netdev - [RFC PATCH net-next v2] ppp: use IFF_NO

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Message-ID: <20250225032857.2932213-1-dqfext@gmail.com>
Date: Tue, 25 Feb 2025 11:28:55 +0800
From: Qingfang Deng <dqfext@...il.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
	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>,
	Michal Ostrowski <mostrows@...thlink.net>,
	James Chapman <jchapman@...alix.com>,
	Simon Horman <horms@...nel.org>,
	linux-ppp@...r.kernel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH net-next v2] ppp: use IFF_NO_QUEUE in virtual interfaces

For PPPoE, PPTP, and PPPoL2TP, the start_xmit() function directly
forwards packets to the underlying network stack and never returns
anything other than 1. So these interfaces do not require a qdisc,
and the IFF_NO_QUEUE flag should be set.

Introduces a direct_xmit flag in struct ppp_channel to indicate when
IFF_NO_QUEUE should be applied. The flag is set in ppp_connect_channel()
for relevant protocols.

Signed-off-by: Qingfang Deng <dqfext@...il.com>
---
RFC v1 -> v2: Conditionally set the flag for relevant protocols.

I'm not sure if ppp_connect_channel can be invoked while the device
is still up. As a qdisc is attached in dev_activate() called by
dev_open(), setting the IFF_NO_QUEUE flag on a running device will have
no effect.

 drivers/net/ppp/ppp_generic.c | 4 ++++
 drivers/net/ppp/pppoe.c       | 1 +
 drivers/net/ppp/pptp.c        | 1 +
 include/linux/ppp_channel.h   | 1 +
 net/l2tp/l2tp_ppp.c           | 1 +
 5 files changed, 8 insertions(+)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 6220866258fc..815108c98b78 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -3493,6 +3493,10 @@ ppp_connect_channel(struct channel *pch, int unit)
 		ret = -ENOTCONN;
 		goto outl;
 	}
+	if (pch->chan->direct_xmit)
+		ppp->dev->priv_flags |= IFF_NO_QUEUE;
+	else
+		ppp->dev->priv_flags &= ~IFF_NO_QUEUE;
 	spin_unlock_bh(&pch->downl);
 	if (pch->file.hdrlen > ppp->file.hdrlen)
 		ppp->file.hdrlen = pch->file.hdrlen;
diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index 2ea4f4890d23..68e631718ab0 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -693,6 +693,7 @@ static int pppoe_connect(struct socket *sock, struct sockaddr *uservaddr,
 		po->chan.mtu = dev->mtu - sizeof(struct pppoe_hdr) - 2;
 		po->chan.private = sk;
 		po->chan.ops = &pppoe_chan_ops;
+		po->chan.direct_xmit = true;
 
 		error = ppp_register_net_channel(dev_net(dev), &po->chan);
 		if (error) {
diff --git a/drivers/net/ppp/pptp.c b/drivers/net/ppp/pptp.c
index 689687bd2574..5feaa70b5f47 100644
--- a/drivers/net/ppp/pptp.c
+++ b/drivers/net/ppp/pptp.c
@@ -465,6 +465,7 @@ static int pptp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	po->chan.mtu -= PPTP_HEADER_OVERHEAD;
 
 	po->chan.hdrlen = 2 + sizeof(struct pptp_gre_header);
+	po->chan.direct_xmit = true;
 	error = ppp_register_channel(&po->chan);
 	if (error) {
 		pr_err("PPTP: failed to register PPP channel (%d)\n", error);
diff --git a/include/linux/ppp_channel.h b/include/linux/ppp_channel.h
index 45e6e427ceb8..3b50802d66fc 100644
--- a/include/linux/ppp_channel.h
+++ b/include/linux/ppp_channel.h
@@ -44,6 +44,7 @@ struct ppp_channel {
 	int		speed;		/* transfer rate (bytes/second) */
 	/* the following is not used at present */
 	int		latency;	/* overhead time in milliseconds */
+	bool		direct_xmit;	/* no qdisc, xmit directly */
 };
 
 #ifdef __KERNEL__
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 53baf2dd5d5d..fc5c2fd8f34c 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -806,6 +806,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	po->chan.private = sk;
 	po->chan.ops	 = &pppol2tp_chan_ops;
 	po->chan.mtu	 = pppol2tp_tunnel_mtu(tunnel);
+	po->chan.direct_xmit	= true;
 
 	error = ppp_register_net_channel(sock_net(sk), &po->chan);
 	if (error) {
-- 
2.43.0