* Re: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment [not found] <60A73B7A161A82449A7DB7B382BFD1553712AF1C@SZXEMA504-MBS.china.huawei.com> @ 2015-07-30 4:26 ` Jason Wang 2015-07-30 5:01 ` Eric Dumazet 0 siblings, 1 reply; 4+ messages in thread From: Jason Wang @ 2015-07-30 4:26 UTC (permalink / raw) To: Zhangjie (HZ), linux-kernel@vger.kernel.org, mst@redhat.com Cc: Qinchuanyu, Yewudi, liuyongan 00175866, netdev@vger.kernel.org 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? > > > > > > Thanks > > Zhangjie > ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment 2015-07-30 4:26 ` [Question]Why a skb with frag_list causes BUG_ON in function skb_segment Jason Wang @ 2015-07-30 5:01 ` Eric Dumazet 2015-07-30 5:56 ` Zhangjie (HZ) 0 siblings, 1 reply; 4+ messages in thread From: Eric Dumazet @ 2015-07-30 5:01 UTC (permalink / raw) To: Jason Wang Cc: Zhangjie (HZ), linux-kernel@vger.kernel.org, mst@redhat.com, Qinchuanyu, Yewudi, liuyongan 00175866, netdev@vger.kernel.org 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. ^ permalink raw reply [flat|nested] 4+ messages in thread
* RE: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment 2015-07-30 5:01 ` Eric Dumazet @ 2015-07-30 5:56 ` Zhangjie (HZ) 0 siblings, 0 replies; 4+ messages in thread From: Zhangjie (HZ) @ 2015-07-30 5:56 UTC (permalink / raw) To: Eric Dumazet, Jason Wang Cc: linux-kernel@vger.kernel.org, mst@redhat.com, Qinchuanyu, Yewudi, liuyongan 00175866, netdev@vger.kernel.org [-- Warning: decoded text below may be mangled, UTF-8 assumed --] [-- Attachment #1: Type: text/plain; charset="utf-8", Size: 1896 bytes --] 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.dumazet@gmail.com] Sent: Thursday, July 30, 2015 1:01 PM To: Jason Wang Cc: Zhangjie (HZ); linux-kernel@vger.kernel.org; mst@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. ÿôèº{.nÇ+‰·Ÿ®‰†+%ŠËÿ±éݶ\x17¥Šwÿº{.nÇ+‰·¥Š{±þG«éÿŠ{ayº\x1dʇڙë,j\a¢f£¢·hšïêÿ‘êçz_è®\x03(階ŠÝ¢j"ú\x1a¶^[m§ÿÿ¾\a«þG«éÿ¢¸?™¨èÚ&£ø§~á¶iO•æ¬z·švØ^\x14\x04\x1a¶^[m§ÿÿÃ\fÿ¶ìÿ¢¸?–I¥ ^ permalink raw reply [flat|nested] 4+ messages in thread
* RE: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment @ 2015-07-30 5:56 ` Zhangjie (HZ) 0 siblings, 0 replies; 4+ messages in thread From: Zhangjie (HZ) @ 2015-07-30 5:56 UTC (permalink / raw) To: Eric Dumazet, Jason Wang Cc: linux-kernel@vger.kernel.org, mst@redhat.com, Qinchuanyu, Yewudi, liuyongan 00175866, netdev@vger.kernel.org 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.dumazet@gmail.com] Sent: Thursday, July 30, 2015 1:01 PM To: Jason Wang Cc: Zhangjie (HZ); linux-kernel@vger.kernel.org; mst@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. ^ permalink raw reply [flat|nested] 4+ messages in thread
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2015-07-30 4:26 ` [Question]Why a skb with frag_list causes BUG_ON in function skb_segment Jason Wang
2015-07-30 5:01 ` Eric Dumazet
2015-07-30 5:56 ` Zhangjie (HZ)
2015-07-30 5:56 ` Zhangjie (HZ)
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