Re: [PATCH nvme-cli] nvme: warn about attaching a namespace to unknown controller

[Nilay Shroff <nilay@linux.ibm.com>]

On 6/21/24 20:38, Keith Busch wrote:
> On Thu, Jun 20, 2024 at 06:25:45PM +0530, Nilay Shroff wrote:
>> Sometime it's possible for a multi-controller NVMe disk to have only
>> one of its controller discovered by the kernel. And if this happens
>> then it's also possible for a user to create and then attach a namespace
>> to a controller which has not been discovered by the kernel. In such a
>> case the attached namespace can't be used for IO because there's no path
>> to reach such namespace from the kernel.
> 
> Isn't that a pretty normal thing to do, though? Like for an sriov
> situation, a primary controller attaches namespaces to secondaries, but
> not to itself.
Yes correct, but in case of sriov, AFAIK, the "nvme list-secondary ...." shall 
not list any secondary controller unless the secondary controller's corresponding 
VF is enabled (i.e. /sys/class/nvme/nvmeX/device/sriov_numvfs set to at least the 
secondary controller's VF number). 

So it means that before attaching namespace to secondaries, user would first set 
sriov_numvfs. Setting numvfs would then enable the kernel to discover all secondary
controllers. 

However, in the proposed patch we're trying to address a case where "nvme list-ctrl ..."
shows multiple controller entries however only one of those controllers is discovered
by kernel. All controllers shown in the output of "nvme list-ctrl ..." are physical
controllers on the nvme disk. In this case attaching a namespace to any of the 
undiscovered controller has a side effect due which the namespace rendered unusable.  
This patch tries to address this issue by showing a WARNING to the user and optionally
allowing user to cancel the attach operation. However if user still prefers to go 
ahead without cancelling the attach operation then nvme-cli would execute this command
as usual. 
For sriov use case, as secondary controllers have been already discovered by kernel 
before we attach namespace to secondaries, it doesn't have any side effect. 

Thanks,
--Nilay