[PATCH 3/4] io_uring: add IOSQE2_POLL_FIRST flag

[Jens Axboe <axboe@kernel.dk>]

For some operations, the application fully expects that no data or space
is available, and that the request will be punted to our poll handler
after the initial transfer attempt. For this case, it's a potentially big
waste of time to first attempt the operation, only to get -EAGAIN and
need to arm poll.

Add IOSQE2_POLL_FIRST for this case, allowing the application to request
that the handler checks poll first before attempting the operation.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c                 |  5 ++++-
 include/uapi/linux/io_uring.h | 11 +++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 06afe4db5a9a..eb5f77bde98d 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -111,7 +111,7 @@
 			IOSQE_IO_DRAIN | IOSQE_CQE_SKIP_SUCCESS | \
 			IOSQE_FLAGS2)
 
-#define SQE_VALID_FLAGS2	0
+#define SQE_VALID_FLAGS2	(IOSQE2_POLL_FIRST)
 
 #define IO_REQ_CLEAN_FLAGS (REQ_F_BUFFER_SELECTED | REQ_F_NEED_CLEANUP | \
 				REQ_F_POLLED | REQ_F_CREDS | REQ_F_ASYNC_DATA)
@@ -792,6 +792,7 @@ enum {
 	REQ_F_BUFFER_SELECT_BIT	= IOSQE_BUFFER_SELECT_BIT,
 	REQ_F_CQE_SKIP_BIT	= IOSQE_CQE_SKIP_SUCCESS_BIT,
 	REQ_F_FLAGS2_BIT	= IOSQE_FLAGS2_BIT,
+	REQ_F_POLL_FIRST_BIT	= IOSQE_FLAGS2_BIT + 1,
 
 	/* first bits are taken by user flags, shift it to not overlap */
 	REQ_F_FAIL_BIT		= 9,
@@ -837,6 +838,8 @@ enum {
 	REQ_F_CQE_SKIP		= BIT(REQ_F_CQE_SKIP_BIT),
 	/* IOSQE_FLAGS2 */
 	REQ_F_FLAGS2		= BIT(REQ_F_FLAGS2_BIT),
+	/* IOSQE2_POLL_FIRST */
+	REQ_F_POLL_FIRST	= BIT(REQ_F_POLL_FIRST_BIT),
 
 	/* fail rest of links */
 	REQ_F_FAIL		= BIT(REQ_F_FAIL_BIT),
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 622f6e27a444..fe5d8cae7e7d 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -66,6 +66,7 @@ struct io_uring_sqe {
 	__u64	__pad2[2];
 };
 
+/* sqe->flags */
 enum {
 	IOSQE_FIXED_FILE_BIT,
 	IOSQE_IO_DRAIN_BIT,
@@ -77,6 +78,11 @@ enum {
 	IOSQE_FLAGS2_BIT,
 };
 
+/* sqe->flags2, if IOSQE_FLAGS2 is set in sqe->flags */
+enum {
+	IOSQE2_POLL_FIRST_BIT,
+};
+
 /*
  * sqe->flags
  */
@@ -97,6 +103,11 @@ enum {
 /* flags2 is valid and set */
 #define IOSQE_FLAGS2		(1U << IOSQE_FLAGS2_BIT)
 
+/*
+ * sqe->flags2
+ */
+#define IOSQE2_POLL_FIRST	(1U << IOSQE2_POLL_FIRST_BIT)
+
 /*
  * io_uring_setup() flags
  */
-- 
2.35.1