linux-kernel - Re: [PATCH] nvme-core: Fix subsystem instance mismatches

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Message-ID: <d71107d2-c133-9f51-c271-688516641703@deltatee.com>
Date:   Tue, 3 Sep 2019 12:13:24 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Keith Busch <keith.busch@...el.com>, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>, Jens Axboe <axboe@...com>,
        Hannes Reinecke <hare@...e.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH] nvme-core: Fix subsystem instance mismatches



On 2019-09-03 10:46 a.m., Keith Busch wrote:
> On Tue, Sep 03, 2019 at 10:08:01AM -0600, Logan Gunthorpe wrote:
>> On 2019-08-31 9:29 a.m., Keith Busch wrote:
>>> On Fri, Aug 30, 2019 at 06:01:39PM -0600, Logan Gunthorpe wrote:
>>>> To fix this, assign the subsystem's instance based on the instance
>>>> number of the controller's instance that first created it. There should
>>>> always be fewer subsystems than controllers so the should not be a need
>>>> to create extra subsystems that overlap existing controllers.
>>>
>>> The subsystem's lifetime is not tied to the controller's. When the
>>> controller is removed and releases its instance, the next controller
>>> to take that available instance will create naming collisions with the
>>> subsystem still using it.
>>>
>>
>> Hmm, yes, ok.
>>
>> So perhaps we can just make the subsystem prefer the ctrl's instance
>> when allocating the ID? Then at least, in the common case, the
>> controller numbers will match the subsystem numbers. Only when there's
>> random hot-plugs would the numbers get out of sync.
> 
> I really don't know about a patch that works only on static
> configurations. Connects and disconnects do happen on live systems,
> so the numerals will inevitably get out of sync.

Well this depends on how big a problem we think the number mismatch is.
Right now it's pretty annoying because numbers aren't matching for
non-CMIC controllers in simple setups on boot. I think having a small
patch that makes it more consistent for the static would be worth it and
if CMIC controllers with significant hot-plug events have mismatches
that seems more understandable to me.

> Could we possibly make /dev/nvmeX be a subsystem handle without causing
> trouble for anyone? This would essentially be the same thing as today
> for non-CMIC controllers with a device-per-controller and only affects
> the CMIC ones.

Well then we'd have to be able to do everything that's possible with a
controller via the subsystem and it would have to multiplex all admin
commands for CMIC ones, etc to a sensible controller. This might make
sense in the long term but it sounds like a larger project than I have
time to take on.

Logan