Everything is a File

文件描述符可以访问 “一切”

• 操作系统内核是如何实现的？
• 如你所料：一个 switch-case

if(f->type == FD_PIPE){
  r = piperead(f->pipe, addr, n);
} else if(f->type == FD_DEVICE){
  if(f->major < 0 || f->major >= NDEV || !devsw[f->major].read)
    return -1;
  r = devsw[f->major].read(1, addr, n);
} else if(f->type == FD_INODE){
  ilock(f->ip);
  if((r = readi(f->ip, 1, addr, f->off, n)) > 0)
    f->off += r;
  iunlock(f->ip);
} else {
  panic("fileread");
}

如果是 /proc/[pid]/maps?

syscall 时，我们有 current

• current 指向了 memory areas 的数据结构
  • $<$ 6.1: rbtree
  • $\ge$ 6.1: maple tree (B-tree)
• 要记得上锁
• 上锁就可能出性能问题
  • per-vma locks in userfaultfd

文件 = 实现了文件操作的 “Anything”

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 (*read_iter) (struct kiocb *, struct iov_iter *);
 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
 int (*iopoll)(struct kiocb *kiocb, struct io_comp_batch *, uint flags);
 int (*iterate_shared) (struct file *, struct dir_context *);
 __poll_t (*poll) (struct file *, struct poll_table_struct *);
 long (*unlocked_ioctl) (struct file *, uint, ul);
 long (*compat_ioctl) (struct file *, uint, ul);
 int (*mmap) (struct file *, struct vm_area_struct *);
 ul mmap_supported_flags;
 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 (*fasync) (int, struct file *, int);
 int (*lock) (struct file *, int, struct file_lock *);
 ul (*get_unmapped_area)(struct file *, ul, ul, ul, ul);
 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, uint);
 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, uint);
 void (*splice_eof)(struct file *file);
 int (*setlease)(struct file *, int, struct file_lease **, void **);
 long (*fallocate)(struct file *file, int mode, loff_t offset, loff_t len);
 void (*show_fdinfo)(struct seq_file *m, struct file *f);
 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, uint);
 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, struct file *file_out,
   loff_t pos_out, loff_t len, uint remap_flags);
 int (*fadvise)(struct file *, loff_t, loff_t, int);
 int (*uring_cmd)(struct io_uring_cmd *ioucmd, uint issue_flags);
 int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
    uint poll_flags);
} __randomize_layout;