RE: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment
Get it.
Eric, thanks for your explaination. :-)
Jason, thank you! I will remember cc net...@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-kernel@vger.kernel.org; m...@redhat.com; Qinchuanyu;
Yewudi; liuyongan 00175866; net...@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.
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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.
--
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Re: [Question]Why a skb with frag_list causes BUG_ON in function skb_segment
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
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
