Linux Kernel FileSystems Sub-system – Introduction – register_filesystem() APIs – data-structures

Refer:
Wiki File system ↗

Linux Kernel Source:
/fs/filesystems.c ↗
register_filesystem(), unregister_filesystem() APIs ↗
struct file_system_type data-structure ↗

Linux Kernel – ext4 filesystem type:
static struct file_system_type ext2_fs_type ↗
static struct file_system_type ext3_fs_type ↗
static struct file_system_type ext4_fs_type ↗
static int __init ext4_init_fs(void) ↗

Here is the register_filesystem(), unregister_filesystem() APIs (/fs/filesystems.c) from the Kernel-source version 6.5.9 for quick reference:

/**
 *	register_filesystem - register a new filesystem
 *	@fs: the file system structure
 *
 *	Adds the file system passed to the list of file systems the kernel
 *	is aware of for mount and other syscalls. Returns 0 on success,
 *	or a negative errno code on an error.
 *
 *	The &struct file_system_type that is passed is linked into the kernel 
 *	structures and must not be freed until the file system has been
 *	unregistered.
 */
 
int register_filesystem(struct file_system_type * fs)
{
	int res = 0;
	struct file_system_type ** p;

	if (fs->parameters &&
	    !fs_validate_description(fs->name, fs->parameters))
		return -EINVAL;

	BUG_ON(strchr(fs->name, '.'));
	if (fs->next)
		return -EBUSY;
	write_lock(&file_systems_lock);
	p = find_filesystem(fs->name, strlen(fs->name));
	if (*p)
		res = -EBUSY;
	else
		*p = fs;
	write_unlock(&file_systems_lock);
	return res;
}

EXPORT_SYMBOL(register_filesystem);

/**
 *	unregister_filesystem - unregister a file system
 *	@fs: filesystem to unregister
 *
 *	Remove a file system that was previously successfully registered
 *	with the kernel. An error is returned if the file system is not found.
 *	Zero is returned on a success.
 *	
 *	Once this function has returned the &struct file_system_type structure
 *	may be freed or reused.
 */
 
int unregister_filesystem(struct file_system_type * fs)
{
	struct file_system_type ** tmp;

	write_lock(&file_systems_lock);
	tmp = &file_systems;
	while (*tmp) {
		if (fs == *tmp) {
			*tmp = fs->next;
			fs->next = NULL;
			write_unlock(&file_systems_lock);
			synchronize_rcu();
			return 0;
		}
		tmp = &(*tmp)->next;
	}
	write_unlock(&file_systems_lock);

	return -EINVAL;
}

EXPORT_SYMBOL(unregister_filesystem);

Here is the struct file_system_type data-structure (/include/linux/fs.h) from the Kernel-source version 6.5.9 for quick reference:

struct file_system_type {
	const char *name;
	int fs_flags;
#define FS_REQUIRES_DEV		1 
#define FS_BINARY_MOUNTDATA	2
#define FS_HAS_SUBTYPE		4
#define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
#define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
#define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
#define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
	int (*init_fs_context)(struct fs_context *);
	const struct fs_parameter_spec *parameters;
	struct dentry *(*mount) (struct file_system_type *, int,
		       const char *, void *);
	void (*kill_sb) (struct super_block *);
	struct module *owner;
	struct file_system_type * next;
	struct hlist_head fs_supers;

	struct lock_class_key s_lock_key;
	struct lock_class_key s_umount_key;
	struct lock_class_key s_vfs_rename_key;
	struct lock_class_key s_writers_key[SB_FREEZE_LEVELS];

	struct lock_class_key i_lock_key;
	struct lock_class_key i_mutex_key;
	struct lock_class_key invalidate_lock_key;
	struct lock_class_key i_mutex_dir_key;
};