This patch mitigates the two-reallocations issue with rpl_iptunnel by
providing the dst_entry (in the cache) to the first call to
skb_cow_head(). As a result, the very first iteration would still
trigger two reallocations (i.e., empty cache), while next iterations
would only trigger a single reallocation.

Performance tests before/after applying this patch, which clearly shows
there is no impact (it even shows improvement):
- before: https://ibb.co/nQJhqwc
- after: https://ibb.co/4ZvW6wV

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Cc: Alexander Aring <aahringo@redhat.com>
---
 net/ipv6/rpl_iptunnel.c | 67 +++++++++++++++++++++++------------------
 1 file changed, 38 insertions(+), 29 deletions(-)

diff --git a/net/ipv6/rpl_iptunnel.c b/net/ipv6/rpl_iptunnel.c
index db3c19a42e1c..c518728460a2 100644
--- a/net/ipv6/rpl_iptunnel.c
+++ b/net/ipv6/rpl_iptunnel.c
@@ -124,8 +124,17 @@ static void rpl_destroy_state(struct lwtunnel_state *lwt)
 	dst_cache_destroy(&rpl_lwt_lwtunnel(lwt)->cache);
 }
 
+static inline int dev_overhead(struct dst_entry *dst, struct sk_buff *skb)
+{
+	if (likely(dst))
+		return LL_RESERVED_SPACE(dst->dev);
+
+	return skb->mac_len;
+}
+
 static int rpl_do_srh_inline(struct sk_buff *skb, const struct rpl_lwt *rlwt,
-			     const struct ipv6_rpl_sr_hdr *srh)
+			     const struct ipv6_rpl_sr_hdr *srh,
+			     struct dst_entry *dst)
 {
 	struct ipv6_rpl_sr_hdr *isrh, *csrh;
 	const struct ipv6hdr *oldhdr;
@@ -153,7 +162,7 @@ static int rpl_do_srh_inline(struct sk_buff *skb, const struct rpl_lwt *rlwt,
 
 	hdrlen = ((csrh->hdrlen + 1) << 3);
 
-	err = skb_cow_head(skb, hdrlen + skb->mac_len);
+	err = skb_cow_head(skb, hdrlen + dev_overhead(dst, skb));
 	if (unlikely(err)) {
 		kfree(buf);
 		return err;
@@ -186,36 +195,35 @@ static int rpl_do_srh_inline(struct sk_buff *skb, const struct rpl_lwt *rlwt,
 	return 0;
 }
 
-static int rpl_do_srh(struct sk_buff *skb, const struct rpl_lwt *rlwt)
+static int rpl_do_srh(struct sk_buff *skb, const struct rpl_lwt *rlwt,
+		      struct dst_entry *dst)
 {
-	struct dst_entry *dst = skb_dst(skb);
 	struct rpl_iptunnel_encap *tinfo;
 
 	if (skb->protocol != htons(ETH_P_IPV6))
 		return -EINVAL;
 
-	tinfo = rpl_encap_lwtunnel(dst->lwtstate);
+	tinfo = rpl_encap_lwtunnel(skb_dst(skb)->lwtstate);
 
-	return rpl_do_srh_inline(skb, rlwt, tinfo->srh);
+	return rpl_do_srh_inline(skb, rlwt, tinfo->srh, dst);
 }
 
 static int rpl_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
-	struct dst_entry *orig_dst = skb_dst(skb);
-	struct dst_entry *dst = NULL;
+	struct dst_entry *dst;
 	struct rpl_lwt *rlwt;
 	int err;
 
-	rlwt = rpl_lwt_lwtunnel(orig_dst->lwtstate);
-
-	err = rpl_do_srh(skb, rlwt);
-	if (unlikely(err))
-		goto drop;
+	rlwt = rpl_lwt_lwtunnel(skb_dst(skb)->lwtstate);
 
 	local_bh_disable();
 	dst = dst_cache_get(&rlwt->cache);
 	local_bh_enable();
 
+	err = rpl_do_srh(skb, rlwt, dst);
+	if (unlikely(err))
+		goto drop;
+
 	if (unlikely(!dst)) {
 		struct ipv6hdr *hdr = ipv6_hdr(skb);
 		struct flowi6 fl6;
@@ -237,15 +245,15 @@ static int rpl_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		local_bh_disable();
 		dst_cache_set_ip6(&rlwt->cache, dst, &fl6.saddr);
 		local_bh_enable();
+
+		err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
+		if (unlikely(err))
+			goto drop;
 	}
 
 	skb_dst_drop(skb);
 	skb_dst_set(skb, dst);
 
-	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
-	if (unlikely(err))
-		goto drop;
-
 	return dst_output(net, sk, skb);
 
 drop:
@@ -255,36 +263,37 @@ static int rpl_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 static int rpl_input(struct sk_buff *skb)
 {
-	struct dst_entry *orig_dst = skb_dst(skb);
-	struct dst_entry *dst = NULL;
+	struct dst_entry *dst;
 	struct rpl_lwt *rlwt;
 	int err;
 
-	rlwt = rpl_lwt_lwtunnel(orig_dst->lwtstate);
-
-	err = rpl_do_srh(skb, rlwt);
-	if (unlikely(err))
-		goto drop;
+	rlwt = rpl_lwt_lwtunnel(skb_dst(skb)->lwtstate);
 
 	local_bh_disable();
 	dst = dst_cache_get(&rlwt->cache);
+	local_bh_enable();
+
+	err = rpl_do_srh(skb, rlwt, dst);
+	if (unlikely(err))
+		goto drop;
 
 	if (!dst) {
 		ip6_route_input(skb);
 		dst = skb_dst(skb);
 		if (!dst->error) {
+			local_bh_disable();
 			dst_cache_set_ip6(&rlwt->cache, dst,
 					  &ipv6_hdr(skb)->saddr);
+			local_bh_enable();
 		}
+
+		err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
+		if (unlikely(err))
+			goto drop;
 	} else {
 		skb_dst_drop(skb);
 		skb_dst_set(skb, dst);
 	}
-	local_bh_enable();
-
-	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
-	if (unlikely(err))
-		goto drop;
 
 	return dst_input(skb);
 
-- 
2.34.1