Re: [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 13, 2022 at 11:07:57AM +0530, Kanchan Joshi wrote:
> > > > > > The way I would do this that in nvme_ioucmd_failover_req (or in the
> > > > > > retry driven from command retriable failure) I would do the above,
> > > > > > requeue it and kick the requeue work, to go over the requeue_list and
> > > > > > just execute them again. Not sure why you even need an explicit retry
> > > > > > code.
> > > > > During retry we need passthrough command. But passthrough command is not
> > > > > stable (i.e. valid only during first submission). We can make it stable
> > > > > either by:
> > > > > (a) allocating in nvme (b) return -EAGAIN to io_uring, and
> > > > > it will do allocate + deferral
> > > > > Both add a cost. And since any command can potentially fail, that
> > > > > means taking that cost for every IO that we issue on mpath node. Even if
> > > > > no failure (initial or subsquent after IO) occcured.
> > > > 
> > > > As mentioned, I think that if a driver consumes a command as queued,
> > > > it needs a stable copy for a later reformation of the request for
> > > > failover purposes.
> > > 
> > > So what do you propose to make that stable?
> > > As I mentioned earlier, stable copy requires allocating/copying in fast
> > > path. And for a condition (failover) that may not even occur.
> > > I really think currrent solution is much better as it does not try to make
> > > it stable. Rather it assembles pieces of passthrough command if retry
> > > (which is rare) happens.
> > 
> > Well, I can understand that io_uring_cmd is space constrained, otherwise
> > we wouldn't be having this discussion.
> 
> Indeed. If we had space for keeping passthrough command stable for
> retry, that would really have simplified the plumbing. Retry logic would
> be same as first submission.
> 
> > However io_kiocb is less
> > constrained, and could be used as a context to hold such a space.
> > 
> > Even if it is undesired to have io_kiocb be passed to uring_cmd(), it
> > can still hold a driver specific space paired with a helper to obtain it
> > (i.e. something like io_uring_cmd_to_driver_ctx(ioucmd) ). Then if the
> > space is pre-allocated it is only a small memory copy for a stable copy
> > that would allow a saner failover design.
> 
> I am thinking along the same lines, but it's not about few bytes of
> space rather we need 80 (72 to be precise). Will think more, but
> these 72 bytes really stand tall in front of my optimism.
> 
> Do you see anything is possible in nvme-side?
> Now also passthrough command (although in a modified form) gets copied
> into this preallocated space i.e. nvme_req(req)->cmd. This part -

I understand it can't be allocated in nvme request which is freed
during retry, and looks the extra space has to be bound with
io_uring_cmd.


thanks, 
Ming