netdev - Re: [PATCH net-next 4/7] net: packet: use skb->dev as storage for skb orig len instead of skb->cb[]

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Date:	Thu, 26 Feb 2015 20:38:05 +0200
From:	Eyal Birger <eyal.birger@...il.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Shmulik Ladkani <shmulik.ladkani@...il.com>,
	Marcel Holtmann <marcel@...tmann.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 4/7] net: packet: use skb->dev as storage for skb
 orig len instead of skb->cb[]

Hi,

On Thu, Feb 26, 2015 at 7:26 PM, Willem de Bruijn <willemb@...gle.com> wrote:
> On Wed, Feb 25, 2015 at 9:10 PM, Eyal Birger <eyal.birger@...il.com> wrote:
>> As part of an effort to move skb->dropcount to skb->cb[], 4 bytes
>> of additional room are needed in skb->cb[] in packet sockets.
>>
>> Store the skb original length in skb->dev instead of skb->cb[] for
>> this purpose.
>
> Another option is to delay preparation of the full sockaddr_ll struct until
> when it's needed. It often is not used at all (if msg_name == NULL).
>
> sll_family, sll_protocol and sll_pkttype can be derived when needed
> in packet_recvmsg. The first two are the head of sockaddr_ll, so
> a shorter struct filled in in packet_rcv only from sll_ifindex onwards
> would save the 4B needed for origlen. Actually, a union is simpler:
>
> struct __packet_cb_sockaddr_ll {
>         union {
>                 struct {
>                         unsigned short  sll_family;
>                         __be16          sll_protocol;
>                  }
>                  unsigned int sll_origlen;
>         }
>     /* etc.. */
> };
>
> The family and protocol can just overwrite sll_origlen in packet_recvmsg
> before the memcpy into msg->msg_name.
>

That's an interesting option.
My personal inclination is not to inline sockaddr_ll in the packet cb structure.
IMHO it would probably make sense to do it as part of your below suggestion.

> Also interesting would be to avoid copying the address in the fast path
> as it may never be used, especially for long addresses (INFINIBAND_ALEN
> and FWNET_ALEN). From what I can tell, all but one header_ops.parse
> implementations return bytes from inside the packet, so could take an
> unsigned char ** as argument, store that pointer and postpone the
> memcpy to packet_recvmsg (handling skb_trim correctly). The exception is
> fwnet_header_parse, which would require some workaround.

I think it may be interesting to pursue this option, though I don't
think it should
be part of this specific effort.

Thanks!
Eyal.
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