netdev - Re: [PATCH RFC 6/9] veth: Add ndo_xdp

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Message-ID: <da8cb809-ab79-1373-f69b-9eed323b99f6@lab.ntt.co.jp>
Date:   Thu, 26 Apr 2018 19:52:40 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH RFC 6/9] veth: Add ndo_xdp_xmit

On 2018/04/26 5:24, Jesper Dangaard Brouer wrote:
> On Tue, 24 Apr 2018 23:39:20 +0900
> Toshiaki Makita <toshiaki.makita1@...il.com> 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.

> 
> 
>> +		if (unlikely(veth_forward_skb(rcv, skb, false) != NET_RX_SUCCESS))
>> +			return -ENXIO;
>> +
>> +		/* Put page ref on success */
>> +		page_frag_free(frame->data);
>> +	}
>> +
>> +	return err;
>> +}
> 
> 

-- 
Toshiaki Makita