netdev - Re: MTU does not change

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Date:   Wed, 5 Oct 2022 09:53:44 -0400
From:   Slade Watkins <srw@...dewatkins.net>
To:     Вячеслав Сальников 
        <snordicstr16@...il.com>
Cc:     linux-newbie@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: MTU does not change

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