Re: [PATCH net-next V6 2/3] net/mlx5e: Avoid copying payload to the skb's linear part

[Amery Hung]

On Thu, May 7, 2026 at 4:50 PM Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
>
>
> Hi Amery,
>
> On 07.05.26 15:53, Amery Hung wrote:
> > [...]
> > Am I understanding correctly that the better performance comes with
> > the assumption that the XDP does not change headers?
> >
> > headlen is determined before the XDP program runs. If it push/pop
> > headers, there could be headers in frags or data in the linear region
> > after __pskb_pull_tail().
> >
> That's right.
>
> >>                         if (__test_and_clear_bit(MLX5E_RQ_FLAG_XDP_XMIT, rq->flags)) {
> >>                                 struct mlx5e_frag_page *pfp;
> >> @@ -2060,8 +2066,7 @@ mlx5e_skb_from_cqe_mpwrq_nonlinear(struct mlx5e_rq *rq, struct mlx5e_mpw_info *w
> >>                                 pagep->frags++;
> >>                         while (++pagep < frag_page);
> >>
> >> -                       headlen = min_t(u16, MLX5E_RX_MAX_HEAD - len,
> >> -                                       skb->data_len);
> >> +                       headlen = min_t(u16, headlen - len, skb->data_len);
> >
> > headlen - len can underflow but will be capped by skb->data_len, so
> > this should be okay, right?
> It is safe. But it might trigger an extra allocation in the pull when
> len > headlen. We could also skip the pull in that case. Or do a
> min(headlen - len, min(skb->data_len, MLX5E_RX_MAX_HEAD)). WDYT?

Make sense, but this line took me a bit to understand. Maybe consider
checking len < headlen first?

if (len < headlen) {
        headlen = min_t(u32, headlen - len, skb->data_len);
        __pskb_pull_tail(skb, headlen);
}

Another clarifying question. So this patch will improve the
performance when the XDP programs don't change header length. For
those that encap/decap, they should precisely pull only headers into
the linear area for optimal performance. Is it correct?

>
> Thanks,
> Dragos
>