mmap can be called either for file-backed memory or for anonymous memory
in which case no fd is provided. This patch allows an io_uring command
to request optional files for io_uring.  If a fd is provided, io_uring
loads it in the regular paths. Otherwise, req->file stays NULL.

At the SQE level, Use the ancient mmap semantics of fd == -1 to indicate
no fd.  This feels more useful than a flag or using 0. The later because
I'd expect 0 to be commonly used for direct FDs.  We can abstract this
details in liburing.  It is a bit ugly and it could be a flag elsewhere,
but we don't need to waste a flag on that.

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
---
 io_uring/io_uring.c | 15 ++++++++++-----
 io_uring/opdef.h    |  2 ++
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 87a87396e940..158e9823a72a 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1756,10 +1756,17 @@ static __cold void io_drain_req(struct io_kiocb *req)
 		ctx->drain_active = false;
 }
 
+static inline bool op_wants_file(const struct io_issue_def *def,
+				    struct io_kiocb *req)
+{
+	return (def->needs_file ||
+		(def->opt_file && req->cqe.fd != -1));
+}
+
 static bool io_assign_file(struct io_kiocb *req, const struct io_issue_def *def,
 			   unsigned int issue_flags)
 {
-	if (req->file || !def->needs_file)
+	if (req->file || !op_wants_file(def, req))
 		return true;
 
 	if (req->flags & REQ_F_FIXED_FILE)
@@ -2200,11 +2207,9 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	if (!def->iopoll && (ctx->flags & IORING_SETUP_IOPOLL))
 		return io_init_fail_req(req, -EINVAL);
 
-	if (def->needs_file) {
+	req->cqe.fd = READ_ONCE(sqe->fd);
+	if (op_wants_file(def, req)) {
 		struct io_submit_state *state = &ctx->submit_state;
-
-		req->cqe.fd = READ_ONCE(sqe->fd);
-
 		/*
 		 * Plug now if we have more than 2 IO left after this, and the
 		 * target is potentially a read/write to block based storage.
diff --git a/io_uring/opdef.h b/io_uring/opdef.h
index aa37846880ff..5b81f82c2359 100644
--- a/io_uring/opdef.h
+++ b/io_uring/opdef.h
@@ -5,6 +5,8 @@
 struct io_issue_def {
 	/* needs req->file assigned */
 	unsigned		needs_file : 1;
+	/* Optional req->file assigned, if available. */
+	unsigned		opt_file : 1;
 	/* should block plug */
 	unsigned		plug : 1;
 	/* supports ioprio */
-- 
2.52.0