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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
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