On 04.06.2021 18:03, Stefano Garzarella wrote:

On Fri, Jun 04, 2021 at 04:12:23PM +0300, Arseny Krasnov wrote:

quoted

On 03.06.2021 17:45, Stefano Garzarella wrote:

quoted

On Thu, May 20, 2021 at 10:17:58PM +0300, Arseny Krasnov wrote:

quoted

Callback fetches RW packets from rx queue of socket until whole record
is copied(if user's buffer is full, user is not woken up). This is done
to not stall sender, because if we wake up user and it leaves syscall,
nobody will send credit update for rest of record, and sender will wait
for next enter of read syscall at receiver's side. So if user buffer is
full, we just send credit update and drop data.

Signed-off-by: Arseny Krasnov <redacted>
---
v9 -> v10:
1) Number of dequeued bytes incremented even in case when
   user's buffer is full.
2) Use 'msg_data_left()' instead of direct access to 'msg_hdr'.
3) Rename variable 'err' to 'dequeued_len', in case of error
   it has negative value.

include/linux/virtio_vsock.h            |  5 ++
net/vmw_vsock/virtio_transport_common.c | 65 +++++++++++++++++++++++++
2 files changed, 70 insertions(+)

diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index dc636b727179..02acf6e9ae04 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h

@@ -80,6 +80,11 @@ virtio_transport_dgram_dequeue(struct vsock_sock *vsk,

			       struct msghdr *msg,
			       size_t len, int flags);

+ssize_t
+virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
+				   struct msghdr *msg,
+				   int flags,
+				   bool *msg_ready);
s64 virtio_transport_stream_has_data(struct vsock_sock *vsk);
s64 virtio_transport_stream_has_space(struct vsock_sock *vsk);

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index ad0d34d41444..61349b2ea7fe 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c

@@ -393,6 +393,59 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,

	return err;
}

+static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
+						 struct msghdr *msg,
+						 int flags,
+						 bool *msg_ready)
+{
+	struct virtio_vsock_sock *vvs = vsk->trans;
+	struct virtio_vsock_pkt *pkt;
+	int dequeued_len = 0;
+	size_t user_buf_len = msg_data_left(msg);
+
+	*msg_ready = false;
+	spin_lock_bh(&vvs->rx_lock);
+
+	while (!*msg_ready && !list_empty(&vvs->rx_queue) && dequeued_len >= 0) {

I'

quoted

+		size_t bytes_to_copy;
+		size_t pkt_len;
+
+		pkt = list_first_entry(&vvs->rx_queue, struct virtio_vsock_pkt, list);
+		pkt_len = (size_t)le32_to_cpu(pkt->hdr.len);
+		bytes_to_copy = min(user_buf_len, pkt_len);
+
+		if (bytes_to_copy) {
+			/* sk_lock is held by caller so no one else can dequeue.
+			 * Unlock rx_lock since memcpy_to_msg() may sleep.
+			 */
+			spin_unlock_bh(&vvs->rx_lock);
+
+			if (memcpy_to_msg(msg, pkt->buf, bytes_to_copy))
+				dequeued_len = -EINVAL;

I think here is better to return the error returned by memcpy_to_msg(),
as we do in the other place where we use memcpy_to_msg().

I mean something like this:
			err = memcpy_to_msgmsg, pkt->buf, bytes_to_copy);
			if (err)
				dequeued_len = err;

Ack

quoted

quoted

+			else
+				user_buf_len -= bytes_to_copy;
+
+			spin_lock_bh(&vvs->rx_lock);
+		}
+

Maybe here we can simply break the cycle if we have an error:
		if (dequeued_len < 0)
			break;

Or we can refactor a bit, simplifying the while() condition and also the
code in this way (not tested):

	while (!*msg_ready && !list_empty(&vvs->rx_queue)) {
		...

		if (bytes_to_copy) {
			int err;

			/* ...
			*/
			spin_unlock_bh(&vvs->rx_lock);
			err = memcpy_to_msgmsg, pkt->buf, bytes_to_copy);
			if (err) {
				dequeued_len = err;
				goto out;
			}
			spin_lock_bh(&vvs->rx_lock);

			user_buf_len -= bytes_to_copy;
		}

		dequeued_len += pkt_len;

		if (le32_to_cpu(pkt->hdr.flags) & VIRTIO_VSOCK_SEQ_EOR)
			*msg_ready = true;

		virtio_transport_dec_rx_pkt(vvs, pkt);
		list_del(&pkt->list);
		virtio_transport_free_pkt(pkt);
	}

out:
	spin_unlock_bh(&vvs->rx_lock);

	virtio_transport_send_credit_update(vsk);

	return dequeued_len;
}

I think we can't do 'goto out' or break, because in case of error, we still need
to free packet.

Didn't we have code that remove packets from a previous message?
I don't see it anymore.

For example if we have 10 packets queued for a message (the 10th packet 
has the EOR flag) and the memcpy_to_msg() fails on the 2nd packet, with 
you proposal we are freeing only the first 2 packets, the rest is there 
and should be freed when reading the next message, but I don't see that 
code.

The same can happen if the recvmsg syscall is interrupted. In that case 
we report that nothing was copied, but we freed the first N packets, so 
they are lost but the other packets are still in the queue.

Please check also the patch where we implemented 
__vsock_seqpacket_recvmsg().

I thinks we should free packets only when we are sure we copied them to 
the user space.

Hm, yes, this is problem. To solve it i can restore previous approach

with seqbegin/seqend. In that case i can detect unfinished record and

drop it's packets. Seems seqbegin will be a bit like VIRTIO_VSOCK_SEQ_EOR in flags

field of header(e.g. VIRTIO_VSOCK_SEQ_BEGIN). Message id and length are unneeded,

as channel considedered lossless. What do You think?


Thank You

quoted

It is possible to do something like this:

		virtio_transport_dec_rx_pkt(vvs, pkt);
		list_del(&pkt->list);
		virtio_transport_free_pkt(pkt);

		if (dequeued_len < 0)
			break;

quoted