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 Sun, Jan 15, 2017 at 10:35:43AM +0200, Leon Romanovsky wrote:
> On Fri, Jan 13, 2017 at 02:27:21PM -0700, Jason Gunthorpe wrote:
> > On Fri, Jan 13, 2017 at 10:31:27PM +0200, Leon Romanovsky wrote:
> >
> > > > This is about what happens if the multicast MTU < unicast MTU - which
> > > > is *exactly* the same case on RC and UD.
> > > >
> > > > IPoIB cannot send a 64k multicast MTU on RC either.
> > >
> > > Multicast is sent in datagram despite being in connected mode and for
> > > datagram the MTU is limited by IB
> >
> > I am aware of that. Read you patch again:
> >
> > > +	if (priv->mcast_mtu < priv->admin_mtu)
> > > +		ipoib_warn(priv, "MTU must be smaller than mcast_mtu (%u)\n",
> >
> > This check is fails in RC mode as well.
> >
> > And we already check if the requested MTU is beyond the port
> > capability:
> >
> > +        if (new_mtu > IPOIB_UD_MTU(priv->max_ib_mtu))
> > +                return -EINVAL;
> >
> > So I have *no idea* why you'd want to check it against the multicast
> > group too..
> >
> 
> Port MTU capability can be higher than link MTU. In this flow, the user
> can try to set manually MTU which will be higher than SM provided.
> 
> Immediately after that print, we have a following piece of code:
>  236         dev->mtu = min(priv->mcast_mtu, priv->admin_mtu);

Maybe the better fix is to take that min out than generate a
warning.

Jason
--
To unsubscribe from this list: send the line "unsubscribe linux-rdma" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html