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Message-ID: <20250904021328.24329-1-dqfext@gmail.com>
Date: Thu, 4 Sep 2025 10:13:28 +0800
From: Qingfang Deng <dqfext@...il.com>
To: Andrew Lunn <andrew+netdev@...n.ch>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
linux-ppp@...r.kernel.org,
netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Cc: Felix Fietkau <nbd@....name>
Subject: [RFC PATCH net-next] ppp: enable TX scatter-gather
When chan->direct_xmit is true, and no compressors are in use, PPP
prepends its header to a skb, and calls dev_queue_xmit directly. In this
mode the skb does not need to be linearized.
Enable NETIF_F_SG and NETIF_F_FRAGLIST if chan->direct_xmit is true, so
the networking core can transmit non-linear skbs directly. The
compressors still require a linear buffer so call skb_linearize() before
passing skb->data to them.
This is required to support PPPoE GSO.
Signed-off-by: Qingfang Deng <dqfext@...il.com>
---
RFC:
This depends on a pending fix:
https://lore.kernel.org/netdev/20250903100726.269839-1-dqfext@gmail.com/
There are also alternative approaches:
- set SG and FRAGLIST unconditionally, and use skb_linearize()
on !chan->direct_xmit paths.
- don't use skb_linearize(), instead fix the compressors to handle
non-linear sk_buffs.
- conditionally set SG and FRAGLIST based on whether compressors are
in use.
drivers/net/ppp/ppp_generic.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index f9f0f16c41d1..3bf37871a1aa 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -1710,6 +1710,12 @@ 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;
+ /* Until we fix the compressor need to make sure data portion is
+ * linear.
+ */
+ if (skb_linearize(skb))
+ return NULL;
+
new_skb = alloc_skb(new_skb_size, GFP_ATOMIC);
if (!new_skb) {
if (net_ratelimit())
@@ -1797,6 +1803,12 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
case PPP_IP:
if (!ppp->vj || (ppp->flags & SC_COMP_TCP) == 0)
break;
+ /* Until we fix the compressor need to make sure data portion
+ * is linear.
+ */
+ 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);
@@ -3516,10 +3528,13 @@ ppp_connect_channel(struct channel *pch, int unit)
ret = -ENOTCONN;
goto outl;
}
- if (pch->chan->direct_xmit)
+ if (pch->chan->direct_xmit) {
ppp->dev->priv_flags |= IFF_NO_QUEUE;
- else
+ ppp->dev->features |= NETIF_F_SG | NETIF_F_FRAGLIST;
+ } else {
ppp->dev->priv_flags &= ~IFF_NO_QUEUE;
+ ppp->dev->features &= ~(NETIF_F_SG | NETIF_F_FRAGLIST);
+ }
spin_unlock_bh(&pch->downl);
if (pch->file.hdrlen > ppp->file.hdrlen)
ppp->file.hdrlen = pch->file.hdrlen;
--
2.43.0
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