VFS笔记

Table of Contents

时间:2014-02-23

1 为什么有VFS

因为Linux系统支持各种文件系统,甚至微软的NTFS,不同的文件系统操作方法不一致,为了保持一致,VFS诞生,使得开发者只管调用write、read的函数,每种文件系统的细节都被VFS隐藏了。

2 VFS运用了面向对象思想。

VFS提供外部接口,结构定义在include/linux/fs.h里:

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 (*write) (struct file *, const char __user *, size_t, loff_t *);
  ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
  ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
  int (*readdir) (struct file *, void *, filldir_t);
  unsigned int (*poll) (struct file *, struct poll_table_struct *);
  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 *, fl_owner_t id);
  int (*release) (struct inode *, struct file *);
  int (*fsync) (struct file *, loff_t, loff_t, 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 (*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 (*flock) (struct file *, int, struct file_lock *);
  ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
  ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
  int (*setlease)(struct file *, long, struct file_lock **);
  long (*fallocate)(struct file *file, int mode, loff_t offset,
		    loff_t len);
};

里面有各种操作方法,当open一个文件的时候,返回一个file结构体,这个结构体中有一个fop指针,就指向了fileoperations。file定义如下:

struct file {
  /*
   * fu_list becomes invalid after file_free is called and queued via
   * fu_rcuhead for RCU freeing
   */
  union {
    struct list_head   fu_list;
    struct rcu_head    fu_rcuhead;
  } f_u;
  struct path   f_path;
#define f_dentry   f_path.dentry
#define f_vfsmnt   f_path.mnt
  const struct file_operations   *f_op;

  /*
   * Protects f_ep_links, f_flags, f_pos vs i_size in lseek SEEK_CUR.
   * Must not be taken from IRQ context.
   */
  spinlock_t    f_lock;
#ifdef CONFIG_SMP
  int      f_sb_list_cpu;
#endif
  atomic_long_t     f_count;
  unsigned int      f_flags;
  fmode_t      f_mode;
  loff_t       f_pos;
  struct fown_struct f_owner;
  const struct cred  *f_cred;