Subject: Re: [PATCH v3 net-next 05/16] tcp: switch back to proper tcp_skb_cb size check in tcp_init()

From: Lance Richardson <[email-masked]>
Date: Mon, 19 Sep 2016 10:37:06 -0400 (EDT)

> From: "Neal Cardwell" <[email-masked]>
> To: "David Miller" <[email-masked]>
> Cc: [email-masked], "Eric Dumazet" <[email-masked]>, "Soheil Hassas Yeganeh" <[email-masked]>, "Neal
> Cardwell" <[email-masked]>, "Yuchung Cheng" <[email-masked]>
> Sent: Sunday, September 18, 2016 6:03:42 PM
> Subject: [PATCH v3 net-next 05/16] tcp: switch back to proper tcp_skb_cb size check in tcp_init()
> 
> From: Eric Dumazet <[email-masked]>
> 
> Revert to the tcp_skb_cb size check that tcp_init() had before commit
> b4772ef879a8 ("net: use common macro for assering skb->cb[] available
> size in protocol families"). As related commit 744d5a3e9fe2 ("net:
> move skb->dropcount to skb->cb[]") explains, the
> sock_skb_cb_check_size() mechanism was added to ensure that there is
> space for dropcount, "for protocol families using it". But TCP is not
> a protocol using dropcount, so tcp_init() doesn't need to provision
> space for dropcount in the skb->cb[], and thus we can revert to the
> older form of the tcp_skb_cb size check. Doing so allows TCP to use 4
> more bytes of the skb->cb[] space.
> 
> Fixes: b4772ef879a8 ("net: use common macro for assering skb->cb[] available
> size in protocol families")
> Signed-off-by: Eric Dumazet <[email-masked]>
> Signed-off-by: Soheil Hassas Yeganeh <[email-masked]>
> Signed-off-by: Neal Cardwell <[email-masked]>
> Signed-off-by: Yuchung Cheng <[email-masked]>
> ---
>  net/ipv4/tcp.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 5b0b49c..53798e1 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -3244,11 +3244,12 @@ static void __init tcp_init_mem(void)
>  
>  void __init tcp_init(void)
>  {
> -	unsigned long limit;
>  	int max_rshare, max_wshare, cnt;
> +	unsigned long limit;
> +	struct sk_buff *skb;
>  	unsigned int i;
>  
> -	sock_skb_cb_check_size(sizeof(struct tcp_skb_cb));
> +	BUILD_BUG_ON(sizeof(struct tcp_skb_cb) > sizeof(skb->cb));
>  
The skb local variable could be avoided via:

        BUILD_BUG_ON(sizeof(struct tcp_skb_cb) > FIELD_SIZEOF(struct sk_buff, cb));

>  	percpu_counter_init(&tcp_sockets_allocated, 0, GFP_KERNEL);
>  	percpu_counter_init(&tcp_orphan_count, 0, GFP_KERNEL);
> --
> 2.8.0.rc3.226.g39d4020
> 
>