netdev - Re: Why vlan_dev can not follow real_dev mtu change from smaller to bigger

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Date:   Thu, 17 Feb 2022 20:56:06 -0700
From:   David Ahern <dsahern@...nel.org>
To:     "Ziyang Xuan (William)" <william.xuanziyang@...wei.com>,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, Jakub Kicinski <kuba@...nel.org>,
        pablo@...filter.org, keescook@...omium.org, alobakin@...me,
        nbd@....name, herbert@...dor.apana.org.au
Subject: Re: Why vlan_dev can not follow real_dev mtu change from smaller to
 bigger

On 2/16/22 11:44 PM, Ziyang Xuan (William) wrote:
>> Hello,
>>
>> Recently, I did some tests about mtu change for vlan device
>> and real_dev. I found that vlan_dev's mtu could not follow its
>> real_dev's mtu change from smaller to bigger.
>>
>> For example:
>> Firstly, change real_dev's mtu from 1500 to 256, vlan_dev's mtu
>> follow change from 1500 to 256.
>> Secondly, change real_dev's mtu from 256 to 1500, but vlan_dev's
>> mtu is still 256.
>>
>> I fond the code as following. But I could not understand the
>> limitations. Is there anyone can help me?
>>
>> static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>> 			     void *ptr)
>> {
>> 	...
>>
>> 	case NETDEV_CHANGEMTU:
>> 		vlan_group_for_each_dev(grp, i, vlandev) {
>> 			if (vlandev->mtu <= dev->mtu)
>> 				continue;
>>
>> 			dev_set_mtu(vlandev, dev->mtu);
>> 		}
>> 		break;
>> 	...
>> }
>>
>> Thank you for your reply.
>>
> commit: 2e477c9bd2bb ("vlan: Propagate physical MTU changes") wanted
> to ensure that all existing VLAN device MTUs do not exceed the new
> underlying MTU. Make VLAN device MTUs equal to the new underlying MTU
> follow this rule. So I think the following modification is reasonable
> and can solve the above problem.
> 
> @@ -418,12 +418,8 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>                 break;
> 
>         case NETDEV_CHANGEMTU:
> -               vlan_group_for_each_dev(grp, i, vlandev) {
> -                       if (vlandev->mtu <= dev->mtu)
> -                               continue;
> -
> +               vlan_group_for_each_dev(grp, i, vlandev)
>                         dev_set_mtu(vlandev, dev->mtu);
> -               }
>                 break;
> 

vlans must work within the mtu of the underlying device, so shrinking
the mtu of the vlan device when the real device changes is appropriate.

vlan devices do not necessarily have to operate at the same mtu as the
real device, so when the real device mtu is increased it might not be
appropriate to raise the mtu of the vlan devices. Admin needs to manage
that.