VFS - Virtual File System

static struct file_system_type *file_systems;

$ cat /proc/filesystems       
nodev   sysfs
nodev   tmpfs
nodev   bdev
nodev   proc
nodev   cgroup
nodev   cgroup2
nodev   cpuset
nodev   devtmpfs
nodev   configfs
nodev   debugfs
nodev   tracefs
nodev   securityfs
nodev   sockfs
...

struct dentry {
    ...
    struct hlist_bl_node d_hash;
    struct dentry *d_parent;
    struct qstr d_name;
    struct inode *d_inode;
    unsigned char d_iname[DNAME_INLINE_LEN
    ...
    const struct dentry_operations *d_op;
    struct super_block *d_sb;
    ...
    struct list_head d_child;
    struct list_head d_subdirs;
    ...
};

struct dentry_operations {
    int (*d_revalidate)(struct dentry *, unsigned int);
    int (*d_weak_revalidate)(struct dentry *, unsigned int);
    int (*d_hash)(const struct dentry *, struct qstr *);
    int (*d_compare)(const struct dentry *,
                     unsigned int, const char *, const struct qstr *);
    int (*d_delete)(const struct dentry *);
    int (*d_init)(struct dentry *);
    void (*d_release)(struct dentry *);
    void (*d_prune)(struct dentry *);
    void (*d_iput)(struct dentry *, struct inode *);
    char *(*d_dname)(struct dentry *, char *, int);
    struct vfsmount *(*d_automount)(struct path *);
    int (*d_manage)(const struct path *, bool);
    struct dentry *(*d_real)(struct dentry *, const struct inode *);
}

struct super_block {
    ...
    unsigned char		s_blocksize_bits;
    unsigned long		s_blocksize;
    loff_t			s_maxbytes;	/* Max file size */
    struct file_system_type	*s_type;
    const struct super_operations	*s_op;
    unsigned long		s_magic;
    struct dentry		*s_root;
    void			*s_fs_info;	/* Filesystem private info */
    char			s_id[32];	/* Informational name */
    uuid_t			s_uuid;		/* UUID */
    fmode_t			s_mode;
    const struct dentry_operations *s_d_op; /* default d_op for dentries */
    struct list_head	s_inodes;	/* all inodes */
    struct list_head	s_inodes_wb;	/* writeback inodes */
    ...
};

struct super_operations {
    struct inode *(*alloc_inode)(struct super_block *sb);
    void (*destroy_inode)(struct inode *);
    void (*dirty_inode) (struct inode *, int flags);
    int (*write_inode) (struct inode *, int);
    void (*drop_inode) (struct inode *);
    void (*delete_inode) (struct inode *);
    void (*put_super) (struct super_block *);
    int (*sync_fs)(struct super_block *sb, int wait);
    int (*freeze_fs) (struct super_block *);
    ...
};

struct dentry *__d_lookup(const struct dentry *parent, const struct qstr *name)
{
	unsigned int hash = name->hash;
	struct hlist_bl_head *b = d_hash(hash);
	struct hlist_bl_node *node;
	struct dentry *found = NULL;
	struct dentry *dentry;

	rcu_read_lock();
	
	hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {

		if (dentry->d_name.hash != hash)
			continue;

		spin_lock(&dentry->d_lock);
		if (dentry->d_parent != parent)
			goto next;
		if (d_unhashed(dentry))
			goto next;

		if (!d_same_name(dentry, parent, name))
			goto next;

		dentry->d_lockref.count++;
		found = dentry;
		spin_unlock(&dentry->d_lock);
		break;
next:
		spin_unlock(&dentry->d_lock);
 	}
 	rcu_read_unlock();

 	return found;
}

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;
};

#include <linux/fs.h>

extern int register_filesystem(struct file_system_type *);
extern int unregister_filesystem(struct file_system_type *);

module_init(ext4_init_fs)
module_exit(ext4_exit_fs)

static int __init ext4_init_fs(void)
{
    ...
    err = register_filesystem(&ext4_fs_type);
    if (err)
    	goto out;
    return 0;
    ...
}


static void __exit ext4_exit_fs(void)
{
    ...
    unregister_filesystem(&ext4_fs_type);
    ...
}

