[Bugme-new] [Bug 13467] New: Cannot set larger mtu on vlan then on underlying untagged device

[bugzilla-daemon at bugzilla.kernel.org]

http://bugzilla.kernel.org/show_bug.cgi?id=13467

           Summary: Cannot set larger mtu on vlan then on underlying
                    untagged device
           Product: Networking
           Version: 2.5
    Kernel Version: 2.6.27.24-170.2.68.fc10.i686
          Platform: All
        OS/Version: Linux
              Tree: Fedora
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Other
        AssignedTo: acme at ghostprotocols.net
        ReportedBy: tomek at jot23.org
        Regression: No


I want to create vlan interface which uses jumbo frames while underlying
network device is still using standerd 1500 bytes frames. So I tried:

tkepczyx-mobl1:~# ip link show dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP
qlen 100
    link/ether 00:0d:60:79:e0:9d brd ff:ff:ff:ff:ff:ff
tkepczyx-mobl1:~# vconfig add eth0 12
Added VLAN with VID == 12 to IF -:eth0:-
tkepczyx-mobl1:~# ip link set dev eth0.12 mtu 9000
RTNETLINK answers: Numerical result out of range

So I made sure that eth0 supports 9000 bytes mtu:

tkepczyx-mobl1:~# ip link set dev eth0 mtu 9000
tkepczyx-mobl1:~# ip link show dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc pfifo_fast state UP
qlen 100
    link/ether 00:0d:60:79:e0:9d brd ff:ff:ff:ff:ff:ff

It apparently does. So I gave it another try:

tkepczyx-mobl1:~# ip link set dev eth0.12 mtu 9000
tkepczyx-mobl1:~# ip link show dev eth0.12
4: eth0.12 at eth0: <BROADCAST,MULTICAST> mtu 9000 qdisc noop state DOWN
    link/ether 00:0d:60:79:e0:9d brd ff:ff:ff:ff:ff:ff

Now it worked. So lets decrease mtu on eth0:

tkepczyx-mobl1:~# ip link set dev eth0 mtu 1500
tkepczyx-mobl1:~# ip link show dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP
qlen 100
    link/ether 00:0d:60:79:e0:9d brd ff:ff:ff:ff:ff:ff
tkepczyx-mobl1:~# ip link show dev eth0.12
4: eth0.12 at eth0: <BROADCAST,MULTICAST> mtu 9000 qdisc noop state DOWN
    link/ether 00:0d:60:79:e0:9d brd ff:ff:ff:ff:ff:ff

It also worked.
Now - if there is a reason which prevents me to increase mtu on vlan over mtu
on underlyng device, the very same reason should have prevented me from
decreasing mtu on underlying eth0 below mtu on eth0.12 vlan. This is a fault.

I started digging a bit in sources and there is one thing which rings a bell:
vlan_dev_change_mtu function in net/8021q/vlan_dev.c:

static int vlan_dev_change_mtu(struct net_device *dev, int new_mtu)
{
        /* TODO: gotta make sure the underlying layer can handle it,
         * maybe an IFF_VLAN_CAPABLE flag for devices?
         */
        if (vlan_dev_info(dev)->real_dev->mtu < new_mtu)
                return -ERANGE;

        dev->mtu = new_mtu;

        return 0;
}

My guess is that check in if() statement above is against currently configured
mtu while it probably should check what physically device can transmit.

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