netdev - [PATCH net-next 26/27] ipv6: improve opt-less __ip6_make

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Date:   Sun,  3 Apr 2022 14:06:38 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, Wei Liu <wei.liu@...nel.org>,
        Paul Durrant <paul@....org>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH net-next 26/27] ipv6: improve opt-less __ip6_make_skb()

We do a bit of a network header pointer shuffling in __ip6_make_skb()
expecting that ipv6_push_*frag_opts() might change the layout. Avoid it
with associated overhead when there are no opts.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 net/ipv6/ip6_output.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 3c37b07cbfae..f7c092af64f5 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1882,22 +1882,20 @@ struct sk_buff *__ip6_make_skb(struct sock *sk,
 
 	/* Allow local fragmentation. */
 	skb->ignore_df = ip6_sk_ignore_df(sk);
-	__skb_pull(skb, skb_network_header_len(skb));
-
 	final_dst = &fl6->daddr;
 	if (v6_cork->opt) {
 		struct ipv6_txoptions *opt = v6_cork->opt;
 
+		__skb_pull(skb, skb_network_header_len(skb));
 		if (opt->opt_flen)
 			ipv6_push_frag_opts(skb, opt, &proto);
 		if (opt->opt_nflen)
 			ipv6_push_nfrag_opts(skb, opt, &proto, &final_dst, &fl6->saddr);
+		skb_push(skb, sizeof(struct ipv6hdr));
+		skb_reset_network_header(skb);
 	}
 
-	skb_push(skb, sizeof(struct ipv6hdr));
-	skb_reset_network_header(skb);
 	hdr = ipv6_hdr(skb);
-
 	ip6_flow_hdr(hdr, v6_cork->tclass,
 		     ip6_make_flowlabel(net, skb, fl6->flowlabel,
 					ip6_autoflowlabel(net, np), fl6));
-- 
2.35.1