io_uring: optimise fixed rw rsrc node setting

Move fixed rw io_req_set_rsrc_node() from rw prep into
io_import_fixed(), if we're using fixed buffers it will always be called
during submission as we save the state in advance,
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/68c06f66d5aa9661f1e4b88d08c52d23528297ec.1634314022.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2871,12 +2871,7 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		kiocb->ki_complete = io_complete_rw;
 	}
 
-	if (req->opcode == IORING_OP_READ_FIXED ||
-	    req->opcode == IORING_OP_WRITE_FIXED) {
-		req->imu = NULL;
-		io_req_set_rsrc_node(req, ctx);
-	}
-
+	req->imu = NULL;
 	req->rw.addr = READ_ONCE(sqe->addr);
 	req->rw.len = READ_ONCE(sqe->len);
 	req->buf_index = READ_ONCE(sqe->buf_index);
@@ -3005,13 +3000,15 @@ static int __io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter
 
 static int io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter)
 {
-	struct io_ring_ctx *ctx = req->ctx;
 	struct io_mapped_ubuf *imu = req->imu;
 	u16 index, buf_index = req->buf_index;
 
 	if (likely(!imu)) {
+		struct io_ring_ctx *ctx = req->ctx;
+
 		if (unlikely(buf_index >= ctx->nr_user_bufs))
 			return -EFAULT;
+		io_req_set_rsrc_node(req, ctx);
 		index = array_index_nospec(buf_index, ctx->nr_user_bufs);
 		imu = READ_ONCE(ctx->user_bufs[index]);
 		req->imu = imu;