Re: [PATCH net-next] seg6: reallocate the skb head on L2 encapsulation only when needed

[Yuya Kusakabe]

On Wed, Sep 2, 2026 at 11:00 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> This is a nice improvement for forwarded traffic, but locally
> generated TCP traffic
> is still hitting two expensive reallocations (second one in
> __seg6_do_srh_encap())
>
> Can we combine needed headrooms so that only a single re-alloc occurs?
Thanks for looking at this.  I will send a v2 with the combined
headroom.

One clarification on the locally generated case: seg6_build_state()
does not redirect the output path for the L2 modes, so locally
generated traffic never reaches this branch:

	newts->flags |= LWTUNNEL_STATE_INPUT_REDIRECT;

	if (tuninfo->mode != SEG6_IPTUN_MODE_L2ENCAP &&
	    tuninfo->mode != SEG6_IPTUN_MODE_L2ENCAP_RED)
		newts->flags |= LWTUNNEL_STATE_OUTPUT_REDIRECT;

Two reallocations do happen on the forwarded path, though: with a
header-cloned skb whose headroom is below the total, asking only for
skb->mac_len leaves the second reallocation to __seg6_do_srh_encap(),
because the cow that unclones the skb does not also make room for the
outer header.  So the combined headroom is the right shape either way.