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?
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。
1 dev_t s_dev;
s_dev:设备标识符
1 unsigned char s_dirt; 2 unsigned char s_blocksize_bits; 3 unsigned long s_blocksize; 4 loff_t s_maxbytes; /* Max file size */
s_dev:包含该具体文件系统的块设备标识符。例如,对于 /dev/hda1,其设备标识符为 0x301
s_blocksize:文件系统中数据块大小,以字节单位
s_blocksize_bits:上面的size大小占用位数,例如512字节就是9 bits
s_dirt:脏位,标识是否超级块被修改
1 loff_t s_maxbytes; /* Max file size */
s_maxbytes:允许的最大的文件大小(字节数)
1 struct file_system_type *s_type;
struct file_system_type *s_type:文件系统类型(也就是当前这个文件系统属于哪个类型?ext2还是fat32)要区分“文件系统”和“文件系统类型”不一样!一个文件系统类型下可以包括很多文件系统即很多的super_block。
1 const struct super_operations *s_op; 2 const struct dquot_operations *dq_op;
struct super_operations *s_op:指向某个特定的具体文件系统的用于超级块操作的函数集合。
struct dquot_operations *dq_op:指向某个特定的具体文件系统用于限额操作的函数集合。
1 const struct quotactl_ops *s_qcop;
struct quotactl_ops *s_qcop:用于配置磁盘限额的的方法,处理来自用户空间的请求。
1 const struct export_operations *s_export_op;
struct export_operations *s_export_op:导出方法
1 unsigned long s_flags;
s_flags:安装标识
1 unsigned long s_magic;
s_magic:区别于其他文件系统的标识
1 struct dentry *s_root;
s_root:指向该具体文件系统安装目录的目录项
1 struct rw_semaphore s_umount;
s_umount:对超级块读写时进行同步
1 struct mutex s_lock;
s_lock:锁标志位,若置该位,则其它进程不能对该超级块操作
1 int s_count;
s_count:对超级块的使用计数
1 atomic_t s_active;
s_active:引用计数
s_dirty:已修改的索引节点inode形成的链表,一个文件系统中有很多的inode,有些inode节点的内容会被修改,那么会先被记录,然后写回磁盘。
s_locked_inodes:要进行同步的索引节点形成的链表
s_files:所有的已经打开文件的链表,这个file和实实在在的进程相关的
s_bdev:指向文件系统被安装的块设备
u:u 联合体域包括属于具体文件系统的超级块信息
s_instances:具体的意义后来会说的!(同一类型的文件系统通过这个子墩将所有的super_block连接起来)
s_dquot:磁盘限额相关选项
Reference:
http://www.linuxidc.com/Linux/2011-02/32127.htm
http://blog.csdn.net/shanshanpt/article/details/38943731