linux-kernel - Re: [PATCH v2] skbuff: skb_segment, Call zero copy functions before using skbuff frags

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Date:   Thu, 31 Aug 2023 09:43:14 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mohamed Khalfella <mkhalfella@...estorage.com>
Cc:     willemdebruijn.kernel@...il.com, alexanderduyck@...com,
        bpf@...r.kernel.org, brouer@...hat.com, davem@...emloft.net,
        dhowells@...hat.com, keescook@...omium.org, kuba@...nel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        pabeni@...hat.com, willemb@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH v2] skbuff: skb_segment, Call zero copy functions before
 using skbuff frags

On Thu, Aug 31, 2023 at 9:30 AM Mohamed Khalfella
<mkhalfella@...estorage.com> wrote:
>
> On 2023-08-31 08:58:51 +0200, Eric Dumazet wrote:
> > On Thu, Aug 31, 2023 at 1:28 AM Mohamed Khalfella
> > <mkhalfella@...estorage.com> wrote:
> > >         do {
> > >                 struct sk_buff *nskb;
> > >                 skb_frag_t *nskb_frag;
> > > @@ -4465,6 +4471,10 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
> > >                     (skb_headlen(list_skb) == len || sg)) {
> > >                         BUG_ON(skb_headlen(list_skb) > len);
> > >
> > > +                       nskb = skb_clone(list_skb, GFP_ATOMIC);
> > > +                       if (unlikely(!nskb))
> > > +                               goto err;
> > > +
> >
> > This patch is quite complex to review, so I am asking if this part was
> > really needed ?
>
> Unfortunately the patch is complex because I try to avoid calling
> skb_orphan_frags() in the middle of processing these frags. Otherwise
> it would be much harder to implement because as reallocated frags do not
> map 1:1 with existing frags as Willem mentioned.
>
> > <1>  : You moved here <2> and <3>
>
> <2> was moved here because skb_clone() calls skb_orphan_frags(). By

Oh right, I think we should amend skb_clone() documentation, it is
slightly wrong.

( I will take care of this change)

> moving this up we do not need to call skb_orphan_frags() for list_skb
> and we can start to use nr_frags and frags without worrying their value
> is going to change.
>
> <3> was moved here because <2> was moved here. Fail fast if we can not
> clone list_skb.
>
> >
> > If this is not strictly needed, please keep the code as is to ease
> > code review...
> >
> > >                         i = 0;
> > >                         nfrags = skb_shinfo(list_skb)->nr_frags;
> > >                         frag = skb_shinfo(list_skb)->frags;
> > > @@ -4483,12 +4493,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
> > >                                 frag++;
> > >                         }
> > >
> > > -                       nskb = skb_clone(list_skb, GFP_ATOMIC);
> >
> > <2>
> >
> > >                         list_skb = list_skb->next;
> > >
> > > -                       if (unlikely(!nskb))
> > > -                               goto err;
> > > -
> >
> > <3>
> >
> > >                         if (unlikely(pskb_trim(nskb, len))) {
> > >                                 kfree_skb(nskb);
> > >                                 goto err;
> > > @@ -4564,12 +4570,16 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
> > >                 skb_shinfo(nskb)->flags |= skb_shinfo(head_skb)->flags &
> > >                                            SKBFL_SHARED_FRAG;
> > >
> > > -               if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
> > > -                   skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
> > > +               if (skb_zerocopy_clone(nskb, list_skb, GFP_ATOMIC))
> >
> > Why using list_skb here instead of frag_skb ?
> > Again, I have to look at the whole thing to understand why you did this.
>
> Oops, this is a mistake. It should be frag_skb. Will fix it run the test
> one more time and post v3.