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

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Message-ID: <20190903164620.GA20847@localhost.localdomain>
Date:   Tue, 3 Sep 2019 10:46:20 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
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 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.

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.