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

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Date:	Mon, 23 Feb 2015 21:25:29 +0200
From:	Eyal Birger <eyal.birger@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Shmulik Ladkani <shmulik.ladkani@...il.com>,
	David Miller <davem@...emloft.net>,
	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 Mon, Feb 23, 2015 at 9:10 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2015-02-23 at 20:56 +0200, Shmulik Ladkani wrote:
>> Hi,
>>
>> On Mon, 23 Feb 2015 19:52:03 +0200 Eyal Birger <eyal.birger@...il.com> wrote:
>> > Commit 977750076d98 ("af_packet: add interframe drop cmsg (v6)")
>> > unionized skb->mark and skb->dropcount in order to allow recording
>> > of the socket drop count while maintaining struct sk_buff size.
>> >
>> > In order to allow for the skb->mark to be fetched by user-space code
>> > it needs to be detached from this union; skb->priority is used
>> > instead.
>> >
>> > The purpose of overloading skb->priority is solely for retaining
>> > struct sk_buff size; skb->priority is not used after the skb is
>> > queued to the socket.
>> >
>> > Use of aliased fields, skb->dropcount and skb->reserved_tailroom,
>> > is either done after the skb is cloned (e.g. in packet_rcv()) or
>> > before skb->priority is used (e.g. in IGMPv3/MLD/TCP).
>>
>> Preserving a small skb size is an important cause.
>>
>> In this case however, it seems that aliasing 'priority' sacrifices
>> maintainability: 'priority' does not seem "naturally orthogonal" to
>> 'drop_count' or 'reserved_tailroom'.
>>
>> One might arm 'priority' in various code flows (future and existing),
>> which may accidentally clash with the aliases. It is unexpected, hard to
>> remember (especially since 'priority' wasn't formerly aliased), and
>> sometimes it's simply not trivial to assure there's no clash.
>>
>> May I suggest to unalias 'mark' out of the union?
>
> My suggestion would be to move dropcount in skb->cb[] instead.
>
> mark & reserved_tailroom can stay in the union.
>
>

The original commit introducing dropcount for af_packet
(see http://marc.info/?l=linux-netdev&m=125450261121971) also claimed
the proper location would in fact be skb->cb[]. However, it was also
claimed that
skb->cb[] is all used up.

Things are even more complicated now as dropcount became a socket
level feature and skb->cb[] is handled differently in each protocol family.
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