Get it. Eric, thanks for your explaination. :-) Jason, thank you! I will remember cc netdev@vger.kernel.org next time.
-----Original Message----- From: Eric Dumazet [mailto:eric.duma...@gmail.com] Sent: Thursday, July 30, 2015 1:01 PM To: Jason Wang Cc: Zhangjie (HZ); linux-ker...@vger.kernel.org; m...@redhat.com; Qinchuanyu; Yewudi; liuyongan 00175866; netdev@vger.kernel.org Subject: Re: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment On Thu, 2015-07-30 at 12:26 +0800, Jason Wang wrote: > cc netdev for more experts > > On 07/28/2015 04:53 PM, Zhangjie (HZ) wrote: > > > > Hi, > > > > I generate a skb as follows: > > > > It has a linear data region, 17 frags and the last fragment is in > > skb_shinfo(skb)->frag_list. > > > > Before this skb is sent to driver, dev_hard_start_xmit() will > > segment it first(skb has frag_list, > > > > so we get true from netif_needs_gso()), then the skb is passed to > > function skb_segment(). > > > > Then, BUG_ON() happened. > > > > while (pos < offset + len) { > > > > if (i >= nfrags) { > > > > BUG_ON(skb_headlen(list_skb)); (skbuff.c:3120) > > > > … > > > > } > > > > … > > > > } > > > > A skb that has no frags but frag_list also causes BUG_ON(). > > > > I wonder if skb like follows is legal? Could skb in frag_list have > > linear data region? The answer is : skb_segment() is very complex but does not handle all possible cases. skb found in skb_shinfo(skb)->frag_list must not have anything in their skb->head. This would require very expensive logic and memory allocations and copies. Make sure you follow this rule in your driver, or even better leave this work to GRO engine. N�����r��y����b�X��ǧv�^�){.n�+���z�^�)����w* jg��� �����ݢj/���z�ޖ��2�ޙ����&�)ߡ�a���� �G���h��j:+v���w��٥
