title: linux2.6 epoll epoll_wait why not use mmap
body: <p>copy data to user space，why? linux 2.6</p>
  <pre><code>if (__put_user(revents, &amp;uevent-&gt;events) || __put_user(epi-&gt;event.data, &amp;uevent-&gt;data)) {
      list_add(&amp;epi-&gt;rdllink, head);
      return eventcnt ? eventcnt : -EFAULT;
  }
  </code></pre>