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: <CAHsH6GtW1d9PvaOuTPxWqK_dWVZHxY7j7qgSJ8sChYsVYFj2UQ@mail.gmail.com>
Date:	Wed, 25 Feb 2015 23:15:14 +0200
From:	Eyal Birger <eyal.birger@...il.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Shmulik Ladkani <shmulik.ladkani@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Network Development <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 Wed, Feb 25, 2015 at 10:49 PM, Willem de Bruijn <willemb@...gle.com> wrote:
>>> 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.
>
> Please consider another field than priority. There are use
> cases that want to receive priority, similar to reading mark.
>
> Indeed, I have a draft packet auxdata patch for one user who wants
> to see the iptables changes (and infer tc effects) on egress packets.
> Fwiw, that patch also exposes mark, and runs into exactly this issue
> that you're fixing, so thanks :)
>
> Fields that are consistently NULL on enqueue, such as the dev field
> that Eric proposed, do not have this drawback.

Thanks! I was wondering whether someone would want to fetch skb->priority.

I had already conceded that aliasing skb->priority won't hold.

I plan to submit a patch series for moving skb->dropcount to skb->cb[].
Following that I will re-submit the patch for fetching skb->mark in auxdata.
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