Re: [PATCH net-next v7 3/4] virtio/vsock: fix logic which reduces credit update messages

[Michael S. Tsirkin]

On Thu, Dec 07, 2023 at 12:18:48AM +0300, Arseniy Krasnov wrote:
> 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).
> Also handle case when 'fwd_cnt' wraps, while 'last_fwd_cnt' is still
> not.
> 
> Fixes: b89d882dc9fc ("vsock/virtio: reduce credit update messages")
> Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
> ---
>  Changelog:
>  v6 -> v7:
>   * Handle wrap of 'fwd_cnt'.
>   * Do to send credit update when 'fwd_cnt' == 'last_fwd_cnt'.
> 
>  net/vmw_vsock/virtio_transport_common.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index e137d740804e..39f8660d825d 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -558,6 +558,8 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>  	struct virtio_vsock_sock *vvs = vsk->trans;
>  	size_t bytes, total = 0;
>  	struct sk_buff *skb;
> +	u32 fwd_cnt_delta;
> +	bool low_rx_bytes;
>  	int err = -EFAULT;
>  	u32 free_space;
>  
> @@ -601,7 +603,15 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>  		}
>  	}
>  
> -	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
> +	/* Handle wrap of 'fwd_cnt'. */
> +	if (vvs->fwd_cnt < vvs->last_fwd_cnt)
> +		fwd_cnt_delta = vvs->fwd_cnt + (U32_MAX - vvs->last_fwd_cnt);

Are you sure there's no off by one here? for example if fwd_cnt is 0
and last_fwd_cnt is 0xfffffffff then apparently delta is 0.


> +	else
> +		fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;

I actually don't see what is wrong with just
	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt
32 bit unsigned math will I think handle wrap around correctly.

And given buf_alloc is also u32 - I don't see where the bug is in
the original code.


> +
> +	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);
>  
> @@ -611,9 +621,11 @@ 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;
> -- 
> 2.25.1