Subject: Re: [PATCH RFC 6/9] veth: Add ndo_xdp_xmit

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 30 Apr 2018 19:27:09 +0200

On Thu, 26 Apr 2018 19:52:40 +0900
Toshiaki Makita <[email-masked]> wrote:

> On 2018/04/26 5:24, Jesper Dangaard Brouer wrote:
> > On Tue, 24 Apr 2018 23:39:20 +0900
> > Toshiaki Makita <[email-masked]> wrote:
> >   
> >> +static int veth_xdp_xmit(struct net_device *dev, struct xdp_frame *frame)
> >> +{
> >> +	struct veth_priv *rcv_priv, *priv = netdev_priv(dev);
> >> +	int headroom = frame->data - (void *)frame;
> >> +	struct net_device *rcv;
> >> +	int err = 0;
> >> +
> >> +	rcv = rcu_dereference(priv->peer);
> >> +	if (unlikely(!rcv))
> >> +		return -ENXIO;
> >> +
> >> +	rcv_priv = netdev_priv(rcv);
> >> +	/* xdp_ring is initialized on receive side? */
> >> +	if (rcu_access_pointer(rcv_priv->xdp_prog)) {
> >> +		err = xdp_ok_fwd_dev(rcv, frame->len);
> >> +		if (unlikely(err))
> >> +			return err;
> >> +
> >> +		err = veth_xdp_enqueue(rcv_priv, veth_xdp_to_ptr(frame));
> >> +	} else {
> >> +		struct sk_buff *skb;
> >> +
> >> +		skb = veth_build_skb(frame, headroom, frame->len, 0);
> >> +		if (unlikely(!skb))
> >> +			return -ENOMEM;
> >> +
> >> +		/* Get page ref in case skb is dropped in netif_rx.
> >> +		 * The caller is responsible for freeing the page on error.
> >> +		 */
> >> +		get_page(virt_to_page(frame->data));  
> > 
> > I'm not sure you can make this assumption, that xdp_frames coming from
> > another device driver uses a refcnt based memory model. But maybe I'm
> > confused, as this looks like an SKB receive path, but in the
> > ndo_xdp_xmit().  
> 
> I find this path similar to cpumap, which creates skb from redirected
> xdp frame. Once it is converted to skb, skb head is freed by
> page_frag_free, so anyway I needed to get the refcount here regardless
> of memory model.

Yes I know, I wrote cpumap ;-)

First of all, I don't want to see such xdp_frame to SKB conversion code
in every driver.  Because that increase the chances of errors.  And
when looking at the details, then it seems that you have made the
mistake of making it possible to leak xdp_frame info to the SKB (which
cpumap takes into account).

Second, I think the refcnt scheme here is wrong. The xdp_frame should
be "owned" by XDP and have the proper refcnt to deliver it directly to
the network stack.

Third, if we choose that we want a fallback, in-case XDP is not enabled
on egress dev (but it have an ndo_xdp_xmit), then it should be placed
in the generic/core code.  E.g. __bpf_tx_xdp_map() could look at the
return code from dev->netdev_ops->ndo_xdp() and create an SKB.  (Hint,
this would make it easy to implement TX bulking towards the dev).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

+static struct sk_buff *veth_build_skb(void *head, int headroom, int len,
+				      int buflen)
+{
+	struct sk_buff *skb;
+
+	if (!buflen) {
+		buflen = SKB_DATA_ALIGN(headroom + len) +
+			 SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+	}
+	skb = build_skb(head, buflen);
+	if (!skb)
+		return NULL;
+
+	skb_reserve(skb, headroom);
+	skb_put(skb, len);
+
+	return skb;
+}