[RFC PATCH 1/1] io_uring: honor I/O nowait flag for read/write

[Chaitanya Kulkarni <kch@nvidia.com>]

When IO_URING_F_NONBLOCK is set on io_kiocb req->flag in io_write() or
io_read() IOCB_NOWAIT is set for kiocb when passed it to the respective
rw_iter callback. This sets REQ_NOWAIT for underlaying I/O. The result
is low level driver always sees block layer request as REQ_NOWAIT even
if user has submitted request with nowait = 0 e.g. fio nowait=0.

That is not consistent behaviour with other fio ioengine such as
libaio as it will issue non REQ_NOWAIT request with REQ_NOWAIT:-

libaio nowait = 0:-
null_blk: fio:null_handle_rq 1288 *NOWAIT=FALSE* REQ_OP_WRITE

libaio nowait = 1:-
null_blk: fio:null_handle_rq 1288 *NOWAIT=TRUE* REQ_OP_WRITE

* Without this patch with fio ioengine io_uring :-
---------------------------------------------------

iouring nowait = 0:-
null_blk: fio:null_handle_rq 1288 *NOWAIT=TRUE* REQ_OP_WRITE

iouring nowait = 1:-
null_blk: fio:null_handle_rq 1288 *NOWAIT=TRUE* REQ_OP_WRITE

* With this patch with fio ioengine io_uring :-
---------------------------------------------------

iouring nowait = 0:-
null_blk: fio:null_handle_rq 1307 *REQ_NOWAIT=FALSE* WRITE

iouring nowait = 1:
null_blk: fio:null_handle_rq 1307 *REQ_NOWAIT=TRUE* WRITE

Instead of only relying on IO_URING_F_NONBLOCK blindly in io_read() and
io_write(), also make sure io_kiocb->io_rw->flags is set to RWF_NOWAIT
before we mark kiocb->ki_flags = IOCB_NOWAIT.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
---
 io_uring/rw.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/io_uring/rw.c b/io_uring/rw.c
index 3f118ed46e4f..4b3a2c1df5f2 100644
--- a/io_uring/rw.c
+++ b/io_uring/rw.c
@@ -745,7 +745,7 @@ int io_read(struct io_kiocb *req, unsigned int issue_flags)
 	}
 	req->cqe.res = iov_iter_count(&s->iter);
 
-	if (force_nonblock) {
+	if (force_nonblock && (rw->flags & RWF_NOWAIT)) {
 		/* If the file doesn't support async, just async punt */
 		if (unlikely(!io_file_supports_nowait(req))) {
 			ret = io_setup_async_rw(req, iovec, s, true);
@@ -877,7 +877,7 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags)
 	}
 	req->cqe.res = iov_iter_count(&s->iter);
 
-	if (force_nonblock) {
+	if (force_nonblock && (rw->flags & RWF_NOWAIT)) {
 		/* If the file doesn't support async, just async punt */
 		if (unlikely(!io_file_supports_nowait(req)))
 			goto copy_iov;
-- 
2.40.0