Re: [PATCH] nvme: optimize passthrough IOPOLL completion for local ring context

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

On Thu, Jan 15, 2026 at 11:21:41AM -0700, Keith Busch wrote:
> On Thu, Jan 15, 2026 at 04:59:52PM +0800, Ming Lei wrote:
> > +	if (blk_rq_is_poll(req) && req->poll_ctx == io_uring_cmd_ctx_handle(ioucmd)) {
> 
> ...
> 
> > @@ -677,8 +691,14 @@ int nvme_ns_chr_uring_cmd_iopoll(struct io_uring_cmd *ioucmd,
> >  	struct nvme_uring_cmd_pdu *pdu = nvme_uring_cmd_pdu(ioucmd);
> >  	struct request *req = pdu->req;
> >  
> > -	if (req && blk_rq_is_poll(req))
> > +	if (req && blk_rq_is_poll(req)) {
> > +		/*
> > +		 * Store the polling context in the request so end_io can
> > +		 * detect if it's completing in the local ring's context.
> > +		 */
> > +		req->poll_ctx = iob ? iob->poll_ctx : NULL;
> 
> I don't think this works. The io_uring polling always polls from a
> single ctx's iopoll_list, so it's redundant to store the ctx in the iob
> since it will always match the ctx of the ioucmd passed in.

Yeah, the patch looks totally wrong, what it should record is the
io_ring_ctx for completing the request, instead of the context which owns
the uring_cmd, which can be obtained always from the uring_cmd.

> 
> Which then leads to the check at the top: if req->poll_ctx was ever set,
> then it should always match its ioucmd ctx too, right? If it was set
> once before, but the polling didn't find the completion, then another
> ctx polling does find it, we won't complete it in the iouring task as
> needed.
> 
> I think you want to save off ctx that called
> 'nvme_ns_chr_uring_cmd_iopoll()', but there doesn't seem to be an
> immediate way to refer back to that from 'nvme_uring_cmd_end_io'. Maybe
> stash it in current->io_uring->last instead, then check if
> io_uring_cmd_ctx_handle(ioucmd)) equals that.

One easy way is to add `iob` to rq_end_io_fn, then pass it via
blk_mq_end_request_batch().


Thanks,
Ming