[PATCH V6 1/2] nvme: enable char device per namespace

[Christoph Hellwig]

On Mon, Mar 01, 2021 at 08:24:51PM +0100, javier at javigon.com wrote:
> From: Javier González <javier.gonz at samsung.com>
> 
> Create a char device per NVMe namespace. This char device is always
> initialized, independently of whether the features implemented by the
> device are supported by the kernel. User-space can therefore always
> issue IOCTLs to the NVMe driver using the char device.
> 
> The char device is presented as /dev/nvme-generic-XcYnZ. This naming
> scheme follows the convention of the hidden device (nvmeXcYnZ). Support
> for multipath will follow.

So I'm a little worried about the "support for multipath will follow" as
this has implications for the naming scheme, and our policy of how we
allow access to a namespace.

Ignoring some of the deprecated historic mistakes I think the policy
should be:

 - admin commands that often are controller specific should usually
   go to a controller-specific device, the existing /dev/nvmeX
   devices
 - I/O commands and admin command that do specific a nsid should go
   through a per-namespace node that is multipath aware and not
   controller specific

Which also makes me wonder about patch 2 in the series that seems
somewhat dangerous.   Can we clearly state the policy implemented?