So first thing first to get you back up and running run the following command
"setenforce 0"
 This will set selinux into permissive mode then restart mysql.
The next step is to reliable your file system a quick Google search can tell you how to do this.
The next steps are a little more complicated but you should be start with the audit2why command 
Sorry I'm not giving you a full how to right now but I'm answering from my cell phone so I don't have the full set of commands avaliable to me at the moment to give you proper examples.
What I can tell you is selinux isn't that hard to deal with once you know the basic tools.


  Original Message  
From: [log in to unmask]
Sent: May 16, 2018 8:34 PM
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Re: Trouble with MySQL Server

It looks like we have a winner. chown doesn’t work, but checking selinux, I get a number of denied {write} notices for that directory.

It looks like selinux is preventing writing to that directory. Is there a way to change that? I confess selinux is utterly opaque to me.

Eric

> On May 16, 2018, at 2:36 AM, Paddy Doyle <[log in to unmask]> wrote:
> Maybe instead of the chmod, just make the dir owned by the mysql user:
> 
>  chown mysql.mysql /home/mysqltmp
> 
> Or check if selinux is enabled and is preventing writing to that directory.
> 
>  getenforce
>  grep mysqltmp /var/log/audit/audit.log
> 
> Paddy