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

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Date:   Tue, 15 Feb 2022 16:08:22 +0800
From:   "Ziyang Xuan (William)" <william.xuanziyang@...wei.com>
To:     <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: Why vlan_dev can not follow real_dev mtu change from smaller to
 bigger

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.