早上下的 rubychina 代码，按照指示，安装虚拟机和各类包，直到启动，报错如下：

vagrant@ruby-china-dev:/vagrant$ rails s -b 0.0.0.0
=> Booting Puma
=> Rails 5.0.0.beta3 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
=> Memory usage: 156.73 MB
[24430] Puma starting in cluster mode...
[24430] * Version 3.0.2 (ruby 2.3.0-p0), codename: Plethora of Penguin Pinatas
[24430] * Min threads: 8, max threads: 16
[24430] * Environment: development
[24430] * Process workers: 4
[24430] * Preloading application
[24430] * Listening on tcp://0.0.0.0:3000
[24430] * Daemonizing...
Exiting
Exiting
/home/vagrant/.rvm/gems/ruby-2.3.0/gems/rack-2.0.0.alpha/lib/rack/server.rb:362:in `delete': No such file or directory @ unlink_internal - /vagrant/tmp/pids/server.pid (Errno::ENOENT)
    from /home/vagrant/.rvm/gems/ruby-2.3.0/gems/rack-2.0.0.alpha/lib/rack/server.rb:362:in `block in write_pid'

尝试

$ mkdir -p /vagrant/tmp/pids/ 

无效，是何原因？