linux kernel文件系统

任务管理、内存管理和文件系统是内核最为核心的部分。其中内存管理最为复杂，文件系统最为庞大。内核通过引入虚拟文件系统，从而支持数十种不同的文件系统，不得不让人感叹Linux以及它显示出的开源的力量的强大。

虚拟文件系统(Virtual Filesystem)，也可以称为虚拟文件系统转换(Virual Filesystem Switch ，简称VFS)，是Linux强大的集中体现。Linux通过VFS可以处理几十种特定的文件系统，它在内核中的位置如下：

1. 数据结构
   虚拟文件系统所隐含的主要思想在于引入了一个通用的文件模型，这个模型能够表示所有支持的文件系统。该模型严格遵守传统Unix文件系统提供的文件模型。通用文件模型由下列关键对象组成：
   (1) 超级块（superblock）对象：
   存放系统中已安装文件系统的有关信息。对于基于磁盘的文件系统，这类对象通常对应于存放在磁盘上的文件系统控制块,也就是说,每个文件系统都有一个超级块对象。
   (2) 索引节点（inode）对象：
   存放关于具体文件的一般信息。对于基于磁盘的文件系统，这类对象通常对应于存放在磁盘上的文件控制块(FCB),也就是说,每个文件都有一个索引节点对象。每个索引节点对象都有一个索引节点号,这个号唯一地标识某个文件系统中的指定文件。
   (3) 目录项（dentry）对象：
   存放目录项与对应文件进行链接的信息。VFS把每个目录看作一个由若干子目录和文件组成的常规文件。
   (4) 文件(file)对象：
   存放打开文件与进程之间进行交互的有关信息。这类信息仅当进程访问文件期间存在于内存中。
   进程与VFS之间的交互：
   

2、超级块对象

//VFS超级块(fs/fs.h)struct super_block {    struct list_head    s_list;        /* Keep this first,超级块链表*/    dev_t            s_dev;        /* search index; _not_ kdev_t,设备号*/    unsigned long        s_blocksize; //块大小(以字节为单位)    unsigned long        s_old_blocksize;    unsigned char        s_blocksize_bits; //以位为单位的块大小    unsigned char        s_dirt;                 /*脏位，若置该位，表明该超级块已被修改*/    unsigned long long    s_maxbytes;    /* Max file size, 文件的最大长度 */    struct file_system_type    *s_type;   //文件系统类型    struct super_operations    *s_op; //超级块操作集合    struct dquot_operations    *dq_op;     struct quotactl_ops    *s_qcop;    struct export_operations *s_export_op;    unsigned long        s_flags;    unsigned long        s_magic;    struct dentry        *s_root; /*指向该具体文件系统安装目录的目录项。*/     struct rw_semaphore    s_umount;//卸载所用的信号量    struct semaphore    s_lock;  //超级块信号量       int            s_count;      //引用计数    int            s_syncing;  //索引节点同步标志    int            s_need_sync_fs; //文件系统同步标志    atomic_t        s_active;    void                    *s_security;    struct xattr_handler    **s_xattr;  struct list_head    s_inodes;    /* all inodes,所有索引节点*/    struct list_head    s_dirty;    /* dirty inodes, 已修改的索引节点形成的链表 */    struct list_head    s_io;        /* parked for writeback,等待写回磁盘的索引节点 */    struct hlist_head    s_anon;        /* anonymous dentries for (nfs) exporting */    struct list_head    s_files;  //文件对象的链表    struct block_device    *s_bdev; //指向块设备驱动程序描述符指针    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,指向特定文件系统超级块 */    /*     * The next field is for VFS *only*. No filesystems have any business     * even looking at it. You had been warned.     */    struct semaphore s_vfs_rename_sem;    /* Kludge */        /* Granuality of c/m/atime in ns.       Cannot be worse than a second */    u32           s_time_gran;};//超级块链表(fs/super.c)LIST_HEAD(super_blocks);//超级块操作函数struct super_operations {       struct inode *(*alloc_inode)(struct super_block *sb);    void (*destroy_inode)(struct inode *);    void (*read_inode) (struct inode *);       void (*dirty_inode) (struct inode *);    int (*write_inode) (struct inode *, int);    void (*put_inode) (struct inode *);    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);    void (*write_super_lockfs) (struct super_block *);    void (*unlockfs) (struct super_block *);    int (*statfs) (struct super_block *, 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 *);    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);};//include/linux/fs.h//文件系统类型struct file_system_type {    const char *name;  //文件系统名称    int fs_flags;   //文件系统类型的标志    struct super_block *(*get_sb) (struct file_system_type *, int,                       const char *, void *); //读超级块的方法    void (*kill_sb) (struct super_block *); //删除超级块的方法    struct module *owner;//实现文件系统的模块    struct file_system_type * next; //文件系统类型链表的下一个元素    struct list_head fs_supers; //具有相同文件系统类型的超级块对象链表头};

