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

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Message-ID: <20250310132813.GA8956@lst.de>
Date: Mon, 10 Mar 2025 14:28:13 +0100
From: Christoph Hellwig <hch@....de>
To: Keith Busch <kbusch@...nel.org>
Cc: Christoph Hellwig <hch@....de>, 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 05:46:46PM -0700, Keith Busch wrote:
> 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.

PArt of it is about that.  A private namespace can't have another
path.


> 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.

The point is that different devices want different behavior.  Think of
a fabrics attached array vs a usb4 dongle used by the admin.