[PATCH] nvme-multipath: report "none" iopolicy for discovery subsystems

[Keith Busch]

On Wed, Sep 30, 2026 at 09:06:58PM +0530, Martin George wrote:
> A discovery subsystem never exposes namespaces, so it has no multipath
> head and no I/O paths to select between. Nevertheless the per-subsystem
> iopolicy attribute is registered unconditionally and reads back the
> driver-wide default which is 'numa' for the discovery subsystems too.
> This can be misleading as it suggests a path selection policy is in
> effect when nothing is ever selected, and writing the attribute silently
> mutates state that can never be used.
> 
> Report "none" instead for a discovery subsystem, and reject writes
> with -EOPNOTSUPP. Report "none" from iopolicies as well, so that the
> value read from iopolicy always remains a member of the set advertised
> by iopolicies; otherwise userspace validating the current policy
> against the available ones would reject its own subsystem.
> 
> And while at it, document iopolicies too, which was not covered when it
> was added previously.

Thanks, applied to nvme-7.4.