {
    .su_group = {
        .cg_item = {
            .ci_namebuf = "hello",
        },
    },
};

static int __init setup(void)
{
    config_group_init(&subsys.su_group);
    return configfs_register_subsystem(&subsys); // -22 EINVAL
}

static void __exit cleanup(void)
{
    configfs_unregister_subsystem(&subsys);
}

module_init(setup);
module_exit(cleanup);

It appears to be incorrect:

configfs_register_subsystem() fails and returns -22 (-EINVAL) right after insmod mymod. This is unexpected, I hoped the module to load without error and a directory named /sys/kernel/config/mymod to appear.