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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Message-ID: <CAHsH6Gvr=g_htFbJfhu4aYbG+bKsF7QWZMBUfvZVZKSfrbKD8Q@mail.gmail.com>
Date:	Thu, 26 Feb 2015 20:55:34 +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[]

On Thu, Feb 26, 2015 at 8:51 PM, Willem de Bruijn <willemb@...gle.com> wrote:
> On Thu, Feb 26, 2015 at 1:38 PM, Eyal Birger <eyal.birger@...il.com> wrote:
>> Hi,
>>
>> On Thu, Feb 26, 2015 at 7:26 PM, Willem de Bruijn <willemb@...gle.com> wrote:
>>>
>>> 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.
>
> It already is, isn't it?
>

I meant I prefer not to break out sockaddr_ll's individual fields in
packet_skb_cb.
Sorry for being ambiguous.

>> 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.
>
> Absolutely. This is a lot more work. Too much to fold into this patchset.
> I was more speculating in general. I may look into this at some point.
>
>>
>> Thanks!
>> Eyal.
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