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
>
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.
Get it.
Eric, thanks for your explaination. :-)
Jason, thank you! I will remember cc [email protected] next time.
-----Original Message-----
From: Eric Dumazet [mailto:[email protected]]
Sent: Thursday, July 30, 2015 1:01 PM
To: Jason Wang
Cc: Zhangjie (HZ); [email protected]; [email protected]; Qinchuanyu; Yewudi; liuyongan 00175866; [email protected]
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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