From mboxrd@z Thu Jan 1 00:00:00 1970 From: Leon Romanovsky Subject: Re: [PATCH rdma-next V1 1/9] IB/ipoib: Add warning message when changing the MTU in UD over the max range Date: Fri, 13 Jan 2017 17:08:24 +0200 Message-ID: <20170113150824.GQ20392@mtr-leonro.local> References: <20161228124728.26619-1-leon@kernel.org> <20161228124728.26619-2-leon@kernel.org> <1484246776.123135.19.camel@redhat.com> <20170112193508.GO20392@mtr-leonro.local> <20170112201622.GA14584@obsidianresearch.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="dT+85zccSFkyJC53" Return-path: Content-Disposition: inline In-Reply-To: <20170112201622.GA14584-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org> Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Jason Gunthorpe Cc: Doug Ledford , linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Feras Daoud , Noa Osherovich List-Id: linux-rdma@vger.kernel.org --dT+85zccSFkyJC53 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Thu, Jan 12, 2017 at 01:16:22PM -0700, Jason Gunthorpe wrote: > 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? RC mode doesn't have limitation on MTU size and it allows messages larger than IB link-layer MTU. > > > 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. Multicast traffic is sent in datagram mode hence the limit will imply. Maybe, I went too far by calling it error, but IMHO absence of indication to user of decreased MTU is not good either. > > Jason --dT+85zccSFkyJC53 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEkhr/r4Op1/04yqaB5GN7iDZyWKcFAlh47WgACgkQ5GN7iDZy WKeXJw//WxN7xdEIlRqkfo7DNVbVvWZPV7qWBhhu1Ei03NTChJM/zw0KdhBFD5Id LI4Uhot2+ddUigC4TsuUI+YgFM92lEXw98XrmWY5nnhzRDR5yM4b/Qy0r7F/duMU l9gV5J9nP5or1DT8Zs0EfHiDAcS0u5TEbDSjo67SXXbTnAnQVxPny3rPfduAH05l FuEznTnhjzoW8Sz0qdD39ge7AkpRy6BDe6+IOQf27BvynrMZsKEddsZ2o9mq/6K4 e9ST6rrOD8jPy6VzOLtoSL08hQalL6cKtbaGODRdMHeZ8A6OB09ECcXJsfduZNKD QqgwV3WS6fqzKXoMTtG1rbGyoioJ/UI/4EMif/5LLLbXtNhRlPhBeIaAXGd3PdXa Ez1zAhRqlMNXyQCNWdmkjhm7h/a15XSy0YD0/NBtcVIyXHbe8WcaidjPmoUIjc9v Vwr5oQsP3KB1qrX01VcZlSgDcVKmKLO7xg/XUQArtYpABkbhRHNNaOCZQKS7Ekpu LKAEHMNuxyKIhk2ZLqvLCi3Unp6h/HybE6TRzytdfJhdJR5BdGLRopFtCRN5SDgX vld3aP/cdKGoWEt4hupUy/8vBv2CdbUsD1qhkYHTZYsmUmL+KlTbj2b6+K3HswHy bYixunTSEoG4x0QxbZG/FgJ8GcjfMERXzXGevZUDPwi3UhN35jY= =FMpW -----END PGP SIGNATURE----- --dT+85zccSFkyJC53-- -- 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