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

[Arseniy Krasnov]

On 07.12.2023 00:53, Michael S. Tsirkin wrote:
> 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.

Seems yes, I need +1 here

> 
> 
>> +	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.

I think problem is when fwd_cnt wraps, while last_fwd_cnt is not. In this
case fwd_cnt_delta will be too big, so we won't send credit update which
leads to stall for sender

Thanks, Arseniy

> 
> 
>> +
>> +	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
>