linux-kernel - Re: [PATCH net-next] net: vrf: don't down netif when add slave

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Message-ID: <aItcuFGx1S7ySE3y@shredder>
Date: Thu, 31 Jul 2025 15:08:24 +0300
From: Ido Schimmel <idosch@...sch.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: dsahern@...nel.org, andrew+netdev@...n.ch, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: vrf: don't down netif when add slave

On Thu, Jul 31, 2025 at 07:22:19PM +0800, Menglong Dong wrote:
> For now, cycle_netdev() will be called to flush the neighbor cache when
> add slave by downing and upping the slave netdev. When the slave has
> vlan devices, the data transmission can interrupted.
> 
> Optimize it by notifying the NETDEV_CHANGEADDR instead, which will also
> flush the neighbor cache. It's a little ugly, and maybe we can introduce
> a new event to do such flush :/

Cycling the netdev is not only about neighbors, but also about moving
routes to the correct table (see the comment above the function):

ip link add name dummy1 up type dummy
sysctl -wq net.ipv6.conf.dummy1.keep_addr_on_down=1
ip address add 192.0.2.1/24 dev dummy1
ip address add 2001:db8:1::1/64 dev dummy1
ip link add name vrf1 up type vrf table 100
ip link set dev dummy1 master vrf1
ip -4 route show table 100
192.0.2.0/24 dev dummy1 proto kernel scope link src 192.0.2.1 
local 192.0.2.1 dev dummy1 proto kernel scope host src 192.0.2.1 
broadcast 192.0.2.255 dev dummy1 proto kernel scope link src 192.0.2.1 
ip -6 route show table 100
local 2001:db8:1::1 dev dummy1 proto kernel metric 0 pref medium
2001:db8:1::/64 dev dummy1 proto kernel metric 256 pref medium
local fe80::f877:f7ff:fecb:bfb dev dummy1 proto kernel metric 0 pref medium
fe80::/64 dev dummy1 proto kernel metric 256 pref medium
multicast ff00::/8 dev dummy1 proto kernel metric 256 pref medium

And it doesn't happen with your patch:

ip link add name dummy1 up type dummy
sysctl -wq net.ipv6.conf.dummy1.keep_addr_on_down=1
ip address add 192.0.2.1/24 dev dummy1
ip address add 2001:db8:1::1/64 dev dummy1
ip link add name vrf1 up type vrf table 100
ip link set dev dummy1 master vrf1
ip -4 route show table 100
ip -6 route show table 100

You can try configuring the VLAN devices with "loose_binding on".