超级块和文件系统类型的关系如下：

3、inode对象

//索引节点对象(fs/fs.h)struct inode {    struct hlist_node    i_hash; //散列链表    struct list_head    i_list;   //索引结点链表    struct list_head    i_sb_list;     struct list_head    i_dentry; //引用索引节点的目录项对象链表    unsigned long        i_ino;  //索引节点号    atomic_t        i_count;     //引用计数    umode_t            i_mode;    unsigned int        i_nlink; //硬链接数量    uid_t            i_uid;    gid_t            i_gid;    dev_t            i_rdev; //实设备标识    loff_t            i_size;  //文件的字节数    struct timespec        i_atime;    struct timespec        i_mtime;    struct timespec        i_ctime;    unsigned int        i_blkbits;  //块的位数    unsigned long        i_blksize;  //块的字节数    unsigned long        i_version;    unsigned long        i_blocks;  //文件的块数    unsigned short          i_bytes; //文件中最后一块的字节数    unsigned char        i_sock;    spinlock_t        i_lock;    /* i_blocks, i_bytes, maybe i_size */    struct semaphore    i_sem;    struct rw_semaphore    i_alloc_sem;    struct inode_operations    *i_op; //索引节点的操作函数    struct file_operations    *i_fop;    /* former ->i_op->default_file_ops,默认文件操作函数 */    struct super_block    *i_sb; //指向超级块    struct file_lock    *i_flock; //文件锁    struct address_space    *i_mapping;    struct address_space    i_data;#ifdef CONFIG_QUOTA    struct dquot        *i_dquot[MAXQUOTAS];#endif    /* These three should probably be a union */    struct list_head    i_devices;    struct pipe_inode_info    *i_pipe;    struct block_device    *i_bdev;  //块设备    struct cdev        *i_cdev;        //字符设备    int            i_cindex;    __u32            i_generation;#ifdef CONFIG_DNOTIFY    unsigned long        i_dnotify_mask; /* Directory notify events */    struct dnotify_struct    *i_dnotify; /* for directory notifications */#endif    unsigned long        i_state;    unsigned long        dirtied_when;    /* jiffies of first dirtying */    unsigned int        i_flags;    atomic_t        i_writecount;    void            *i_security;    union {        void        *generic_ip;    } u;#ifdef __NEED_I_SIZE_ORDERED    seqcount_t        i_size_seqcount;#endif};//fs/inode.cLIST_HEAD(inode_in_use); //正在使用的索引节点链表LIST_HEAD(inode_unused); //有效未使用的索引节点链表//索引节点对象存放的散列表static struct hlist_head *inode_hashtable; //索引节点缓存static kmem_cache_t * inode_cachep; //索引节点操作函数struct inode_operations {    int (*create) (struct inode *,struct dentry *,int, struct nameidata *);    struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);    int (*link) (struct dentry *,struct inode *,struct dentry *);    int (*unlink) (struct inode *,struct dentry *);    int (*symlink) (struct inode *,struct dentry *,const char *);    int (*mkdir) (struct inode *,struct dentry *,int);    int (*rmdir) (struct inode *,struct dentry *);    int (*mknod) (struct inode *,struct dentry *,int,dev_t);    int (*rename) (struct inode *, struct dentry *,            struct inode *, struct dentry *);    int (*readlink) (struct dentry *, char __user *,int);    int (*follow_link) (struct dentry *, struct nameidata *);    void (*put_link) (struct dentry *, struct nameidata *);    void (*truncate) (struct inode *);    int (*permission) (struct inode *, int, struct nameidata *);    int (*setattr) (struct dentry *, struct iattr *);    int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);    int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);    ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);    ssize_t (*listxattr) (struct dentry *, char *, size_t);    int (*removexattr) (struct dentry *, const char *);};

索引节点与超级块的关系：

4、目录项对象

//include/linux/dcache.hstruct dentry {    atomic_t d_count;  //引用计数    unsigned int d_flags;        /* protected by d_lock */    spinlock_t d_lock;        /* per dentry lock */    struct inode *d_inode;        /* Where the name belongs to - NULL is                     * negative,相关的索引节点*/    /*     * The next three fields are touched by __d_lookup.  Place them here     * so they all fit in a 16-byte range, with 16-byte alignment.     */    struct dentry *d_parent;    /* parent directory,父目录的目录对象*/    struct qstr d_name;  //文件名    struct list_head d_lru;        /* LRU list,用于未使用目录项链表*/    struct list_head d_child;    /* child of parent list,同一父目录的目录项链表*/    struct list_head d_subdirs;    /* our children,子目录链表*/    struct list_head d_alias;    /* inode alias list,连接同一索引节点的目录项链表*/    unsigned long d_time;        /* used by d_revalidate */    struct dentry_operations *d_op; //目录操作函数    struct super_block *d_sb;    /* The root of the dentry tree,指向文件的超级对象*/    void *d_fsdata;            /* fs-specific data */     struct rcu_head d_rcu;    struct dcookie_struct *d_cookie; /* cookie, if any */    struct hlist_node d_hash;    /* lookup hash list,目录项对象散列表*/        int d_mounted;    unsigned char d_iname[DNAME_INLINE_LEN_MIN];    /* small names */};//dcache.c//目录项对象缓存static kmem_cache_t *dentry_cache; //目录项对象散列表static struct hlist_head *dentry_hashtable;/***所有"未使用"目录项对象都存放在一个LRU双向链表中.该变量存放LRU链表的首元素和尾元素的地址.*/static LIST_HEAD(dentry_unused);struct dentry_operations {    int (*d_revalidate)(struct dentry *, struct nameidata *);    int (*d_hash) (struct dentry *, struct qstr *);    int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);    int (*d_delete)(struct dentry *);    void (*d_release)(struct dentry *);    void (*d_iput)(struct dentry *, struct inode *);};

