netdev - [PATCH 1/4] net: arp: introduce arp_evict

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Date:   Wed, 13 Oct 2021 15:27:07 -0700
From:   James Prestwood <prestwoj@...il.com>
To:     netdev@...r.kernel.org
Cc:     James Prestwood <prestwoj@...il.com>
Subject: [PATCH 1/4] net: arp: introduce arp_evict_nocarrier sysctl parameter

This change introduces a new sysctl parameter, arp_evict_nocarrier.
When set (default) the ARP cache will be cleared on a NOCARRIER event.
This new option has been defaulted to '1' which maintains existing
behavior.

Clearing the ARP cache on NOCARRIER is relatively new, introduced by:

commit 859bd2ef1fc1110a8031b967ee656c53a6260a76
Author: David Ahern <dsahern@...il.com>
Date:   Thu Oct 11 20:33:49 2018 -0700

    net: Evict neighbor entries on carrier down

The reason for this changes is to prevent the ARP cache from being
cleared when a wireless device roams. Specifically for wireless roams
the ARP cache should not be cleared because the underlying network has not
changed. Clearing the ARP cache in this case can introduce significant
delays sending out packets after a roam.

A user reported such a situation here:

https://lore.kernel.org/linux-wireless/CACsRnHWa47zpx3D1oDq9JYnZWniS8yBwW1h0WAVZ6vrbwL_S0w@mail.gmail.com/

After some investigation it was found that the kernel was holding onto
packets until ARP finished which resulted in this 1 second delay. It
was also found that the first ARP who-has was never responded to,
which is actually what caues the delay. This change is more or less
working around this behavior, but again, there is no reason to clear
the cache on a roam anyways.

As for the unanswered who-has, we know the packet made it OTA since
it was seen while monitoring. Why it never received a response is
unknown.

Signed-off-by: James Prestwood <prestwoj@...il.com>
---
 include/linux/inetdevice.h  | 1 +
 include/uapi/linux/ip.h     | 1 +
 include/uapi/linux/sysctl.h | 1 +
 net/ipv4/arp.c              | 4 +++-
 net/ipv4/devinet.c          | 4 ++++
 5 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/linux/inetdevice.h b/include/linux/inetdevice.h
index 53aa0343bf69..63180170fdbd 100644
--- a/include/linux/inetdevice.h
+++ b/include/linux/inetdevice.h
@@ -133,6 +133,7 @@ static inline void ipv4_devconf_setall(struct in_device *in_dev)
 #define IN_DEV_ARP_ANNOUNCE(in_dev)	IN_DEV_MAXCONF((in_dev), ARP_ANNOUNCE)
 #define IN_DEV_ARP_IGNORE(in_dev)	IN_DEV_MAXCONF((in_dev), ARP_IGNORE)
 #define IN_DEV_ARP_NOTIFY(in_dev)	IN_DEV_MAXCONF((in_dev), ARP_NOTIFY)
+#define IN_DEV_ARP_EVICT_NOCARRIER(in_dev) IN_DEV_CONF_GET((in_dev), ARP_EVICT_NOCARRIER)
 
 struct in_ifaddr {
 	struct hlist_node	hash;
diff --git a/include/uapi/linux/ip.h b/include/uapi/linux/ip.h
index e42d13b55cf3..e00bbb9c47bb 100644
--- a/include/uapi/linux/ip.h
+++ b/include/uapi/linux/ip.h
@@ -169,6 +169,7 @@ enum
 	IPV4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST,
 	IPV4_DEVCONF_DROP_GRATUITOUS_ARP,
 	IPV4_DEVCONF_BC_FORWARDING,
+	IPV4_DEVCONF_ARP_EVICT_NOCARRIER,
 	__IPV4_DEVCONF_MAX
 };
 
diff --git a/include/uapi/linux/sysctl.h b/include/uapi/linux/sysctl.h
index 1e05d3caa712..6a3b194c50fe 100644
--- a/include/uapi/linux/sysctl.h
+++ b/include/uapi/linux/sysctl.h
@@ -482,6 +482,7 @@ enum
 	NET_IPV4_CONF_PROMOTE_SECONDARIES=20,
 	NET_IPV4_CONF_ARP_ACCEPT=21,
 	NET_IPV4_CONF_ARP_NOTIFY=22,
+	NET_IPV4_CONF_ARP_EVICT_NOCARRIER=23,
 };
 
 /* /proc/sys/net/ipv4/netfilter */
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index 922dd73e5740..50cfe4f37089 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -1247,6 +1247,7 @@ static int arp_netdev_event(struct notifier_block *this, unsigned long event,
 {
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
 	struct netdev_notifier_change_info *change_info;
+	struct in_device *in_dev = __in_dev_get_rcu(dev);
 
 	switch (event) {
 	case NETDEV_CHANGEADDR:
@@ -1257,7 +1258,8 @@ static int arp_netdev_event(struct notifier_block *this, unsigned long event,
 		change_info = ptr;
 		if (change_info->flags_changed & IFF_NOARP)
 			neigh_changeaddr(&arp_tbl, dev);
-		if (!netif_carrier_ok(dev))
+		if (IN_DEV_ARP_EVICT_NOCARRIER(in_dev) &&
+		    !netif_carrier_ok(dev))
 			neigh_carrier_down(&arp_tbl, dev);
 		break;
 	default:
diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index 1c6429c353a9..4ff4403749e0 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -75,6 +75,7 @@ static struct ipv4_devconf ipv4_devconf = {
 		[IPV4_DEVCONF_SHARED_MEDIA - 1] = 1,
 		[IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL - 1] = 10000 /*ms*/,
 		[IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL - 1] =  1000 /*ms*/,
+		[IPV4_DEVCONF_ARP_EVICT_NOCARRIER - 1] = 1,
 	},
 };
 
@@ -87,6 +88,7 @@ static struct ipv4_devconf ipv4_devconf_dflt = {
 		[IPV4_DEVCONF_ACCEPT_SOURCE_ROUTE - 1] = 1,
 		[IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL - 1] = 10000 /*ms*/,
 		[IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL - 1] =  1000 /*ms*/,
+		[IPV4_DEVCONF_ARP_EVICT_NOCARRIER - 1] = 1,
 	},
 };
 
@@ -2527,6 +2529,8 @@ static struct devinet_sysctl_table {
 		DEVINET_SYSCTL_RW_ENTRY(ARP_IGNORE, "arp_ignore"),
 		DEVINET_SYSCTL_RW_ENTRY(ARP_ACCEPT, "arp_accept"),
 		DEVINET_SYSCTL_RW_ENTRY(ARP_NOTIFY, "arp_notify"),
+		DEVINET_SYSCTL_RW_ENTRY(ARP_EVICT_NOCARRIER,
+					"arp_evict_nocarrier"),
 		DEVINET_SYSCTL_RW_ENTRY(PROXY_ARP_PVLAN, "proxy_arp_pvlan"),
 		DEVINET_SYSCTL_RW_ENTRY(FORCE_IGMP_VERSION,
 					"force_igmp_version"),
-- 
2.31.1