ERROR! MySQL is running but PID file could not be found - MariaDB 10.1 - CentOS 7 - Fresh VM
Seems the issue was how it is started.

Using the following, all is well!!

[root@sql ~]# systemctl start mariadb.service
[root@sql ~]# systemctl enable mariadb.service
[root@sql ~]# /etc/init.d/mysql status
SUCCESS! MySQL running (920)


So don't use /etc/init.d/mysql start as stated in the install guide...

https://mariadb.com/kb/en/mariadb/yum/