linux-kernel - MTU does not change

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Message-ID: <CACzz7ux8mGT-d5YPWdkvTw5vg4-LKXUUsunL8sJySOeuYd0G1w@mail.gmail.com>
Date:   Wed, 5 Oct 2022 19:47:39 +0600
From:   Вячеслав Сальников 
        <snordicstr16@...il.com>
To:     linux-newbie@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: MTU does not change

Hi.

I apologize if I wrote in the wrong mail list. I have not found
linux-netdev for questions

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?

Best regards,
Slava.