How to enable "user" namespace in RHEL7 and CentOS7?

The "user" namespace support in RHEL7 is a technology preview feature. The docker has adopted it. The procszoo prefer the "user" namespace, too. Hence, how to enable it on RHEL7/CentOS7?

A simple way is run following command by super user priveleges

 grubby --args="user_namespace.enable=1" --update-kernel="$(grubby --default-kernel)"

Then

 reboot

I think you have known how to disable the "user" namespace. But even we have enabled the "user" namespace, when creating all available namespaces (hence we will create a new user namespace) on the RHEL7/CentOS7, we still need super user privileges.

To revoke what we have done, do as follows

 grubby --remove-args="user_namespace.enable=1" --update-kernel="$(grubby --default-kernel)"