linux-kernel - Re: [PATCH net-next v4] ppp: enable TX scatter-gather

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Message-ID: <03c714f3-edd5-43fb-a0be-8c4a43bf6101@redhat.com>
Date: Tue, 3 Feb 2026 11:55:02 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Qingfang Deng <dqfext@...il.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, linux-ppp@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH net-next v4] ppp: enable TX scatter-gather

On 1/29/26 2:29 AM, Qingfang Deng wrote:
> PPP channels using chan->direct_xmit prepend the PPP header to a skb and
> call dev_queue_xmit() directly. In this mode the skb does not need to be
> linear, but the PPP netdevice currently does not advertise
> scatter-gather features, causing unnecessary linearization and
> preventing GSO.
> 
> Enable NETIF_F_SG and NETIF_F_FRAGLIST on PPP devices. In case a linear
> buffer is required (PPP compression, multilink, and channels without
> direct_xmit), call skb_linearize() explicitly.
> 
> Signed-off-by: Qingfang Deng <dqfext@...il.com>
> ---
> v3 -> v4:
>  Always expose SG and FRAGLIST, and linearize the skb if necessary.
>  - https://lore.kernel.org/linux-ppp/20260123014214.225278-1-dqfext@gmail.com/
> 
>  drivers/net/ppp/ppp_generic.c | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index f9f0f16c41d1..f8814d7be6f1 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -1641,6 +1641,8 @@ static void ppp_setup(struct net_device *dev)
>  	dev->flags = IFF_POINTOPOINT | IFF_NOARP | IFF_MULTICAST;
>  	dev->priv_destructor = ppp_dev_priv_destructor;
>  	dev->pcpu_stat_type = NETDEV_PCPU_STAT_TSTATS;
> +	dev->features = NETIF_F_SG | NETIF_F_FRAGLIST;
> +	dev->hw_features = dev->features;
>  	netif_keep_dst(dev);
>  }
>  
> @@ -1710,6 +1712,10 @@ pad_compress_skb(struct ppp *ppp, struct sk_buff *skb)
>  		ppp->xcomp->comp_extra + ppp->dev->hard_header_len;
>  	int compressor_skb_size = ppp->dev->mtu +
>  		ppp->xcomp->comp_extra + PPP_HDRLEN;
> +
> +	if (skb_linearize(skb))
> +		return NULL;
> +
>  	new_skb = alloc_skb(new_skb_size, GFP_ATOMIC);
>  	if (!new_skb) {
>  		if (net_ratelimit())
> @@ -1797,6 +1803,10 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
>  	case PPP_IP:
>  		if (!ppp->vj || (ppp->flags & SC_COMP_TCP) == 0)
>  			break;
> +
> +		if (skb_linearize(skb))
> +			goto drop;
> +
>  		/* try to do VJ TCP header compression */
>  		new_skb = alloc_skb(skb->len + ppp->dev->hard_header_len - 2,
>  				    GFP_ATOMIC);
> @@ -1894,19 +1904,26 @@ ppp_push(struct ppp *ppp)
>  	}
>  
>  	if ((ppp->flags & SC_MULTILINK) == 0) {
> +		struct ppp_channel *chan;
>  		/* not doing multilink: send it down the first channel */
>  		list = list->next;
>  		pch = list_entry(list, struct channel, clist);
>  
>  		spin_lock(&pch->downl);
> -		if (pch->chan) {
> -			if (pch->chan->ops->start_xmit(pch->chan, skb))
> -				ppp->xmit_pending = NULL;
> -		} else {
> -			/* channel got unregistered */
> +		chan = pch->chan;
> +		if (unlikely(!chan || (!chan->direct_xmit && skb_linearize(skb)))) {
> +			/* channel got unregistered, or it requires a linear
> +			 * skb but linearization failed
> +			 */
>  			kfree_skb(skb);
>  			ppp->xmit_pending = NULL;
> +			goto out;

I'm sorry for missing this point before, but AFAICS channels with
chan->direct_xmit == 0 do not support GSO packets at all, and here such
packets will be transmitted after linearization (but still as GSO).

I think that while transmitting over !chan->direct_xmit you additionally
need to check for GSO packet. If ppp features are recomputed depending
on the channels configuration, you could chose to drop such packets.
Otherwise you will need to segment them before transmission, and that
looks error prone in case of multilink.

/P



>  		}
> +
> +		if (chan->ops->start_xmit(chan, skb))
> +			ppp->xmit_pending = NULL;
> +
> +out:
>  		spin_unlock(&pch->downl);
>  		return;
>  	}
> @@ -1991,6 +2008,8 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
>  		return 0; /* can't take now, leave it in xmit_pending */
>  
>  	/* Do protocol field compression */
> +	if (skb_linearize(skb))
> +		goto err_linearize;
>  	p = skb->data;
>  	len = skb->len;
>  	if (*p == 0 && mp_protocol_compress) {
> @@ -2149,6 +2168,7 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
>  
>   noskb:
>  	spin_unlock(&pch->downl);
> + err_linearize:
>  	if (ppp->debug & 1)
>  		netdev_err(ppp->dev, "PPP: no memory (fragment)\n");
>  	++ppp->dev->stats.tx_errors;