Understanding the Linux Kernel, 3rd Edition

VFS Data Structures

Each VFS object is stored in a suitable data structure, which includes both the object attributes and a pointer to a table of object methods. The kernel may dynamically modify the methods of the object and, hence, it may install specialized behavior for the object. The following sections explain the VFS objects and their interrelationships in detail.

Superblock Objects

A superblock object consists of a super_block structure whose fields are described in Table 12-2.

Table 12-2. The fields of the superblock object

Type	Field	Description
struct list_head	s_list	Pointers for superblock list
dev_t	s_dev	Device identifier
unsigned long	s_blocksize	Block size in bytes
unsigned long	s_old_blocksize	Block size in bytes as reported by the underlying block device driver
unsigned char	s_blocksize_bits	Block size in number of bits
unsigned char	s_dirt	Modified (dirty) flag
unsigned long long	s_maxbytes	Maximum size of the files
struct file_system_type *	s_type	Filesystem type
struct super_operations *	s_op	Superblock methods
struct dquot_operations *	dq_op	Disk quota handling methods
struct quotactl_ops *	s_qcop	Disk quota administration methods
struct export_operations *	s_export_op	Export operations used by network filesystems
unsigned long	s_flags	Mount flags
unsigned long	s_magic	Filesystem magic number
struct dentry *	s_root	Dentry object of the filesystem’s root directory
struct rw_semaphore	s_umount	Semaphore used for unmounting
struct semaphore	s_lock ...