超级块、索引节点与目录项的关系:

5、文件对象

//文件对象(fs/fs.h)struct file {    struct list_head    f_list;      struct dentry        *f_dentry;     //与文件相关的目录项对象    struct vfsmount         *f_vfsmnt; //含有该文件的文件系统的安装点    struct file_operations    *f_op;    atomic_t        f_count;              //引用计数    unsigned int         f_flags;    mode_t            f_mode;    int            f_error;    loff_t            f_pos;           //文件偏移    struct fown_struct    f_owner;    unsigned int        f_uid, f_gid;    struct file_ra_state    f_ra;    size_t            f_maxcount;    unsigned long        f_version;    void            *f_security;    /* needed for tty driver, and maybe others */    void            *private_data;#ifdef CONFIG_EPOLL    /* Used by fs/eventpoll.c to link all the hooks to this file */    struct list_head    f_ep_links;    spinlock_t        f_ep_lock;#endif /* #ifdef CONFIG_EPOLL */    struct address_space    *f_mapping;};struct file_operations {    struct module *owner;    loff_t (*llseek) (struct file *, loff_t, int);    ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);    ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);    ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);    ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t, loff_t);    int (*readdir) (struct file *, void *, filldir_t);    unsigned int (*poll) (struct file *, struct poll_table_struct *);    int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);    long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);    long (*compat_ioctl) (struct file *, unsigned int, unsigned long);    int (*mmap) (struct file *, struct vm_area_struct *);    int (*open) (struct inode *, struct file *);    int (*flush) (struct file *);    int (*release) (struct inode *, struct file *);    int (*fsync) (struct file *, struct dentry *, int datasync);    int (*aio_fsync) (struct kiocb *, int datasync);    int (*fasync) (int, struct file *, int);    int (*lock) (struct file *, int, struct file_lock *);    ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);    ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);    ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);    ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);    unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);    int (*check_flags)(int);    int (*dir_notify)(struct file *filp, unsigned long arg);    int (*flock) (struct file *, int, struct file_lock *);};

超级块、索引节点、目录项和文件对象的关系：

6、与进程相关的数据结构

//include/linux/namespace.h//命名空间struct namespace {    atomic_t        count; //引用计数(共享命名空间的进程数)    struct vfsmount *    root; //命名空间的根目录的安装文件系统描述符    struct list_head    list;  //所有安装文件系统描述符的链表头    struct rw_semaphore    sem; //读写信号量};//include/linux/fs_struct.h//进程的根目录和当前目录struct fs_struct {    atomic_t count; //引用计数(共享该结构的进程的个数)    rwlock_t lock;    int umask;    struct dentry * root, * pwd, * altroot; //根目录对应的目录项,当前工作目录对应的目录项    struct vfsmount * rootmnt, * pwdmnt, * altrootmnt; //根目录的安装点,当前工作目录的安装占};//include/linux/file.h//进程打开的文件表struct files_struct {        atomic_t count;  //共享该表的进程个数        spinlock_t file_lock;     /* Protects all the below members.  Nests inside tsk->alloc_lock */        int max_fds;             /*当前文件对象的最大数*/        int max_fdset;          /*当前文件描述符的最大数*/        int next_fd;               /*已分配的文件描述符加1*/        struct file ** fd;      /* 指向文件对象指针数组的指针 */        fd_set *close_on_exec; /*指向执行exec(  )时需要关闭的文件描述符*/        fd_set *open_fds;       /*指向打开文件描述符的指针*/        fd_set close_on_exec_init;  /* 执行exec(  )时需要关闭的文件描述符的初值集合*/        fd_set open_fds_init; /*文件描述符的初值集合*/        struct file * fd_array[NR_OPEN_DEFAULT]; /*文件对象指针的初始化数组*/};//include/linux/sched.hstruct task_struct {//    struct fs_struct *fs;  //文件系统信息/* open file information */    struct files_struct *files;//打开的文件/* namespace */    struct namespace *namespace;//文件系统命名空间//}