From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [PATCH rdma-next V1 1/9] IB/ipoib: Add warning message when changing the MTU in UD over the max range Date: Thu, 12 Jan 2017 13:16:22 -0700 Message-ID: <20170112201622.GA14584@obsidianresearch.com> References: <20161228124728.26619-1-leon@kernel.org> <20161228124728.26619-2-leon@kernel.org> <1484246776.123135.19.camel@redhat.com> <20170112193508.GO20392@mtr-leonro.local> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170112193508.GO20392-U/DQcQFIOTAAJjI8aNfphQ@public.gmane.org> Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Leon Romanovsky Cc: Doug Ledford , linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Feras Daoud , Noa Osherovich List-Id: linux-rdma@vger.kernel.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 -- 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