netdev - Re: [PATCH net-next v3 3/4] net: use skb->priority for overloading skb->dropcount and skb->reserved

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Message-ID: <CAHsH6GuJENHse1EJXCzsEqcdCcJ=1xcPYpBO+e7fphxtRU8FyQ@mail.gmail.com>
Date:	Tue, 24 Feb 2015 17:24:39 +0200
From:	Eyal Birger <eyal.birger@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Shmulik Ladkani <shmulik.ladkani@...il.com>,
	Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v3 3/4] net: use skb->priority for overloading
 skb->dropcount and skb->reserved_tailroom instead of skb->mark

On Tue, Feb 24, 2015 at 4:41 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2015-02-24 at 16:07 +0200, Eyal Birger wrote:
>
>> Well, I took a look, then I tried :)
>>
>> It breaks in packet_rcv() in a BUILD_BUG_ON() assertion. The asserted size is:
>>
>> sizeof(*PACKET_SKB_CB(skb)) + MAX_ADDR_LEN - 8
>>
>> This was introduced in ffbc61117d32dc4e768 ("[PACKET]: Fix skb->cb
>> clobbering between aux and sockaddr")
>> It requires the ability to store the maximal possible address length (32).
>
> OK this might require to move origlen elsewhere in skb.
>
> skb->dev for example is set to NULL, same for skb dst.
>
> We have to be a bit creative/hacky to keep skb size small.
>

I agree. I don't think these feature deserve an skb size increase, so
some hackery is required.

Though, as skb->cb[] is somewhat 'owned' by the protocol families on
socket enqueue
I tend to find aliasing skb->priority with skb->dropcount "a lesser
evil" as compared with
partitioning the skb->cb[] space.
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