Re: [PATCH net v4 1/2] bonding: fix initial last_rx vs ARP-monitor slack window

[Hangbin Liu <hangbin.liu@linux.dev>]

On Wed, Oct 28, 2026 at 12:48:07PM +0100, Ramses de Norre via B4 Relay wrote:
> From: Ramses de Norre <ramses@well-founded.dev>
> 
> bond_enslave() seeds a new slave's last_rx so that the ARP monitor does
> not report it UP before a real ARP reply arrives, see commit
> f31c7937c254 ("bonding: start slaves with link down for ARP monitor").
> 
> The seed is one arp_interval (plus a tick) in the past. That was old
> enough when the up test was "jiffies <= last_rx + arp_interval". Commit
> da210f559019 ("bonding: add some slack to arp monitoring time limits")
> added arp_interval/2 of slack to that test without moving the seed, so
> the seed now falls inside the up window. For the first half interval
> after enslavement, bond_ab_arp_inspect() proposes a slave that starts
> DOWN as UP although it has received nothing. The up test always uses a
> single interval, so this does not depend on primary_reselect or
> arp_missed_max.
> 
> If that slave is the configured primary, force_primary makes it the
> active slave. ARP validation fails it shortly after (on SFP+ ports the
> PHY is often still negotiating) and the bond falls back to the backup.
> With primary_reselect=always the bond recovers once the port really
> comes up, but drops traffic until then. With primary_reselect=failure,
> force_primary is spent and the bond stays on the backup.
> 
> Reproducer:
> 
>   ip link add bond0 type bond mode active-backup arp_interval 1000 \
>       arp_validate all arp_ip_target 192.0.2.1 \
>       primary eth0 primary_reselect failure
>   # eth0: SFP+ (slow link-up), eth1: RJ45 (fast link-up)
>   ip link set eth0 master bond0
>   ip link set eth1 master bond0
>   ip link set bond0 up
>   # bond0 lands on eth0 via force_primary, ARP-fails it before the
>   # SFP+ has carrier, falls to eth1, and stays there.
> 
> Move the seed below the initial link state and seed a slave that starts
> DOWN two intervals in the past, outside the up window. A slave that
> starts UP keeps its current seed: the up test does not apply to it, and
> an older seed would only bring its down deadline forward by one
> interval. The initial link state is unchanged.
> 
> Fixes: da210f559019 ("bonding: add some slack to arp monitoring time limits")
> Signed-off-by: Ramses de Norre <ramses@well-founded.dev>
> ---
>  drivers/net/bonding/bond_main.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index 7019e5d8fdc8e..bfa4cb97e647c 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -2122,13 +2122,6 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
>  	if (res && bond_needs_speed_duplex(bond))
>  		new_slave->link = BOND_LINK_DOWN;
>  
> -	new_slave->last_rx = jiffies -
> -		(msecs_to_jiffies(bond->params.arp_interval) + 1);
> -	for (i = 0; i < BOND_MAX_ARP_TARGETS; i++)
> -		new_slave->target_last_arp_rx[i] = new_slave->last_rx;
> -
> -	new_slave->last_tx = new_slave->last_rx;
> -
>  	/* check for initial state */
>  	new_slave->link = BOND_LINK_NOCHANGE;
>  	if (bond->params.miimon) {
> @@ -2159,6 +2152,22 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
>  
>  	if (new_slave->link != BOND_LINK_DOWN)
>  		new_slave->last_link_up = jiffies;

Nit: This part could move down and merge with the "else" part.

> +
> +	/* The ARP monitor's up test includes arp_interval/2 of slack, see
> +	 * bond_time_in_interval(). Seed a slave that starts DOWN outside
> +	 * that window, so it stays DOWN until a real reply arrives.
> +	 */
> +	if (new_slave->link == BOND_LINK_DOWN)
> +		new_slave->last_rx = jiffies -
> +			(2 * msecs_to_jiffies(bond->params.arp_interval) + 1);
> +	else
> +		new_slave->last_rx = jiffies -
> +			(msecs_to_jiffies(bond->params.arp_interval) + 1);
> +	for (i = 0; i < BOND_MAX_ARP_TARGETS; i++)
> +		new_slave->target_last_arp_rx[i] = new_slave->last_rx;
> +
> +	new_slave->last_tx = new_slave->last_rx;
> +
>  	slave_dbg(bond_dev, slave_dev, "Initial state of slave is BOND_LINK_%s\n",
>  		  new_slave->link == BOND_LINK_DOWN ? "DOWN" :
>  		  (new_slave->link == BOND_LINK_UP ? "UP" : "BACK"));
> 
> -- 
> 2.56.0
> 
> 

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>