io_uring/uring_cmd: allow non-iopoll cmds with IORING_SETUP_IOPOLL

Currently, creating an io_uring with IORING_SETUP_IOPOLL requires all
requests issued to it to support iopoll. This prevents, for example,
using ublk zero-copy together with IORING_SETUP_IOPOLL, as ublk
zero-copy buffer registrations are performed using a uring_cmd. There's
no technical reason why these non-iopoll uring_cmds can't be supported.
They will either complete synchronously or via an external mechanism
that calls io_uring_cmd_done(), io_uring_cmd_post_mshot_cqe32(), or
io_uring_mshot_cmd_post_cqe(), so they don't need to be polled.

Allow uring_cmd requests to be issued to IORING_SETUP_IOPOLL io_urings
even if their files don't implement ->uring_cmd_iopoll(). For these
uring_cmd requests, skip initializing struct io_kiocb's iopoll fields,
don't set REQ_F_IOPOLL, and don't set IO_URING_F_IOPOLL in issue_flags.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://patch.msgid.link/20260302172914.2488599-5-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: calebsander

io_uring/uring_cmd.c

@@ -257,9 +257,7 @@ int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
 		issue_flags |= IO_URING_F_CQE32;
 	if (io_is_compat(ctx))
 		issue_flags |= IO_URING_F_COMPAT;
-	if (ctx->flags & IORING_SETUP_IOPOLL) {
-		if (!file->f_op->uring_cmd_iopoll)
-			return -EOPNOTSUPP;
+	if (ctx->flags & IORING_SETUP_IOPOLL && file->f_op->uring_cmd_iopoll) {
 		req->flags |= REQ_F_IOPOLL;
 		issue_flags |= IO_URING_F_IOPOLL;
 		req->iopoll_completed = 0;