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

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Message-ID: <20230831072957.GA3696339@medusa>
Date:   Thu, 31 Aug 2023 00:29:57 -0700
From:   Mohamed Khalfella <mkhalfella@...estorage.com>
To:     Eric Dumazet <edumazet@...gle.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 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
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.