linux-kernel - Re: [PATCH] nvme: remove multipath module parameter

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Message-ID: <Z8pB9jQALxMN6WaA@kbusch-mbp>
Date: Thu, 6 Mar 2025 17:46:46 -0700
From: Keith Busch <kbusch@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Hannes Reinecke <hare@...e.de>, Sagi Grimberg <sagi@...mberg.me>,
	Nilay Shroff <nilay@...ux.ibm.com>,
	John Meneghini <jmeneghi@...hat.com>, bmarzins@...hat.com,
	Bryan Gurney <bgurney@...hat.com>, linux-nvme@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Marco Patalano <mpatalan@...hat.com>,
	axboe@...nel.dk
Subject: Re: [PATCH] nvme: remove multipath module parameter

On Thu, Mar 06, 2025 at 04:16:54PM +0100, Christoph Hellwig wrote:
> On Thu, Mar 06, 2025 at 08:01:19AM -0700, Keith Busch wrote:
> 
> > Or consider a true multiport PCIe where each port connects to a
> > different host. Each host sees a single port so they're not using
> > multipath capabilities, and the admin wants the MD behavior that removes
> > a disk on hot plug. Or even if one host sees both paths of a multiport
> > PCIe, they still might want that hot plug behavior. The module parameter
> > makes that possible, so some equivalent should be available before
> > removing it.
> 
> A module-wide parameter is absolutely the wrong way to configure it.
> You'd ad best want it per-controller or even per-namespace.  One
> tradeoff would be to disable the multipath code for private namespaces,
> although that would cause problems when rescanning changes the flag.

It's not really about private vs. shared namespaces, though. There
really is no programatic way for the driver to know what behavior the
admin needs out of their system without user input. If you don't want a
module parameter, then the driver will just have to default to
something, then the user will have to do something to change it later.
Not very pleasant compared to a simple one time boot parameter.