明明白白我的心--fs series(6)

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作者：GTT

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Linux Version：2.6.33

提示：本文是关于file system 实现的介绍！

 

先把super block的定义代码传上来，如下

struct super_block {
    struct list_head s_list; /* Keep this first */
    dev_t s_dev; /* search index; _not_ kdev_t */
    unsigned long s_blocksize;
    unsigned char s_blocksize_bits;
    unsigned char s_dirt;
    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;
    int s_need_sync;
    atomic_t s_active;
#ifdef CONFIG_SECURITY
    void *s_security;
#endif
    struct xattr_handler **s_xattr;

    struct list_head s_inodes; /* all inodes */
    struct hlist_head s_anon; /* anonymous dentries for (nfs) exporting */
    struct list_head s_files;
/* s_dentry_lru and s_nr_dentry_unused are protected by dcache_lock */
    struct list_head s_dentry_lru; /* unused dentry lru */
    int s_nr_dentry_unused; /* # of dentry on lru */

    struct block_device *s_bdev;
    struct backing_dev_info *s_bdi;
    struct mtd_info *s_mtd;
    struct list_head s_instances;
    struct quota_info s_dquot; /* Diskquota specific options */

    int s_frozen;
    wait_queue_head_t s_wait_unfrozen;

    char s_id[32]; /* Informational name */

    void *s_fs_info; /* Filesystem private info */
    fmode_t s_mode;

/* 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 */

/* Granularity of c/m/atime in ns. Cannot be worse than a second */
    u32 s_time_gran;

/* 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 *s_options;
};

 

super_block描述整个分区的文件系统信息，例如块大小、文件系统版本号、上次mount的时间等等。

 

super_block定义的字段比较多，现在没必要清楚每个字段的具体意义，但其中重要的字段必须了解。
当一个文件系统被mount时,系统将调用文件系统的get_sb，生成一个super block结构，也就是说，每一个
安装的文件系统，都有一个super block 实例与之对应。s_type就是文件系统的定义指针。这样通过s_type
就可以找到文件体统。linux系统利用s_list把所有的super block都链接到super_blocks上。这样安装了多少个文件系统，通过super_blocks链表，遍历一下马上就能知道了。

 

struct super_operations  *s_op是super block 的操作方法，struct super_operations是一个VFT即
virture function table ,虚拟方法表。我是这么翻译的，可能有的书里翻译成虚拟功能表等，一个东西。
直接就是VFT最方便，也不用翻译。因为各个文件系统的super block的操作方法不同，所以定义了这个VFT。
看看他的定义

struct super_operations {
    struct inode *(*alloc_inode)(struct super_block *sb);
    void (*destroy_inode)(struct inode *);

    void (*dirty_inode) (struct inode *);
    int (*write_inode) (struct inode *, int);
    void (*drop_inode) (struct inode *);
    void (*delete_inode) (struct inode *);
    void (*put_super) (struct super_block *);
    void (*write_super) (struct super_block *);
    int (*sync_fs)(struct super_block *sb, int wait);
    int (*freeze_fs) (struct super_block *);
    int (*unfreeze_fs) (struct super_block *);
    int (*statfs) (struct dentry *, struct kstatfs *);
    int (*remount_fs) (struct super_block *, int *, char *);
    void (*clear_inode) (struct inode *);
    void (*umount_begin) (struct super_block *);

    int (*show_options)(struct seq_file *, struct vfsmount *);
    int (*show_stats)(struct seq_file *, struct vfsmount *);
#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);
#endif
    int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
};

 

通过方法名称就基本可以了解要干什么了，
比如alloc_inode，就是分配一个inode 。destroy_inode，就是销毁一个inode等等
super block 要管理inode,所以super block 把所有分配的inode 都链接到s_inodes上。

把所有跟super block 关联的文件都链接到s_files上。

把未使用的dentry链接到s_dentry_lru这个链上。