From: Slade Watkins <srw@sladewatkins.net>
To: "Вячеслав Сальников" <snordicstr16@gmail.com>
Cc: linux-newbie@vger.kernel.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: MTU does not change
Date: Wed, 5 Oct 2022 09:53:44 -0400 [thread overview]
Message-ID: <2e6b2a03-6287-0a23-c187-51fe8e9b7941@sladewatkins.net> (raw)
In-Reply-To: <CACzz7ux8mGT-d5YPWdkvTw5vg4-LKXUUsunL8sJySOeuYd0G1w@mail.gmail.com>
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
prev parent reply other threads:[~2022-10-05 13:53 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-05 13:47 MTU does not change Вячеслав Сальников
2022-10-05 13:53 ` Slade Watkins [this message]
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