From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757929AbcDACNl (ORCPT ); Thu, 31 Mar 2016 22:13:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44045 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752210AbcDACNk (ORCPT ); Thu, 31 Mar 2016 22:13:40 -0400 Subject: Re: [PATCH net-next 1/6] net: skbuff: don't use union for napi_id and sender_cpu To: Eric Dumazet References: <1459403439-6011-1-git-send-email-jasowang@redhat.com> <1459403439-6011-2-git-send-email-jasowang@redhat.com> <1459420341.6473.225.camel@edumazet-glaptop3.roam.corp.google.com> Cc: davem@davemloft.net, mst@redhat.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org From: Jason Wang Message-ID: <56FDD94E.8090908@redhat.com> Date: Fri, 1 Apr 2016 10:13:34 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <1459420341.6473.225.camel@edumazet-glaptop3.roam.corp.google.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >> --- >> 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.