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

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Message-ID: <ca96d675-7ada-42c0-a3b9-6ced9e399ff3@redhat.com>
Date: Tue, 11 Mar 2025 23:08:37 -0400
From: John Meneghini <jmeneghi@...hat.com>
To: Hannes Reinecke <hare@...e.de>, Keith Busch <kbusch@...nel.org>,
 Sagi Grimberg <sagi@...mberg.me>
Cc: Nilay Shroff <nilay@...ux.ibm.com>, hch@....de, 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 2/26/25 4:55 AM, Hannes Reinecke wrote:
> Plus there are some NVMe devices out there which _despite_ being PCIe do report NMIC and CMIC set 
> (I won't name names, if you came across them you'll know). This is causing stacking drivers (most notably MD) 
> to behave vastly different on hotplug. Having the module option is an easy way of debugging (and, in quite 
> some cases, fixing) the issue.

So some MD driver configurations that use PCIe devices that report CMIC and NMIC need to turn off
core.nvme_multipath.  Is that what you're saying?  Are these PCIe devices multiported?
  
> If the module option really causes issues just make it read-only; that
> way you can still set if if absolutely required, and at the same time
> catch installations which try to modify it.

If we do that the parameter can't be used to support use cases like your MD driver, or Kieth's ublk driver.

/John