Send all login requests to the main site in multisite

Hi, is there a way I can handle all the login requests from one login page in a multisite?
I have mysite.com/login, mysite.com/subsite1/login. How do I send request from mysite.com/subsite1/login to mysite.com/login, and send user back to mysite.com/subsite1?
I have tried so far without success:

function tml_action_url_filter( $url, $action, $instance ) {
	if ( 'login' == $action )
		return network_site_url( 'login' );
}
add_filter( 'tml_action_url', 'tml_action_url_filter', 10, 3 );

Any help will be really appreciated.
Thanks

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