Re: [PATCH] nvme: remove multipath module parameter

[Keith Busch <kbusch@kernel.org>]

On Fri, Mar 07, 2025 at 08:49:09PM +0530, Nilay Shroff wrote:
> On 3/7/25 6:16 AM, Keith Busch wrote:
> I think always creating multipath head node even for the disk which doesn't 
> have CMIC/NMIC capability should be useful. That way, we may then be able 
> to remove multipath module parameter? In fact, you already mentioned about
> it in one of your previous message. I see two approaches (one of them you 
> proposed and another one Christoph proposed: 
> https://lore.kernel.org/linux-nvme/Y+1aKcQgbskA2tra@kbusch-mbp.dhcp.thefacebook.com/). 
> 
> Maybe in first cut we should create multipath head disk node always for 
> single/multi ported NVMe disk. Later we may enhance it and allow pinning the 
> head node for hotplug events so that head node dev name remains consistent 
> across disk add/remove hotplug events.

It honestly has potential to solve some real problems, like
re-enumeration triggered by a link reset on an in-use drive. You'd
currently need to close the old handle and open a new on, even though
it's the same device. It may not even be possible to do that if that
device contains your root partition, and then you can only power cycle.

The downside is we wouldn't get the short cut to blk_mq_submit_bio. We'd
instead stack that atop an indirect call, so it's not free.