[RFC net-next v3 27/29] io_uring: allow to override zc tag on flush

[Pavel Begunkov <asml.silence@gmail.com>]

Add a new sendzc flag, IORING_SENDZC_COPY_TAG. When set and the request
flushing a notification, it'll set the notification tag to
sqe->user_data. This adds a bit more flexibility allowing to specify
notification tags on per-request basis.

One use cases is combining the new flag with IOSQE_CQE_SKIP_SUCCESS,
so either the request fails and we expect an CQE with a failure and no
notification, or it succedees, then there will be no request completion
but only a zc notification with an overriden tag. In other words, in the
described scheme it posts only one CQE with user_data set to the current
requests sqe->user_data.

note 1: the flat has no effect if nothing is flushed, e.g. there was
no IORING_SENDZC_FLUSH or the request failed.

note 2: copying sqe->user_data may be not ideal, but we don't have
extra space in SQE to keep a second tag/user_data.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c                 | 9 +++++++--
 include/uapi/linux/io_uring.h | 1 +
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index f5fe2ab5622a..08c98a4d9bd2 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6581,7 +6581,8 @@ static int io_send(struct io_kiocb *req, unsigned int issue_flags)
 	return 0;
 }
 
-#define IO_SENDZC_VALID_FLAGS (IORING_SENDZC_FIXED_BUF|IORING_SENDZC_FLUSH)
+#define IO_SENDZC_VALID_FLAGS (IORING_SENDZC_FIXED_BUF | IORING_SENDZC_FLUSH | \
+			       IORING_SENDZC_OVERRIDE_TAG)
 
 static int io_sendzc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
@@ -6686,7 +6687,11 @@ static int io_sendzc(struct io_kiocb *req, unsigned int issue_flags)
 	ret = sock_sendmsg(sock, &msg);
 
 	if (likely(ret >= min_ret)) {
-		if (req->msgzc.zc_flags & IORING_SENDZC_FLUSH)
+		unsigned zc_flags = req->msgzc.zc_flags;
+
+		if (zc_flags & IORING_SENDZC_OVERRIDE_TAG)
+			notif->tag = req->cqe.user_data;
+		if (zc_flags & IORING_SENDZC_FLUSH)
 			io_notif_slot_flush_submit(notif_slot, 0);
 	} else {
 		if (ret == -EAGAIN && (issue_flags & IO_URING_F_NONBLOCK))
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 7d77d90a5f8a..7533387f25d3 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -280,6 +280,7 @@ enum {
 enum {
 	IORING_SENDZC_FIXED_BUF		= (1U << 0),
 	IORING_SENDZC_FLUSH		= (1U << 1),
+	IORING_SENDZC_OVERRIDE_TAG	= (1U << 2),
 };
 
 /*
-- 
2.36.1