Re: [PATCH net] net: vlan: allow vlan device MTU change follow real device from smaller to bigger

[Eric Dumazet]

CC Herbert Xu, author of blamed commit.

On Mon, Feb 21, 2022 at 4:28 AM Ziyang Xuan
<william.xuanziyang@xxxxxxxxxx> wrote:
>
> vlan device MTU can only follow real device change from bigger to smaller
> but from smaller to bigger under the premise of vlan device MTU not exceed
> the real device MTU.
>
> This issue can be seen using the following commands:
>
> ip link add link eth1 dev eth1.100 type vlan id 100
> ip link set eth1 mtu 256
> ip link set eth1 mtu 1500
> ip link show
>
> Modify to allow vlan device follow real device MTU change from smaller
> to bigger when user has not configured vlan device MTU which is not
> equal to real device MTU. That also ensure user configuration has higher
> priority.
>
> Fixes: 2e477c9bd2bb ("vlan: Propagate physical MTU changes")
> Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
> ---
>  net/8021q/vlan.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> index 788076b002b3..7de4f462525a 100644
> --- a/net/8021q/vlan.c
> +++ b/net/8021q/vlan.c
> @@ -361,6 +361,7 @@ static int __vlan_device_event(struct net_device *dev, unsigned long event)
>  static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>                              void *ptr)
>  {
> +       unsigned int orig_mtu = ((struct netdev_notifier_info_ext *)ptr)->ext.mtu;
>         struct netlink_ext_ack *extack = netdev_notifier_info_to_extack(ptr);
>         struct net_device *dev = netdev_notifier_info_to_dev(ptr);
>         struct vlan_group *grp;
> @@ -419,7 +420,7 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>
>         case NETDEV_CHANGEMTU:
>                 vlan_group_for_each_dev(grp, i, vlandev) {
> -                       if (vlandev->mtu <= dev->mtu)
> +                       if (vlandev->mtu <= dev->mtu && vlandev->mtu != orig_mtu)
>                                 continue;
>
>                         dev_set_mtu(vlandev, dev->mtu);
> --
> 2.25.1
>

Herbert, do you recall why only a decrease was taken into consideration ?

Thanks.