Re: [PATCH net v5 2/2] bonding: give a slave that starts UP a full arp_missed_max window

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

On Fri, Oct 30, 2026 at 03:51:16PM +0100, Ramses de Norre via B4 Relay wrote:
> From: Ramses de Norre <ramses@well-founded.dev>
> 
> The ARP monitor marks an UP slave DOWN once jiffies passes
> last_rx + arp_missed_max * arp_interval + arp_interval/2. bond_enslave()
> seeds last_rx one arp_interval (plus a tick) in the past, which brings
> the down deadline of a slave that starts UP forward by one interval
> compared to what arp_missed_max promises.
> 
> Active-backup mode hides this with a grace period after enslavement in
> bond_ab_arp_inspect(). Load-balance mode has no grace period after
> enslavement. With arp_missed_max=1 the deadline is half an interval
> after enslavement, and the first monitor run checks the slave before it
> sends the first probe. If that run lands in the second half of the
> interval, a healthy slave is reported "interface is now down" and comes
> back UP on the next run.
> 
> Seed a slave that starts UP with last_link_up instead, so it gets a full
> arp_missed_max window for its first reply. bond_arp_rcv() and
> bond_na_rcv() only trust ARPs seen on backup slaves once the active
> slave's last_rx is after its last_link_up. A seed equal to last_link_up
> keeps that check false until a real reply arrives, as it is today.
> 
> This only covers a slave enslaved while the ARP monitor is running.
> When the monitor starts later, from bond_open() or when arp_interval is
> set at runtime, the seed is still as old as the time since enslavement,
> as it was before this patch.
> 
> Fixes: 5944b5abd864 ("Bonding: add arp_missed_max option")
> Signed-off-by: Ramses de Norre <ramses@well-founded.dev>
> ---
>  drivers/net/bonding/bond_main.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index 2f5638f4d3a78..84eafb607c370 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -2158,9 +2158,13 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
>  		new_slave->last_rx = jiffies -
>  			(2 * msecs_to_jiffies(bond->params.arp_interval) + 1);
>  	} else {
> +		/* Give a slave that starts UP a full arp_missed_max window.
> +		 * Not seeding it later than last_link_up keeps bond_arp_rcv()
> +		 * from trusting ARPs on backup slaves before the active one
> +		 * got a reply.
> +		 */
>  		new_slave->last_link_up = jiffies;
> -		new_slave->last_rx = jiffies -
> -			(msecs_to_jiffies(bond->params.arp_interval) + 1);
> +		new_slave->last_rx = new_slave->last_link_up;
>  	}
>  	for (i = 0; i < BOND_MAX_ARP_TARGETS; i++)
>  		new_slave->target_last_arp_rx[i] = new_slave->last_rx;
> 
> -- 
> 2.56.0
> 
> 

Sashiko reported some other issues during arp_target/arp_interval set. But
none of these introduced by this patch. And we haven't received real user
report about these issues.

So for this patch

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