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: <CA+FuTSfG+Se0L0tWdsj2+YHAcZRWLw2_qef6Q5xixwbQ_-OE8Q@mail.gmail.com>
Date:	Thu, 26 Feb 2015 12:26:09 -0500
From:	Willem de Bruijn <willemb@...gle.com>
To:	Eyal Birger <eyal.birger@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, shmulik.ladkani@...il.com,
	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 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.

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.
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