Re: MTU does not change

[Slade Watkins]

Hey there,

On 10/5/22 at 9:47 AM, Вячеслав Сальников wrote:
> Hi.
>
> I apologize if I wrote in the wrong mail list. I have not found
> linux-netdev for questions

I've Cc'd the netdev and stable lists.

> I switched from kernel versions 4.9 to 5.15 and found that the MTU on
> the interfaces in the bridge does not change.
> For example:
> I have the following bridge:
> bridge      interface
> br0          sw1
>                 sw2
>                 sw3
>
> And I change with ifconfig MTU.
> I see that br0 sw1..sw3 has changed MTU from 1500 -> 1982.
>
> But if i send a ping through these interfaces, I get 1500(I added
> prints for output)
> I investigated the code and found the reason:
> The following commit came in the new kernel:
> https://github.com/torvalds/linux/commit/ac6627a28dbfb5d96736544a00c3938fa7ea6dfb
>
> And the behavior of the MTU setting has changed:
> > Kernel 4.9:
> > if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
> >     ip_mtu_locked(dst) ||
> >     !forwarding)  <--- True
> > return dst_mtu(dst) <--- 1982
> >
> >
> > / 'forwarding = true' case should always honour route mtu /
> > mtu = dst_metric_raw(dst, RTAX_MTU);
> > if (mtu)
> > return mtu;
>
>
>
> Kernel 5.15:
> > if (READ_ONCE(net->ipv4.sysctl_ip_fwd_use_pmtu) ||
> >     ip_mtu_locked(dst) ||
> >     !forwarding) { <--- True
> > mtu = rt->rt_pmtu;  <--- 0
> > if (mtu && time_before(jiffies, rt->dst.expires)) <-- False
> > goto out;
> > }
> >
> > / 'forwarding = true' case should always honour route mtu /
> > mtu = dst_metric_raw(dst, RTAX_MTU); <---- 1500
> > if (mtu) <--- True
> > goto out;
>
>
> Why is rt_pmtu now used instead of dst_mtu?
> Why is forwarding = False called with dst_metric_raw?
> Maybe we should add processing when mtu = rt->rt_pmtu == 0?
> Could this be an error?

Cheers,
-srw