netdev - Re: [PATCH net-next 1/6] net: skbuff: don't use union for napi_id and sender

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Date:	Thu, 31 Mar 2016 03:32:21 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	davem@...emloft.net, mst@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/6] net: skbuff: don't use union for napi_id
 and sender_cpu

On Thu, 2016-03-31 at 13:50 +0800, Jason Wang wrote:
> We use a union for napi_id and send_cpu, this is ok for most of the
> cases except when we want to support busy polling for tun which needs
> napi_id to be stored and passed to socket during tun_net_xmit(). In
> this case, napi_id was overridden with sender_cpu before tun_net_xmit()
> was called if XPS was enabled. Fixing by not using union for napi_id
> and sender_cpu.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  include/linux/skbuff.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 15d0df9..8aee891 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -743,11 +743,11 @@ struct sk_buff {
>  	__u32			hash;
>  	__be16			vlan_proto;
>  	__u16			vlan_tci;
> -#if defined(CONFIG_NET_RX_BUSY_POLL) || defined(CONFIG_XPS)
> -	union {
> -		unsigned int	napi_id;
> -		unsigned int	sender_cpu;
> -	};
> +#if defined(CONFIG_NET_RX_BUSY_POLL)
> +	unsigned int		napi_id;
> +#endif
> +#if defined(CONFIG_XPS)
> +	unsigned int		sender_cpu;
>  #endif
>  	union {
>  #ifdef CONFIG_NETWORK_SECMARK

Hmmm...

This is a serious problem.

Making skb bigger (8 bytes because of alignment) was not considered
valid for sender_cpu introduction. We worked quite hard to avoid this,
if you take a look at git history :(

Can you describe more precisely the problem and code path ?