From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [net-next PATCH 2/5] ethernet/intel: Use eth_skb_pad helper Date: Wed, 26 Nov 2014 15:41:13 -0500 (EST) Message-ID: <20141126.154113.1689979442673394078.davem@davemloft.net> References: <1416966185.29427.46.camel@edumazet-glaptop2.roam.corp.google.com> <20141125.221900.653470310152197801.davem@davemloft.net> <1416974473.29427.49.camel@edumazet-glaptop2.roam.corp.google.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: alexander.duyck@gmail.com, alexander.h.duyck@redhat.com, netdev@vger.kernel.org, jeffrey.t.kirsher@intel.com, kuznet@ms2.inr.ac.ru To: eric.dumazet@gmail.com Return-path: Received: from shards.monkeyblade.net ([149.20.54.216]:50561 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752701AbaKZUlP (ORCPT ); Wed, 26 Nov 2014 15:41:15 -0500 In-Reply-To: <1416974473.29427.49.camel@edumazet-glaptop2.roam.corp.google.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Eric Dumazet 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 >> 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.