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;
}

bool fs_validate_description(const char *name,
	const struct fs_parameter_spec *desc)
{
	const struct fs_parameter_spec *param, *p2;
	bool good = true;

	for (param = desc; param->name; param++) {
		/* Check for duplicate parameter names */
		for (p2 = desc; p2 < param; p2++) {
			if (strcmp(param->name, p2->name) == 0) {
				if (is_flag(param) != is_flag(p2))
					continue;
				pr_err("VALIDATE %s: PARAM[%s]: Duplicate\n",
				       name, param->name);
				good = false;
			}
		}
	}
	return good;
}

static struct file_system_type **find_filesystem(const char *name, unsigned len)
{
	struct file_system_type **p;
	for (p = &file_systems; *p; p = &(*p)->next)
		if (strncmp((*p)->name, name, len) == 0 &&
		    !(*p)->name[len])
			break;
	return p;
}

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;
}

struct file_system_type {
    const char *name;
    int fs_flags;
    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 list_head fs_supers;
    struct lock_class_key s_lock_key;
    struct lock_class_key s_umount_key;
    ...
};

static int ramfs_fill_super(struct super_block *sb, void *data, int silent)
{
        struct ramfs_fs_info *fsi;
        struct inode *inode;
        int err;

        save_mount_options(sb, data);

        fsi = kzalloc(sizeof(struct ramfs_fs_info), GFP_KERNEL);
        sb->s_fs_info = fsi;
        if (!fsi)
                return -ENOMEM;

        err = ramfs_parse_options(data, &fsi->mount_opts);
        if (err)
                return err;

        sb->s_maxbytes          = MAX_LFS_FILESIZE;
        sb->s_blocksize         = PAGE_SIZE;
        sb->s_blocksize_bits    = PAGE_SHIFT;
        sb->s_magic             = RAMFS_MAGIC;
        sb->s_op                = &ramfs_ops;
        sb->s_time_gran         = 1;

        inode = ramfs_get_inode(sb, NULL, S_IFDIR | fsi->mount_opts.mode, 0);
        sb->s_root = d_make_root(inode);
        if (!sb->s_root)
                return -ENOMEM;

        return 0;
}

struct dentry *mount_nodev(struct file_system_type *fs_type,
	int flags, void *data,
	int (*fill_super)(struct super_block *, void *, int))
{
	int error;
	struct super_block *s = sget(fs_type, NULL, set_anon_super, flags, NULL);

	if (IS_ERR(s))
		return ERR_CAST(s);

	error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
	if (error) {
		deactivate_locked_super(s);
		return ERR_PTR(error);
	}
	s->s_flags |= SB_ACTIVE;
	return dget(s->s_root);
}
EXPORT_SYMBOL(mount_nodev);

 *	generic_shutdown_super	-	common helper for ->kill_sb()
 *	@sb: superblock to kill
 *
 *	generic_shutdown_super() does all fs-independent work on superblock
 *	shutdown.  Typical ->kill_sb() should pick all fs-specific objects
 *	that need destruction out of superblock, call generic_shutdown_super()
 *	and release aforementioned objects.  Note: dentries and inodes _are_
 *	taken care of and do not need specific handling.

struct buffer_head {
	unsigned long b_state;		/* buffer state bitmap (see above) */
	struct buffer_head *b_this_page;/* circular list of page's buffers */
	union {
		struct page *b_page;	/* the page this bh is mapped to */
		struct folio *b_folio;	/* the folio this bh is mapped to */
	};

	sector_t b_blocknr;		/* start block number */
	size_t b_size;			/* size of mapping */
	char *b_data;			/* pointer to data within the page */

	struct block_device *b_bdev;
	bh_end_io_t *b_end_io;		/* I/O completion */
 	void *b_private;		/* reserved for b_end_io */
	struct list_head b_assoc_buffers; /* associated with another mapping */
	struct address_space *b_assoc_map;	/* mapping this buffer is
						   associated with */
	atomic_t b_count;		/* users using this buffer_head */
	spinlock_t b_uptodate_lock;	/* Used by the first bh in a page, to
					 * serialise IO completion of other
					 * buffers in the page */
};