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 Thu, Nov 05, 2026 at 10:21:15AM +0100, Paolo Abeni wrote:
> On 10/30/26 15:51, Ramses de Norre wrote:
> > 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;
> > 
> 
> @Jay, @Hangbin: can you please have a look here?

Yes, I’ve read this patchset a few times but haven’t thought through it
carefully, since the ARP inspection logic is a bit complex. I’ll try to
finish the review today.

Thanks
Hangbin