[PATCH 4/4] nvmet: expose discovery subsystem

[Sagi Grimberg]

> Add a module option 'expose_discovery_subsys' to expose the
> default discovery subsystem via configfs.
> When this option is enabled the internal discovery subsystem
> is disabled, and the admin has to create a discovery
> subsystem manually.
> The discovery subsystem then has to be linked into the ports
> which should present the discovery subsystem.
> 
> o- /
>    o- ports
>    | o- 2
>    |   o- subsystems
>    |     o- nqn.io-2
>    | o- 3
>    |   o- subsystems
>    |     o- nqn.2014-08.org.nvmexpress.discovery
>    |     o- nqn.io-1
>    o- subsystems
>      o- nqn.2014-08.org.nvmexpress.discovery
>      | o- namespaces
>      o- nqn.io-1
>      | o- namespaces
>      o- nqn.io-2
>        o- namespaces
> 
> So in this example the standard discovery service would be available on
> port 3, presenting information about subsystem nqn.io-1.
> Port 2 would not present a discovery service, but a controller can connect
> to subsystem nqn.io-2 on that port.

So what is the behavior of this param?

expose_discovery_subsys=1:
both default and unique discovery subsystems function and behave
in the same way, i.e. user created, linked to dedicated ports and
answers log page info with subsystems on all ports they are linked to.

expose_discovery_subsys=0:
default discovery subsystem behaves like today but unique discovery
subsystems behave differently (i.e. like first option?)

This will be confusing to users I think.

I would say that in case expose_discovery_subsys=0 unique discovery
subsystems should still behave like the default discovery subsys,
meaning return a log-page with only port local information.

This way default and unique discovery subsystems are consistent with
each other. Then the modparam could be something like:

static int multiport_discovery_log_page;
module_param(multiport_discovery_log_page, int, 0644);
MODULE_PARM_DESC(multiport_discovery_log_page,
		 "Include subsystems from all linked ports in the "
		"discovery log-page information. In this mode the "
		"default discovery subsystem must also be created in "
		"configfs like any other subsystem.");

Thoughts?