VFS超级块
VFS超级块是根据具体文件系统的超级块建立起来的内存结构：
struct super_block {
struct list_head s_list; /* Keep this first */
dev_t s_dev; /* search index; _not_ kdev_t */
unsigned char s_blocksize_bits;
unsigned long s_blocksize;
loff_t s_maxbytes; /* Max file size */
struct file_system_type *s_type;指向对应的文件系统对象
const struct super_operations *s_op;指向具体文件系统超级块操作函数
const struct dquot_operations *dq_op;
const struct quotactl_ops *s_qcop;
const struct export_operations *s_export_op;
unsigned long s_flags;
unsigned long s_magic;
struct dentry *s_root;
struct rw_semaphore s_umount;
int s_count;
atomic_t s_active;
#ifdef CONFIG_SECURITY
void *s_security;
#endif
const struct xattr_handler **s_xattr;
struct list_head s_inodes; /* all inodes */
struct hlist_bl_head s_anon; /* anonymous dentries for (nfs) exporting */
struct list_head s_mounts; /* list of mounts; _not_ for fs use */
struct block_device *s_bdev;
struct backing_dev_info *s_bdi;
struct mtd_info *s_mtd;
struct hlist_node s_instances;
unsigned int s_quota_types; /* Bitmask of supported quota types */
struct quota_info s_dquot; /* Diskquota specific options */
struct sb_writers s_writers;
char s_id[32]; /* Informational name */
u8 s_uuid[16]; /* UUID */
void *s_fs_info; /* Filesystem private info */指向具体文件系统的超级块内存对象，就是ext4_sb_info
unsigned int s_max_links;
fmode_t s_mode;
/* Granularity of c/m/atime in ns.
Cannot be worse than a second */
u32 s_time_gran;
/*
* The next field is for VFS *only*. No filesystems have any business
* even looking at it. You had been warned.
*/
struct mutex s_vfs_rename_mutex; /* Kludge */
/*
* Filesystem subtype. If non-empty the filesystem type field
* in /proc/mounts will be "type.subtype"
*/
char *s_subtype;
/*
* Saved mount options for lazy filesystems using
* generic_show_options()
*/
char __rcu *s_options;
const struct dentry_operations *s_d_op; /* default d_op for dentries */
/*
* Saved pool identifier for cleancache (-1 means none)
*/
int cleancache_poolid;
struct shrinker s_shrink; /* per-sb shrinker handle */
/* Number of inodes with nlink == 0 but still referenced */
atomic_long_t s_remove_count;
/* Being remounted read-only */
int s_readonly_remount;
/* AIO completions deferred from interrupt context */
struct workqueue_struct *s_dio_done_wq;
struct hlist_head s_pins;
/*
* Keep the lru lists last in the structure so they always sit on their
* own individual cachelines.
*/
struct list_lru s_dentry_lru ____cacheline_aligned_in_smp;
struct list_lru s_inode_lru ____cacheline_aligned_in_smp;
struct rcu_head rcu;
/*
* Indicates how deep in a filesystem stack this SB is
*/
int s_stack_depth;
};当内核需要挂载（mount）一个块设备时，可以从分区表中信息得知这个块设备的文件系统类型，从文章EXT4文件系统学习（八）磁盘结构可以看出分区信息中的文件系统类型，也可以从分区的superblock信息中看出文件系统类型。然后从file_systems文件系统对象链表中找到对应的文件系统驱动程序的文件系统对象，调用里面的mount()函数获取具体的文件系统超级块信息。然后根据这些信息初始化VFS超级块，结构中的s_fs_info就指向具体文件系统的超级块内存对象，也就是ext4_sb_info。
由于各个文件系统的超级块不同，所以对操作超级块的方法也不同。为此内核定义了一个super_operations结构，定义如下：
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);分配一个inode结构
void (*destroy_inode)(struct inode *);释放一个inode结构
void (*dirty_inode) (struct inode *, int flags);
int (*write_inode) (struct inode *, struct writeback_control *wbc);
int (*drop_inode) (struct inode *);
void (*evict_inode) (struct inode *);
void (*put_super) (struct super_block *);
int (*sync_fs)(struct super_block *sb, int wait);
int (*freeze_super) (struct super_block *);
int (*freeze_fs) (struct super_block *);
int (*thaw_super) (struct super_block *);
int (*unfreeze_fs) (struct super_block *);
int (*statfs) (struct dentry *, struct kstatfs *);
int (*remount_fs) (struct super_block *, int *, char *);
void (*umount_begin) (struct super_block *);
int (*show_options)(struct seq_file *, struct dentry *);
int (*show_devname)(struct seq_file *, struct dentry *);
int (*show_path)(struct seq_file *, struct dentry *);
int (*show_stats)(struct seq_file *, struct dentry *);
#ifdef CONFIG_QUOTA
ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
struct dquot **(*get_dquots)(struct inode *);
#endif
int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
long (*nr_cached_objects)(struct super_block *,
struct shrink_control *);
long (*free_cached_objects)(struct super_block *,
struct shrink_control *);
};可以看出super_operations结构中的函数指针都是在操作下层文件系统，不同的文件系统super_operations也是不同的。
当内核挂载块设备时，会根据分区表读出文件系统类型信息，然后找到驱动中对应的已经注册过的文件系统对象，并调用它的mount函数设置s_op指针。
ext4文件系统的mount函数是ext4_mount，里面调用了ext4_fill_super函数会把磁盘上数据读出，装载磁盘和内存超级块以及VFS超级块。（装载磁盘和内存超级块可参考11节里面介绍ext4_fill_super函数）
static struct dentry *ext4_mount(struct file_system_type *fs_type, int flags,
const char *dev_name, void *data)
{
return mount_bdev(fs_type, flags, dev_name, data, ext4_fill_super);
}这里比较重要的是设置s_op指针：
static int ext4_fill_super(struct super_block *sb, void *data, int silent)
{
sb->s_op = &ext4_sops;这样就建立起了抽象的VFS超级块对象与具体ext4超级块对象的联系。
操作具体文件系统的操作函数ext4_sops如下：
static const struct super_operations ext4_sops = {
.alloc_inode = ext4_alloc_inode,
.destroy_inode = ext4_destroy_inode,
.write_inode = ext4_write_inode,
.dirty_inode = ext4_dirty_inode,
.drop_inode = ext4_drop_inode,
.evict_inode = ext4_evict_inode,
.put_super = ext4_put_super,
.sync_fs = ext4_sync_fs,
.freeze_fs = ext4_freeze,
.unfreeze_fs = ext4_unfreeze,
.statfs = ext4_statfs,
.remount_fs = ext4_remount,
.show_options = ext4_show_options,
#ifdef CONFIG_QUOTA
.quota_read = ext4_quota_read,
.quota_write = ext4_quota_write,
.get_dquots = ext4_get_dquots,
#endif
.bdev_try_to_free_page = bdev_try_to_free_page,
};ext4_fill_super函数最后会请求读取根目录的inode，调用
#define EXT4_ROOT_INO 2 /* Root inode */
root = ext4_iget(sb, EXT4_ROOT_INO);
继续分析iget函数。。。