Feature request: support fds that have no {read,write}_iter support

[CarterLi]

There is no way to know whether a file descriptor support {read,write}_iter, and sometimes we don't even know the type of fds, for example STDIN/OUT/ERR_FILENO. We have to try IORING_OP_PREADV first. If we get -EINVAL, we have to fall back to IORING_OP_POLL_ADD and plain read, which is inconvenient and slow.

Ref: libuv/libuv#2322 (comment)

[axboe]

Agree, and it should not be hard, only issue is that we always have to punt to async for handling non iter based operations. Five min hack, haven't tested it yet:

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9d8e703bc851..0454565d501e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1298,6 +1298,42 @@ static void io_async_list_note(int rw, struct io_kiocb *req, size_t len)
 	}
 }
 
+static ssize_t loop_rw_iter(int type, struct file *file, struct kiocb *kiocb,
+			   struct iov_iter *iter)
+{
+	ssize_t ret = 0;
+
+	if (kiocb->ki_flags & IOCB_HIPRI)
+		return -EOPNOTSUPP;
+	if (kiocb->ki_flags & IOCB_NOWAIT)
+		return -EAGAIN;
+
+	while (iov_iter_count(iter)) {
+		struct iovec iovec = iov_iter_iovec(iter);
+		ssize_t nr;
+
+		if (type == READ) {
+			nr = file->f_op->read(file, iovec.iov_base,
+					      iovec.iov_len, &kiocb->ki_pos);
+		} else {
+			nr = file->f_op->write(file, iovec.iov_base,
+					       iovec.iov_len, &kiocb->ki_pos);
+		}
+
+		if (nr < 0) {
+			if (!ret)
+				ret = nr;
+			break;
+		}
+		ret += nr;
+		if (nr != iovec.iov_len)
+			break;
+		iov_iter_advance(iter, nr);
+	}
+
+	return ret;
+}
+
 static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
 		   bool force_nonblock)
 {
@@ -1315,8 +1351,6 @@ static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
 
 	if (unlikely(!(file->f_mode & FMODE_READ)))
 		return -EBADF;
-	if (unlikely(!file->f_op->read_iter))
-		return -EINVAL;
 
 	ret = io_import_iovec(req->ctx, READ, s, &iovec, &iter);
 	if (ret < 0)
@@ -1331,7 +1365,11 @@ static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
 	if (!ret) {
 		ssize_t ret2;
 
-		ret2 = call_read_iter(file, kiocb, &iter);
+		if (file->f_op->read_iter)
+			ret2 = call_read_iter(file, kiocb, &iter);
+		else
+			ret2 = loop_rw_iter(READ, file, kiocb, &iter);
+
 		/*
 		 * In case of a short read, punt to async. This can happen
 		 * if we have data partially cached. Alternatively we can
@@ -1376,8 +1414,6 @@ static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
 	file = kiocb->ki_filp;
 	if (unlikely(!(file->f_mode & FMODE_WRITE)))
 		return -EBADF;
-	if (unlikely(!file->f_op->write_iter))
-		return -EINVAL;
 
 	ret = io_import_iovec(req->ctx, WRITE, s, &iovec, &iter);
 	if (ret < 0)
@@ -1415,7 +1451,10 @@ static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
 		}
 		kiocb->ki_flags |= IOCB_WRITE;
 
-		ret2 = call_write_iter(file, kiocb, &iter);
+		if (file->f_op->write_iter)
+			ret2 = call_write_iter(file, kiocb, &iter);
+		else
+			ret2 = loop_rw_iter(WRITE, file, kiocb, &iter);
 		if (!force_nonblock || ret2 != -EAGAIN) {
 			io_rw_done(kiocb, ret2);
 		} else {

[axboe]

Posted it here:

https://lore.kernel.org/linux-block/6e9531fe-9047-7005-4625-652490419cc3@kernel.dk/

and here's a test case as well:

http://git.kernel.dk/cgit/liburing/commit/?id=556960942eaa69fd53544932f00db3fa9f196e00

If you get a chance to test it, please reply back to the original email above with an acknowledgement.

[CarterLi]

That was GREAT! io_uring is getting more feature complete then ever! Thanks for you quick patch!

But sorry, I have no idea how to build the kernel, and have to wait for a distro.

[axboe]

Thanks for prodding me about these issues! Don't worry about not being able to test it, I ran it through some testing here, seems good to me.