io_uring: offload write to async worker in case of -EAGAIN

In case of direct write -EAGAIN will be returned if page cache was
previously populated.  To avoid immediate completion of a request
with -EAGAIN error write has to be offloaded to the async worker,
like io_read() does.
Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1022,6 +1022,8 @@ static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
 
 	ret = rw_verify_area(WRITE, file, &kiocb->ki_pos, iov_count);
 	if (!ret) {
+		ssize_t ret2;
+
 		/*
 		 * Open-code file_start_write here to grab freeze protection,
 		 * which will be released by another thread in
@@ -1036,7 +1038,19 @@ static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
 						SB_FREEZE_WRITE);
 		}
 		kiocb->ki_flags |= IOCB_WRITE;
-		io_rw_done(kiocb, call_write_iter(file, kiocb, &iter));
+
+		ret2 = call_write_iter(file, kiocb, &iter);
+		if (!force_nonblock || ret2 != -EAGAIN) {
+			io_rw_done(kiocb, ret2);
+		} else {
+			/*
+			 * If ->needs_lock is true, we're already in async
+			 * context.
+			 */
+			if (!s->needs_lock)
+				io_async_list_note(WRITE, req, iov_count);
+			ret = -EAGAIN;
+		}
 	}
 out_free:
 	kfree(iovec);