fsnotify

使用 fs/fsnotify.c 作为测试 两个系统调用

inotify_init()                          = 3
inotify_add_watch(3, ".", IN_MODIFY|IN_CREATE|IN_DELETE) = 1

systemd-udevd: inotify_add_watch(7, /dev/sdv, 10) failed: No such file or directory

https://mp.weixin.qq.com/s/VrkPPCs1p5OqNL7PI1eBfQ

notify

1. fsnotify_group 是怎么回事
2. 如何通知支持 inotify 和 dnotify 两个模块的

• 如何实现告知 ? ```c fsnotify_modify : 在 open.c 中间可以查看到 /*
• fsnotify_modify - file was modified */ static inline void fsnotify_modify(struct file *file) { fsnotify_file(file, FS_MODIFY); }

/**

• This is the main call to fsnotify. The VFS calls into hook specific functions
• in linux/fsnotify.h. Those functions then in turn call here. Here will call
• out to all of the registered fsnotify_group. Those groups can then use the

• notification event in whatever means they feel necessary. */ int fsnotify(struct inode *to_tell, __u32 mask, const void *data, int data_is, const struct qstr *file_name, u32 cookie) ```

• 如何让从 notifyFd 中间读出 event

/* inotify syscalls */
static int do_inotify_init(int flags)
{
  struct fsnotify_group *group;
  int ret;

  /* Check the IN_* constants for consistency.  */
  BUILD_BUG_ON(IN_CLOEXEC != O_CLOEXEC);
  BUILD_BUG_ON(IN_NONBLOCK != O_NONBLOCK);

  if (flags & ~(IN_CLOEXEC | IN_NONBLOCK))
    return -EINVAL;

  /* fsnotify_obtain_group took a reference to group, we put this when we kill the file in the end */
  group = inotify_new_group(inotify_max_queued_events);
  if (IS_ERR(group))
    return PTR_ERR(group);

  ret = anon_inode_getfd("inotify", &inotify_fops, group, // 这就是关键吧
          O_RDONLY | flags);
  if (ret < 0)
    fsnotify_destroy_group(group);

  return ret;
}

notify 的工作，和 aio epoll 机制其实都是类似的，相比于 blocking io，每一个 IO 需要一个 thread 阻塞，而切换为 epoll，多个 IO 被阻塞到同一个 thread 而已，还是说其实 aio 是存在各种设计模式的 TODO

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