https://blog.wongcw.com/2018/01/15/centos-%e7%b3%bb%e7%bb%9fswap%e8%99%9a%e6%8b%9f%e5%86%85%e5%ad%98%e6%b7%bb%e5%8a%a0%e4%b8%8e%e5%88%a0%e9%99%a4%e9%85%8d%e7%bd%ae/