Re: RDMA subsystem namespace related questions (was Re: Finding the namespace of a struct ib_device)

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Chuck Lever <chucklever@gmail.com>
Cc: Ka-Cheong Poon <ka-cheong.poon@oracle.com>,
	Leon Romanovsky <leon@kernel.org>,
	linux-rdma@vger.kernel.org
Subject: Re: RDMA subsystem namespace related questions (was Re: Finding the namespace of a struct ib_device)
Date: Fri, 9 Oct 2020 12:07:58 -0300	[thread overview]
Message-ID: <20201009150758.GV5177@ziepe.ca> (raw)
In-Reply-To: <EC7EE276-3529-4374-9F90-F061AAC3B952@gmail.com>

On Fri, Oct 09, 2020 at 11:00:22AM -0400, Chuck Lever wrote:
> 
> 
> > On Oct 9, 2020, at 10:57 AM, Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > 
> > On Fri, Oct 09, 2020 at 10:48:55AM -0400, Chuck Lever wrote:
> >> Hi Jason-
> >> 
> >>> On Oct 9, 2020, at 10:39 AM, Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >>> 
> >>> On Fri, Oct 09, 2020 at 12:49:30PM +0800, Ka-Cheong Poon wrote:
> >>>> As I mentioned before, this is a very serious restriction on how
> >>>> the RDMA subsystem can be used in a namespace environment by kernel
> >>>> module.  The reason given for this restriction is that any kernel
> >>>> socket without a corresponding user space file descriptor is "rogue".
> >>>> All Internet protocol code create a kernel socket without user
> >>>> interaction.  Are they all "rogue"?
> >>> 
> >>> You should work with Chuck to make NFS use namespaces properly and
> >>> then you can propose what changes might be needed with a proper
> >>> justification.
> >> 
> >> The NFS server code already uses namespaces for creating listener
> >> endpoints, already has a user space component that drives the
> >> creation of listeners, and already passes an appropriate struct
> >> net to rdma_create_id. As far as I am aware, it is namespace-aware
> >> and -friendly all the way down to rdma_create_id().
> >> 
> >> What more needs to be done?
> > 
> > I have no idea, if you are able to pass a namespace all the way down
> > to the listening cm_id and everything works right (I'm skeptical) then
> > there is nothing more to worry about - why are we having this thread?
> 
> The thread is about RDS, not NFS. NFS has some useful examples to
> crib, but it's not the main point.
> 
> I don't think NFS/RDMA namespacing works today, but it's not because
> NFS isn't ready. I agree that is another thread.

Exactly, so instead of talking about RDS stuff without any patches,
let's talk about NFS with patches - if you can make NFS work then I
assume RDS will be happy.

NFS has an established model for using namespaces that the other
transports uses, so I'd rather focus on this.

> >>> The rules for lifetime on IB clients are tricky, and the interaction
> >>> with namespaces makes it all a lot more murky.
> >> 
> >> I think what Ka-cheong is asking is for a detailed explanation of
> >> these lifetime rules so we can understand why rdma_create_id bumps
> >> the namespace reference count.
> > 
> > It is because the CM has no code to revoke a CM ID before the
> > namespace goes away and the pointer becomes invalid.
> 
> Is it just a question of "no-one has yet written this code" or is
> there a deeper technical reason why this has not been done?

It is hard to know without spending a big deep look at this
stuff.

Jason