linux虚拟文件系统四大对象：
1）超级块(super block)
2）索引节点(inode)
3）目录项(dentry)
4）文件对象(file)
现在先介绍第一个
一、super_block的含义：
超级块代表了整个文件系统，超级块是文件系统的控制块，有整个文件系统信息，一个文件系统所有的inode都要连接到超级块上，可以说，一个超级块就代表了一个文件系统。
说到inode是啥？参照下一篇博客；
1 struct super_block {
2 struct list_head s_list; /* Keep this first */
3 dev_t s_dev; /* search index; _not_ kdev_t */
4 unsigned char s_dirt;
5 unsigned char s_blocksize_bits;
6 unsigned long s_blocksize;
7 loff_t s_maxbytes; /* Max file size */
8 struct file_system_type *s_type;
9 const struct super_operations *s_op;
10 const struct dquot_operations *dq_op;
11 const struct quotactl_ops *s_qcop;
12 const struct export_operations *s_export_op;
13 unsigned long s_flags;
14 unsigned long s_magic;
15 struct dentry *s_root;
16 struct rw_semaphore s_umount;
17 struct mutex s_lock;
18 int s_count;
19 atomic_t s_active;
20 #ifdef CONFIG_SECURITY
21 void *s_security;
22 #endif
23 const struct xattr_handler **s_xattr;
24
25 struct list_head s_inodes; /* all inodes */
26 struct hlist_bl_head s_anon; /* anonymous dentries for (nfs) exporting */
27 #ifdef CONFIG_SMP
28 struct list_head __percpu *s_files;
29 #else
30 struct list_head s_files;
31 #endif
32 struct list_head s_mounts; /* list of mounts; _not_ for fs use */
33 /* s_dentry_lru, s_nr_dentry_unused protected by dcache.c lru locks */
34 struct list_head s_dentry_lru; /* unused dentry lru */
35 int s_nr_dentry_unused; /* # of dentry on lru */
36
37 /* s_inode_lru_lock protects s_inode_lru and s_nr_inodes_unused */
38 spinlock_t s_inode_lru_lock ____cacheline_aligned_in_smp;
39 struct list_head s_inode_lru; /* unused inode lru */
40 int s_nr_inodes_unused; /* # of inodes on lru */
41
42 struct block_device *s_bdev;
43 struct backing_dev_info *s_bdi;
44 struct mtd_info *s_mtd;
45 struct hlist_node s_instances;
46 struct quota_info s_dquot; /* Diskquota specific options */
47
48 int s_frozen;
49 wait_queue_head_t s_wait_unfrozen;
50
51 char s_id[32]; /* Informational name */
52 u8 s_uuid[16]; /* UUID */
53
54 void *s_fs_info; /* Filesystem private info */
55 unsigned int s_max_links;
56 fmode_t s_mode;
57
58 /* Granularity of c/m/atime in ns.
59 Cannot be worse than a second */
60 u32 s_time_gran;
61
62 /*
63 * The next field is for VFS *only*. No filesystems have any business
64 * even looking at it. You had been warned.
65 */
66 struct mutex s_vfs_rename_mutex; /* Kludge */
67
68 /*
69 * Filesystem subtype. If non-empty the filesystem type field
70 * in /proc/mounts will be "type.subtype"
71 */
72 char *s_subtype;
73
74 /*
75 * Saved mount options for lazy filesystems using
76 * generic_show_options()
77 */
78 char __rcu *s_options;
79 const struct dentry_operations *s_d_op; /* default d_op for dentries */
80
81 /*
82 * Saved pool identifier for cleancache (-1 means none)
83 */
84 int cleancache_poolid;
85
86 struct shrinker s_shrink; /* per-sb shrinker handle */
87
88 /* Number of inodes with nlink == 0 but still referenced */
89 atomic_long_t s_remove_count;
90
91 /* Being remounted read-only */
92 int s_readonly_remount;
93 };
这个数据结构十分庞大，毕竟是聚集了一个文件系统的重要信息，我们关注一些比较重要的信息就行了。
1 struct list_head s_list;
s_list 这是第一个成员，是一个双向循环链表，把所有的super_block连接起来，一个super_block代表一个在linux上的文件系统，这个list上边的就是所有的在linux上记录的文件系统。
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