netdev - [RFC] IP_MAX

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Message-ID: <1356072468.21834.4805.camel@edumazet-glaptop>
Date:	Thu, 20 Dec 2012 22:47:48 -0800
From:	Eric Dumazet <erdnetdev@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>, Rick Jones <rick.jones2@...com>
Subject: [RFC] IP_MAX_MTU value

Hi David

We have the following definition in net/ipv4/route.c

#define IP_MAX_MTU   0xFFF0

This means that "netperf -t UDP_STREAM", using UDP messages of 65507
bytes, are fragmented on loopback interface (while its MTU is now 65536
and should allow those UDP messages being sent without fragments)

I guess Rick chose 65507 bytes in netperf because it was related to the
max IPv4 datagram length :

65507 + 28 = 65535

Changing IP_MAX_MTU from 0xFFF0 to 0x10000 seems safe [1], but I might
miss something really obvious ?

It might be because in old days we reserved 16 bytes for the ethernet
header, and we wanted to avoid kmalloc() round-up to kmalloc-131072
slab ?

If so, we certainly can limit skb->head to 32 or 64 KB and complete with
page fragments the remaining space.

Thanks

[1] performance increase is ~50% 

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