kernel-packages team mailing list archive

[Andy Whitcroft <apw@xxxxxxxxxxxxx>]

Looking at the current mainline code, when you set the MTU is it clamped
by the bridge wide "lowest" mtu:

  static int br_change_mtu(struct net_device *dev, int new_mtu)
  {
        struct net_bridge *br = netdev_priv(dev);
        if (new_mtu < 68 || new_mtu > br_min_mtu(br))
                return -EINVAL;
  [...]
  }

Which is calculated rather simplistically where there are no attached
devices:

  int br_min_mtu(const struct net_bridge *br)
  {
  [...]
        if (list_empty(&br->port_list))
                mtu = ETH_DATA_LEN;
  [...]

It is not entirely clear why we care to clamp the MTU artificially at
this level, as we will reevaluate the mtu and lower it when we do add a
new bridge port.

All of that said, if the MTU was set on the KVM tap before it was added
then the bridge MTU should follow that device, and that appears to be
the expected use model from the way the code is written.

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https://bugs.launchpad.net/bugs/1399064

Title:
  bridges cannot have a mtu > 1500 by themselves

Status in linux package in Ubuntu:
  Triaged

Bug description:
  A linux bridge always adopts the smallest MTU of the enslaved devices.
  When no device are enslaved, it defaults to a MTU of 1500 and refuses
  to use a larger one. This is problematic when using bridges enslaving
  only virtual NICs (vnetX) like it's common with KVM guests.

  Steps to reproduce the problem

  1) sudo ip link add br-test0 type bridge # create an empty bridge
  2) sudo ip link set br-test0 mtu 9000    # attempt to set MTU > 1500
  3) ip link show dev br-test0             # confirm MTU

  Here, 2) returns "RTNETLINK answers: Invalid argument". One
  (cumbersome) way around this is:

  4) sudo modprobe dummy
  5) sudo ip link set dummy0 mtu 9000 master br-test0

  Then the bridge's MTU can be changed from anywhere to 9000.

  This bug is especially annoying for the virtualization case because
  the KVM's tap driver will by default adopt the bridge's MTU on startup
  making it impossible (without the workaround) to use a large MTU on
  the guest VMs.

  Additional information:

  $ lsb_release -rd
  Description:	Ubuntu 14.04.1 LTS
  Release:	14.04
  $ apt-cache policy linux-image-3.13.0-41-generic iproute2
  linux-image-3.13.0-41-generic:
    Installed: 3.13.0-41.70
    Candidate: 3.13.0-41.70
    Version table:
   *** 3.13.0-41.70 0
          500 http://archive.ubuntu.com/ubuntu/ trusty-proposed/main amd64 Packages
          100 /var/lib/dpkg/status
  iproute2:
    Installed: 3.12.0-2
    Candidate: 3.12.0-2
    Version table:
   *** 3.12.0-2 0
          500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
          100 /var/lib/dpkg/status

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: linux-image-3.13.0-41-generic 3.13.0-41.70
  ProcVersionSignature: Ubuntu 3.13.0-41.70-generic 3.13.11.11
  Uname: Linux 3.13.0-41-generic x86_64
  ApportVersion: 2.14.1-0ubuntu3.6
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  simon      4594 F.... pulseaudio
  CurrentDesktop: Unity
  CurrentDmesg: dmesg: klogctl failed: Operation not permitted
  Date: Wed Dec  3 23:02:25 2014
  HibernationDevice: RESUME=UUID=8e7a3220-158f-413b-81b2-55b236bb1f3f
  InstallationDate: Installed on 2014-01-26 (311 days ago)
  InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Alpha amd64 (20140124)
  MachineType: LENOVO 2516CTO
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-3.13.0-41-generic root=/dev/mapper/crypt-root ro quiet splash cryptopts=target=crypt,source=/dev/sda1,lvm=crypt-root possible_cpus=4 nmi_watchdog=0 vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-3.13.0-41-generic N/A
   linux-backports-modules-3.13.0-41-generic  N/A
   linux-firmware                             1.127.10
  RfKill:
   0: phy0: Wireless LAN
    Soft blocked: no
    Hard blocked: no
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 02/14/2013
  dmi.bios.vendor: LENOVO
  dmi.bios.version: 6IET85WW (1.45 )
  dmi.board.name: 2516CTO
  dmi.board.vendor: LENOVO
  dmi.board.version: Not Available
  dmi.chassis.asset.tag: No Asset Information
  dmi.chassis.type: 10
  dmi.chassis.vendor: LENOVO
  dmi.chassis.version: Not Available
  dmi.modalias: dmi:bvnLENOVO:bvr6IET85WW(1.45):bd02/14/2013:svnLENOVO:pn2516CTO:pvrThinkPadT410:rvnLENOVO:rn2516CTO:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
  dmi.product.name: 2516CTO
  dmi.product.version: ThinkPad T410
  dmi.sys.vendor: LENOVO

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