Re: [PATCH rdma-next V1 1/9] IB/ipoib: Add warning message when changing the MTU in UD over the max range

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Thu, Jan 12, 2017 at 09:35:08PM +0200, Leon Romanovsky wrote:

> > > In datagram mode, the IB UD (Unreliable Datagram) transport is used
> > > so the MTU of the interface is equal to the IB L2 MTU minus the
> > > IPoIB encapsulation header. Any request to change the MTU value
> > > above the maximum range will change the MTU to the max allowed, but
> > > will not show any warning message. An ipoib_warn is issued in such
> > > cases, letting the user know that even though the value is legal,
> > > it can't be currently applied.

How does RC mode work then?

> Second, I have a different view from you on the issue. User configured
> some value, which is not correct for IPoIB. In ideal world (without legacy),
> we were supposed to return error to him with proper message, but in our
> case (legacy applications) we can't (we tried and it broke some legacy
> ifcongfig, if I remember well). So it leaves us with one available
> option is to warn user about improper value.

This is a legitimate configuration though, the user could have a mixed
MTU fabric and rely on path MTU discovery, so sets the MTU to the
largest.

AFAIK the way to handle such configurations is with a multicast route that
has a reduced mtu.

Jason
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