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;
struct super_block {
struct list_head s_list; /* Keep this first */
dev_t s_dev; /* search index; _not_ kdev_t */
unsigned char s_dirt;
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;
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;
struct mutex s_lock;
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 */
#ifdef CONFIG_SMP
struct list_head __percpu *s_files;
#else
struct list_head s_files;
#endif
struct list_head s_mounts; /* list of mounts; _not_ for fs use */
/* s_dentry_lru, s_nr_dentry_unused protected by dcache.c lru locks */
struct list_head s_dentry_lru; /* unused dentry lru */
int s_nr_dentry_unused; /* # of dentry on lru */ /* s_inode_lru_lock protects s_inode_lru and s_nr_inodes_unused */
spinlock_t s_inode_lru_lock ____cacheline_aligned_in_smp;
struct list_head s_inode_lru; /* unused inode lru */
int s_nr_inodes_unused; /* # of inodes on lru */ struct block_device *s_bdev;
struct backing_dev_info *s_bdi;
struct mtd_info *s_mtd;
struct hlist_node s_instances;
struct quota_info s_dquot; /* Diskquota specific options */ int s_frozen;
wait_queue_head_t s_wait_unfrozen; char s_id[]; /* Informational name */
u8 s_uuid[]; /* UUID */ void *s_fs_info; /* Filesystem private 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;
};
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。
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。
dev_t s_dev;
s_dev:设备标识符
unsigned char s_dirt;
unsigned char s_blocksize_bits;
unsigned long s_blocksize;
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
loff_t s_maxbytes; /* Max file size */
s_maxbytes:允许的最大的文件大小(字节数)
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。
const struct super_operations *s_op;
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。
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。
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
struct dentry *s_root;
s_root:指向该具体文件系统安装目录的目录项
struct rw_semaphore s_umount;
s_umount:对超级块读写时进行同步
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
int s_count;
s_count:对超级块的使用计数
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: