On Thu, 26 Aug 2010 08:43:42 -0600 (Mountain Daylight Time)
Marc Aurele La France [off-list ref] wrote:

On Thu, 26 Aug 2010, Eric Dumazet wrote:

quoted

Le jeudi 26 août 2010 à 05:40 -0600, Marc Aurele La France a écrit :

quoted

quoted

Steady now.  There's no need to YELL nor be arrogant.  You and I both know
there's a place for NFS over UDP.  That's not changing any time soon.  While
I'm aware of the issue you brought up, it is separate from the one at hand in
this discussion.

quoted

quoted

I do want to thank you, however, for reminding me of TCP.  It's something
20/20 hindsight says I should have checked out before starting this thread.
Logistically, it'll be a few days before I can do so though.  If that allows
me to increase the MTU all the way up to 65520, then this UDP thing will
likely remain unresolved.

quoted

Unfortunately, your infiniband device lacks NETIF_F_SG support.

Oh, the device itself probably has something similar, but ipoib 
(IP-over-Infiniband) doesn't export that capability.

quoted

MTU a bit larger than PAGE_SIZE-overhead will need high order
allocations ?

Right.  And a 65520 MTU allocates sk_buff's with 128K contiguous payloads.

Infiniband device driver needs to be fixed to do SG and checksum offload.
Otherwise it is insane to try and run large MTU over it. I even wonder if
the dev_change_mtu() function should reject > PAGESIZE mtu for devices
that don't do scatter/gather or at least a raise a warning.