netdev - Re: [RFC] IP_MAX

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Date:	Fri, 21 Dec 2012 11:59:10 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	erdnetdev@...il.com
Cc:	netdev@...r.kernel.org, rick.jones2@...com, kuznet@....inr.ac.ru
Subject: Re: [RFC] IP_MAX_MTU value

From: Eric Dumazet <erdnetdev@...il.com>
Date: Thu, 20 Dec 2012 22:47:48 -0800

> 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.

I don't think it has to do with kmalloc() at all.

Maybe something strange to do with the fact that each frag has to
be an 8-byte multiple, or something like that?

Alexey choose this value back in 1998, maybe he remembers the
reason for the strange value.

Alexey?
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