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From: David Miller <davem@davemloft.net>
Subject: Re: Increasing skb->mark size
Date: Wed, 02 Dec 2015 12:29:19 -0500 (EST)
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From: David Ahern <dsa@cumulusnetworks.com>
Date: Tue, 1 Dec 2015 21:42:45 -0800

> On 12/1/15 6:58 PM, Andi Kleen wrote:
>>>> - skb_iff is either skb->dev->iff or 0. so it could be replaced with a
>>>> single bit flag for the 0 case.
>>>
>>> ... and that one wouldn't work on ingress.
>>
>> Please explain.
> 
> skb->dev is dropped fairly early in the Rx path. skb_iif is needed for
> example for IP_CMSG_PKTINFO.

Right, we cannot let skb->dev escape the software interrupt handler, yet
we need some cheap reference to it for socket options et al.

Andi, this is not a 15-20 minute problem to solve.  Many very smart people
have spent days/weeks/months trying to even discover reasonable ways in
which to shrink sk_buff.

The low hanging fruit has been gone for a very long time now.