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

From: Chuck Lever <chucklever@gmail.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
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 11:00:22 -0400	[thread overview]
Message-ID: <EC7EE276-3529-4374-9F90-F061AAC3B952@gmail.com> (raw)
In-Reply-To: <20201009145706.GU5177@ziepe.ca>

> 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.

>>> 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?

--
Chuck Lever
chucklever@gmail.com