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

From: Jason Wang <[email-masked]>
Date: Fri, 1 Apr 2016 10:13:34 +0800

On 03/31/2016 06:32 PM, Eric Dumazet wrote:
> 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 <[email-masked]>
>> ---
>>  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 ?
>

The problem is we want to support busy polling for tun. This needs
napi_id to be passed to tun socket by sk_mark_napi_id() during
tun_net_xmit(). But before reaching this, XPS will set sender_cpu will
make us can't see correct napi_id.