netdev - Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

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Message-ID: <CANn89iJA=mJ+Jz-03p8yBtp6oCBE6ShwW-KW-0bpMx8OdA8whg@mail.gmail.com>
Date:	Thu, 7 Jan 2016 07:54:08 -0500
From:	Eric Dumazet <edumazet@...gle.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	Florian Westphal <fw@...len.de>,
	Thadeu Lima de Souza Cascardo <cascardo@...hat.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

On Thu, Jan 7, 2016 at 7:04 AM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
> On Thu, Jan 7, 2016 at 2:59 PM, Eric Dumazet <edumazet@...gle.com> wrote:
>> On Thu, Jan 7, 2016 at 6:38 AM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
>>>
>>> Also I've found strange thing: reason of expanding skb->cb from 40 to
>>> 48 bypes in 2006
>>> 3e3850e989c5d2eb1aab6f0fd9257759f0f4cbc6 was that struct inet6_skb_parm does
>>> not fit. But it's is only 24 bytes. Does some arches add pad after
>>> each _u16 field?
>>
>> "struct inet6_skb_parm" is part of struct tcp_skb_cb
>>
>> This is why Patrick had to increase skb->cb[]
>
> Whoa. Funny. TCP moves that chunk back and forward instead of just
> putting it at the first place in struct.

You probably want to look at git history to find out why it is done this way.

TCP performance is critical for some of us, and doing such trick avoid
one cache miss per skb in some critical list traversals.
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