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    SubjectRe: [PATCH 4.4 92/97] ip6_vti: adjust vti mtu according to mtu of lower device
    On Thu, Apr 05, 2018 at 05:36:36PM +0200, Stefano Brivio wrote:
    > On Wed, 04 Apr 2018 01:09:16 +0100
    > Ben Hutchings <ben.hutchings@codethink.co.uk> wrote:
    >
    > > On Fri, 2018-03-23 at 10:55 +0100, Greg Kroah-Hartman wrote:
    > > > 4.4-stable review patch.  If anyone has any objections, please let me know.
    > > >
    > > > ------------------
    > > >
    > > > From: Alexey Kodanev <alexey.kodanev@oracle.com>
    > > >
    > > >
    > > > [ Upstream commit 53c81e95df1793933f87748d36070a721f6cb287 ]
    > > [...]
    > >
    > > There are a couple of follow-ups to this:
    > >
    > > c6741fbed6dc vti6: Properly adjust vti6 MTU from MTU of lower device
    > > 7a67e69a339a vti6: Keep set MTU on link creation or change, validate it
    > >
    > > The second of those will fail to build on branches older than 4.10
    > > though. It might be better to revert this one instead.
    >
    > Thanks Ben for spotting this.
    >
    > Actually,
    > 53c81e95df17 ("ip6_vti: adjust vti mtu according to mtu of lower device")
    > alone improves things already, despite being "fixed" by
    > c6741fbed6dc ("vti6: Properly adjust vti6 MTU from MTU of lower device")
    >
    > With just 53c81e95df17 the MTU of a vti6 interface will be somewhat
    > linked to the MTU of the lower layer, but will be underestimated.
    >
    > With c6741fbed6dc the calculation of MTU from lower layer will be
    > accurate instead.
    >
    > However, without
    > 7a67e69a339a ("vti6: Keep set MTU on link creation or change, validate it")
    > but with
    > 53c81e95df17 ("ip6_vti: adjust vti mtu according to mtu of lower device")
    > assignment of MTU on link change is discarded, so this would actually
    > introduce a bug.
    >
    > Fixing
    > 7a67e69a339a ("vti6: Keep set MTU on link creation or change, validate it")
    > for 4.4 up to 4.9 is trivial, we simply need to adjust for the lack of
    > b96f9afee4eb ("ipv4/6: use core net MTU range checking")
    > and reflect the change introduced by
    > f8a554b4aa96 ("vti6: Fix dev->max_mtu setting").
    >
    > So, Greg, here comes the backport of
    > 7a67e69a339a ("vti6: Keep set MTU on link creation or change, validate it")
    > based on latest linux-4.4.y branch, in case you want to keep the existing
    > change and add the follow-ups on top. Please let me know if I should submit
    > it formally.

    Ick, that's a mess. How about I just revert this patch from the stable
    trees, and then someone sends me either a list of git commits to apply,
    or patches, for the different trees if it's really needed?

    thanks,

    greg k-h

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