Patch "virtio/vsock: fix logic which reduces credit update messages" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    virtio/vsock: fix logic which reduces credit update messages

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     virtio-vsock-fix-logic-which-reduces-credit-update-m.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 9bf830baeca12eefb057538b81dbc9755d633b31
Author: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
Date:   Thu Dec 14 15:52:28 2023 +0300

    virtio/vsock: fix logic which reduces credit update messages
    
    [ Upstream commit 93b80887668226180ea5f5349cc728ca6dc700ab ]
    
    Add one more condition for sending credit update during dequeue from
    stream socket: when number of bytes in the rx queue is smaller than
    SO_RCVLOWAT value of the socket. This is actual for non-default value
    of SO_RCVLOWAT (e.g. not 1) - idea is to "kick" peer to continue data
    transmission, because we need at least SO_RCVLOWAT bytes in our rx
    queue to wake up user for reading data (in corner case it is also
    possible to stuck both tx and rx sides, this is why 'Fixes' is used).
    
    Fixes: b89d882dc9fc ("vsock/virtio: reduce credit update messages")
    Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
    Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
    Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 00e8b60af0f8..b490f832439e 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -355,6 +355,8 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
 	struct virtio_vsock_pkt *pkt;
 	size_t bytes, total = 0;
 	u32 free_space;
+	u32 fwd_cnt_delta;
+	bool low_rx_bytes;
 	int err = -EFAULT;
 
 	spin_lock_bh(&vvs->rx_lock);
@@ -386,7 +388,10 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
 		}
 	}
 
-	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
+	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
+	free_space = vvs->buf_alloc - fwd_cnt_delta;
+	low_rx_bytes = (vvs->rx_bytes <
+			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
 
 	spin_unlock_bh(&vvs->rx_lock);
 
@@ -396,9 +401,11 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
 	 * too high causes extra messages. Too low causes transmitter
 	 * stalls. As stalls are in theory more expensive than extra
 	 * messages, we set the limit to a high value. TODO: experiment
-	 * with different values.
+	 * with different values. Also send credit update message when
+	 * number of bytes in rx queue is not enough to wake up reader.
 	 */
-	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
+	if (fwd_cnt_delta &&
+	    (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE || low_rx_bytes))
 		virtio_transport_send_credit_update(vsk);
 
 	return total;