netdev - Re: [net-next PATCH 2/5] ethernet/intel: Use eth_skb

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Message-Id: <20141126.154113.1689979442673394078.davem@davemloft.net>
Date:	Wed, 26 Nov 2014 15:41:13 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	alexander.duyck@...il.com, alexander.h.duyck@...hat.com,
	netdev@...r.kernel.org, jeffrey.t.kirsher@...el.com,
	kuznet@....inr.ac.ru
Subject: Re: [net-next PATCH 2/5] ethernet/intel: Use eth_skb_pad helper

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 25 Nov 2014 20:01:13 -0800

[ I am still intrigued, CC:'ing Alexey ]

> On Tue, 2014-11-25 at 22:19 -0500, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Tue, 25 Nov 2014 17:43:05 -0800
>> 
>> > I believe I finally have an idea why we had various + 15 in skb
>> > allocations in TCP stack !
>> 
>> It was so that you could do one level of tunneling with "for
>> free".  Or that is my recollection.
>> 
>> Those + 15 existed way before any of these padto() calls even
>> existed.
> 
> Well, tunneling is added in front of the packet. Thats why we use
> MAX_TCP_HEADER.
> 
> The +15 is in fact because TCP stack wanted to make sure the eventual
> padding (needing tailroom, not headroom) was possible...
> 
> Note that ack packets never used the +15, but other packets did.

Alexey, do you remember exact reason for that +15 everywhere in TCP
packet allocation sizing?

I thought it was for headroom, but as Eric shows that's illogical,
it can only be for tailroom considerations.
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