ublk: refactor auto buffer register in ublk_dispatch_req()

Refactor auto buffer register code and prepare for supporting batch IO
feature, and the main motivation is to put 'ublk_io' operation code
together, so that per-io lock can be applied for the code block.

The key changes are:
- Rename ublk_auto_buf_reg() as ublk_do_auto_buf_reg()
- Introduce an enum `auto_buf_reg_res` to represent the result of
  the buffer registration attempt (FAIL, FALLBACK, OK).
- Split the existing `ublk_do_auto_buf_reg` function into two:
  - `__ublk_do_auto_buf_reg`: Performs the actual buffer registration
    and returns the `auto_buf_reg_res` status.
  - `ublk_do_auto_buf_reg`: A wrapper that calls the internal function
    and handles the I/O preparation based on the result.
- Introduce `ublk_prep_auto_buf_reg_io` to encapsulate the logic for
  preparing the I/O for completion after buffer registration.
- Pass the `tag` directly to `ublk_auto_buf_reg_fallback` to avoid
  recalculating it.

This refactoring makes the control flow clearer and isolates the different
stages of the auto buffer registration process.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -1168,17 +1168,37 @@ static inline void __ublk_abort_rq(struct ublk_queue *ubq,
 }
 
 static void
-ublk_auto_buf_reg_fallback(const struct ublk_queue *ubq, struct ublk_io *io)
+ublk_auto_buf_reg_fallback(const struct ublk_queue *ubq, unsigned tag)
 {
-	unsigned tag = io - ubq->ios;
 	struct ublksrv_io_desc *iod = ublk_get_iod(ubq, tag);
 
 	iod->op_flags |= UBLK_IO_F_NEED_REG_BUF;
 }
 
-static bool ublk_auto_buf_reg(const struct ublk_queue *ubq, struct request *req,
-			      struct ublk_io *io, struct io_uring_cmd *cmd,
-			      unsigned int issue_flags)
+enum auto_buf_reg_res {
+	AUTO_BUF_REG_FAIL,
+	AUTO_BUF_REG_FALLBACK,
+	AUTO_BUF_REG_OK,
+};
+
+static void ublk_prep_auto_buf_reg_io(const struct ublk_queue *ubq,
+				      struct request *req, struct ublk_io *io,
+				      struct io_uring_cmd *cmd,
+				      enum auto_buf_reg_res res)
+{
+	if (res == AUTO_BUF_REG_OK) {
+		io->task_registered_buffers = 1;
+		io->buf_ctx_handle = io_uring_cmd_ctx_handle(cmd);
+		io->flags |= UBLK_IO_FLAG_AUTO_BUF_REG;
+	}
+	ublk_init_req_ref(ubq, io);
+	__ublk_prep_compl_io_cmd(io, req);
+}
+
+static enum auto_buf_reg_res
+__ublk_do_auto_buf_reg(const struct ublk_queue *ubq, struct request *req,
+		       struct ublk_io *io, struct io_uring_cmd *cmd,
+		       unsigned int issue_flags)
 {
 	int ret;
 
@@ -1186,29 +1206,27 @@ static bool ublk_auto_buf_reg(const struct ublk_queue *ubq, struct request *req,
 				      io->buf.auto_reg.index, issue_flags);
 	if (ret) {
 		if (io->buf.auto_reg.flags & UBLK_AUTO_BUF_REG_FALLBACK) {
-			ublk_auto_buf_reg_fallback(ubq, io);
-			return true;
+			ublk_auto_buf_reg_fallback(ubq, req->tag);
+			return AUTO_BUF_REG_FALLBACK;
 		}
 		blk_mq_end_request(req, BLK_STS_IOERR);
-		return false;
+		return AUTO_BUF_REG_FAIL;
 	}
 
-	io->task_registered_buffers = 1;
-	io->buf_ctx_handle = io_uring_cmd_ctx_handle(cmd);
-	io->flags |= UBLK_IO_FLAG_AUTO_BUF_REG;
-	return true;
+	return AUTO_BUF_REG_OK;
 }
 
-static bool ublk_prep_auto_buf_reg(struct ublk_queue *ubq,
-				   struct request *req, struct ublk_io *io,
-				   struct io_uring_cmd *cmd,
-				   unsigned int issue_flags)
+static void ublk_do_auto_buf_reg(const struct ublk_queue *ubq, struct request *req,
+				 struct ublk_io *io, struct io_uring_cmd *cmd,
+				 unsigned int issue_flags)
 {
-	ublk_init_req_ref(ubq, io);
-	if (ublk_support_auto_buf_reg(ubq) && ublk_rq_has_data(req))
-		return ublk_auto_buf_reg(ubq, req, io, cmd, issue_flags);
+	enum auto_buf_reg_res res = __ublk_do_auto_buf_reg(ubq, req, io, cmd,
+			issue_flags);
 
-	return true;
+	if (res != AUTO_BUF_REG_FAIL) {
+		ublk_prep_auto_buf_reg_io(ubq, req, io, cmd, res);
+		io_uring_cmd_done(cmd, UBLK_IO_RES_OK, issue_flags);
+	}
 }
 
 static bool ublk_start_io(const struct ublk_queue *ubq, struct request *req,
@@ -1281,8 +1299,12 @@ static void ublk_dispatch_req(struct ublk_queue *ubq,
 	if (!ublk_start_io(ubq, req, io))
 		return;
 
-	if (ublk_prep_auto_buf_reg(ubq, req, io, io->cmd, issue_flags))
+	if (ublk_support_auto_buf_reg(ubq) && ublk_rq_has_data(req)) {
+		ublk_do_auto_buf_reg(ubq, req, io, io->cmd, issue_flags);
+	} else {
+		ublk_init_req_ref(ubq, io);
 		ublk_complete_io_cmd(io, req, UBLK_IO_RES_OK, issue_flags);
+	}
 }
 
 static void ublk_cmd_tw_cb(struct io_uring_cmd *cmd,