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/ac6627a28dbfb5d96736544a00c3938fa7e...

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