netdev - Re: [PATCH bpf] xsk: do not discard packet when QUEUE_STATE

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Date:   Thu, 9 Jul 2020 10:03:56 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...el.com>
Cc:     bjorn.topel@...el.com, ast@...nel.org, daniel@...earbox.net,
        netdev@...r.kernel.org, A.Zema@...convsystems.com
Subject: Re: [PATCH bpf] xsk: do not discard packet when QUEUE_STATE_FROZEN

On Thu, Jul 09, 2020 at 11:45:51AM +0200, Magnus Karlsson wrote:
> In the skb Tx path, transmission of a packet is performed with
> dev_direct_xmit(). When QUEUE_STATE_FROZEN is set in the transmit
> routines, it returns NETDEV_TX_BUSY signifying that it was not
> possible to send the packet now, please try later. Unfortunately, the
> xsk transmit code discarded the packet and returned EBUSY to the
> application. Fix this unnecessary packet loss, by not discarding the
> packet and return EAGAIN. As EAGAIN is returned to the application, it
> can then retry the send operation and the packet will finally be sent
> as we will likely not be in the QUEUE_STATE_FROZEN state anymore. So
> EAGAIN tells the application that the packet was not discarded from
> the Tx ring and that it needs to call send() again. EBUSY, on the
> other hand, signifies that the packet was not sent and discarded from
> the Tx ring. The application needs to put the packet on the Tx ring
> again if it wants it to be sent.

Doesn't the original code leak the skb if NETDEV_TX_BUSY is returned?
I'm not seeing where it was released.  The new code looks correct.


> Fixes: 35fcde7f8deb ("xsk: support for Tx")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> Reported-by: Arkadiusz Zema <A.Zema@...convsystems.com>
> Suggested-by: Arkadiusz Zema <A.Zema@...convsystems.com>
> ---
>  net/xdp/xsk.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 3700266..5304250 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -376,13 +376,22 @@ static int xsk_generic_xmit(struct sock *sk)
>  		skb->destructor = xsk_destruct_skb;
>  
>  		err = dev_direct_xmit(skb, xs->queue_id);
> -		xskq_cons_release(xs->tx);
>  		/* Ignore NET_XMIT_CN as packet might have been sent */
> -		if (err == NET_XMIT_DROP || err == NETDEV_TX_BUSY) {
> +		if (err == NET_XMIT_DROP) {
>  			/* SKB completed but not sent */
> +			xskq_cons_release(xs->tx);
>  			err = -EBUSY;
>  			goto out;
> +		} else if  (err == NETDEV_TX_BUSY) {

Should be "if (err == ..." here, no else.


> +			/* QUEUE_STATE_FROZEN, tell application to
> +			 * retry sending the packet
> +			 */
> +			skb->destructor = NULL;
> +			kfree_skb(skb);
> +			err = -EAGAIN;
> +			goto out;
>  		}
> +		xskq_cons_release(xs->tx);
>  
>  		sent_frame = true;
>  	}
> -- 
> 2.7.4
>