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Date:   Tue, 21 Jul 2020 22:45:48 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Magnus Karlsson <magnus.karlsson@...el.com>, bjorn.topel@...el.com,
        ast@...nel.org, netdev@...r.kernel.org, jonathan.lemon@...il.com
Cc:     A.Zema@...convsystems.com
Subject: Re: [PATCH bpf v3] xsk: do not discard packet when QUEUE_STATE_FROZEN

On 7/20/20 3:53 PM, 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 in the Tx ring 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.
> 
> 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>
> Suggested-by: Daniel Borkmann <daniel@...earbox.net>
> ---
> v1->v3:
> * Hinder dev_direct_xmit() from freeing and completing the packet to
>    user space by manipulating the skb->users count as suggested by
>    Daniel Borkmann.
> ---
>   net/xdp/xsk.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 3700266..9e95c85 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -375,10 +375,23 @@ static int xsk_generic_xmit(struct sock *sk)
>   		skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
>   		skb->destructor = xsk_destruct_skb;
>   
> +		/* Hinder dev_direct_xmit from freeing the packet and
> +		 * therefore completing it in the destructor
> +		 */
> +		refcount_inc(&skb->users);
>   		err = dev_direct_xmit(skb, xs->queue_id);
> +		if  (err == NETDEV_TX_BUSY) {
> +			/* QUEUE_STATE_FROZEN, tell app to retry the send */
> +			skb->destructor = NULL;
> +			kfree_skb(skb);
> +			err = -EAGAIN;
> +			goto out;
> +		}
> +
>   		xskq_cons_release(xs->tx);
> +		kfree_skb(skb);

What happens if this was properly 'consumed'. If you call kfree_skb() for these pkts,
then doesn't this confuse perf drop monitor with false positives?

>   		/* 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 */
>   			err = -EBUSY;
>   			goto out;
> 

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