From mboxrd@z Thu Jan 1 00:00:00 1970 From: hch@lst.de (Christoph Hellwig) Date: Wed, 4 Oct 2017 09:13:12 +0200 Subject: [PATCH 3/6] nvme: claim block devices In-Reply-To: <19dbdf11-86c4-b6ea-a18c-a78fcae822d5@suse.de> References: <1506952559-1588-1-git-send-email-hare@suse.de> <1506952559-1588-4-git-send-email-hare@suse.de> <20171002164218.GA11886@lst.de> <20171003115506.GC24650@lst.de> <20171004061533.GA20131@lst.de> <19dbdf11-86c4-b6ea-a18c-a78fcae822d5@suse.de> Message-ID: <20171004071312.GA21143@lst.de> On Wed, Oct 04, 2017@08:33:07AM +0200, Hannes Reinecke wrote: > > On Wed, Oct 04, 2017@07:42:00AM +0200, Hannes Reinecke wrote: > >> Hmm. Not sure how you would be doing that. Who should be doing the > >> claiming? Typically the claim is done whenever a device is created on > >> top of the other... > > > > We'd need a callback in the driver if it is claimed, and use that > > to for propagating the claim, or use a shared struture to record the > > claim. I haven't looked into the details yet, though. > > > >> What about an alternative plan: make creation of the subsystem device > >> fully dynamic. > >> But if a subsystem device is created it will always claim the underlying > >> device. Then we can make the creation dependent on the NMIC attribute, > >> and existing setups would not be affected. > > > > This doesn't work because a lot of devices can just set NMIC. E.g. > > every namespace exported by the Linux NVMe target. > > > But as it's fully dynamic we can decide how to handle each device on a > device-by-device basis, with a common default policy. What do you mean with fully dynamic? > My idea is to have a default policy (create/not create subsystem devices > if NMIC is set) set via kernel command-line, and udev rules for devices > requiring separate handling. Yikes. That's exactly where I do _not_ want to go. No more arcane magic setup that you need to be in the in group for to know like dm-multipath. Things must just work, period. The only other option I could think of would be to turn names around: make /dev/nvmeX (chardev) and /dev/nvmeXnY the per-subsystem devices that are multipathed if available. We'd then need new devices for the invdividual controllers. With a little luck we'd get away with just creating character devices. This would however create a problem for dm-multipath users, which mostly is you and your paterners/customers. Note that in general dm-multipath should continue working but it would generally just see one path.