net: ipv6: ioam6_iptunnel: mitigate 2-realloc issue

This patch mitigates the two-reallocations issue with ioam6_iptunnel by
providing the dst_entry (in the cache) to the first call to
skb_cow_head(). As a result, the very first iteration may 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
the improvement:
- inline mode:
  - before: https://ibb.co/LhQ8V63
  - after: https://ibb.co/x5YT2bS
- encap mode:
  - before: https://ibb.co/3Cjm5m0
  - after: https://ibb.co/TwpsxTC
- encap mode with tunsrc:
  - before: https://ibb.co/Gpy9QPg
  - after: https://ibb.co/PW1bZFT

This patch also fixes an incorrect behavior: after the insertion, the
second call to skb_cow_head() makes sure that the dev has enough
headroom in the skb for layer 2 and stuff. In that case, the "old"
dst_entry was used, which is now fixed. After discussing with Paolo, it
appears that both patches can be merged into a single one -this one-
(for the sake of readability) and target net-next.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv6/ioam6_iptunnel.c

@@ -253,14 +253,15 @@ static int ioam6_do_fill(struct net *net, struct sk_buff *skb)
 }
 
 static int ioam6_do_inline(struct net *net, struct sk_buff *skb,
-			   struct ioam6_lwt_encap *tuninfo)
+			   struct ioam6_lwt_encap *tuninfo,
+			   struct dst_entry *cache_dst)
 {
 	struct ipv6hdr *oldhdr, *hdr;
 	int hdrlen, err;
 
 	hdrlen = (tuninfo->eh.hdrlen + 1) << 3;
 
-	err = skb_cow_head(skb, hdrlen + skb->mac_len);
+	err = skb_cow_head(skb, hdrlen + dst_dev_overhead(cache_dst, skb));
 	if (unlikely(err))
 		return err;
 
@@ -291,7 +292,8 @@ static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
 			  struct ioam6_lwt_encap *tuninfo,
 			  bool has_tunsrc,
 			  struct in6_addr *tunsrc,
-			  struct in6_addr *tundst)
+			  struct in6_addr *tundst,
+			  struct dst_entry *cache_dst)
 {
 	struct dst_entry *dst = skb_dst(skb);
 	struct ipv6hdr *hdr, *inner_hdr;
@@ -300,7 +302,7 @@ static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
 	hdrlen = (tuninfo->eh.hdrlen + 1) << 3;
 	len = sizeof(*hdr) + hdrlen;
 
-	err = skb_cow_head(skb, len + skb->mac_len);
+	err = skb_cow_head(skb, len + dst_dev_overhead(cache_dst, skb));
 	if (unlikely(err))
 		return err;
 
@@ -334,7 +336,7 @@ static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
 
 static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
-	struct dst_entry *dst = skb_dst(skb);
+	struct dst_entry *dst = skb_dst(skb), *cache_dst;
 	struct in6_addr orig_daddr;
 	struct ioam6_lwt *ilwt;
 	int err = -EINVAL;
@@ -352,6 +354,10 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 	orig_daddr = ipv6_hdr(skb)->daddr;
 
+	local_bh_disable();
+	cache_dst = dst_cache_get(&ilwt->cache);
+	local_bh_enable();
+
 	switch (ilwt->mode) {
 	case IOAM6_IPTUNNEL_MODE_INLINE:
 do_inline:
@@ -359,7 +365,7 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		if (ipv6_hdr(skb)->nexthdr == NEXTHDR_HOP)
 			goto out;
 
-		err = ioam6_do_inline(net, skb, &ilwt->tuninfo);
+		err = ioam6_do_inline(net, skb, &ilwt->tuninfo, cache_dst);
 		if (unlikely(err))
 			goto drop;
 
@@ -369,7 +375,7 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		/* Encapsulation (ip6ip6) */
 		err = ioam6_do_encap(net, skb, &ilwt->tuninfo,
 				     ilwt->has_tunsrc, &ilwt->tunsrc,
-				     &ilwt->tundst);
+				     &ilwt->tundst, cache_dst);
 		if (unlikely(err))
 			goto drop;
 
@@ -387,16 +393,7 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		goto drop;
 	}
 
-	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
+	if (unlikely(!cache_dst)) {
-	if (unlikely(err))
-		goto drop;
-
-	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr)) {
-		local_bh_disable();
-		dst = dst_cache_get(&ilwt->cache);
-		local_bh_enable();
-
-		if (unlikely(!dst)) {
 		struct ipv6hdr *hdr = ipv6_hdr(skb);
 		struct flowi6 fl6;
 
@@ -407,21 +404,25 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		fl6.flowi6_mark = skb->mark;
 		fl6.flowi6_proto = hdr->nexthdr;
 
-			dst = ip6_route_output(net, NULL, &fl6);
+		cache_dst = ip6_route_output(net, NULL, &fl6);
-			if (dst->error) {
+		if (cache_dst->error) {
-				err = dst->error;
+			err = cache_dst->error;
-				dst_release(dst);
+			dst_release(cache_dst);
 			goto drop;
 		}
 
 		local_bh_disable();
-			dst_cache_set_ip6(&ilwt->cache, dst, &fl6.saddr);
+		dst_cache_set_ip6(&ilwt->cache, cache_dst, &fl6.saddr);
 		local_bh_enable();
+
+		err = skb_cow_head(skb, LL_RESERVED_SPACE(cache_dst->dev));
+		if (unlikely(err))
+			goto drop;
 	}
 
+	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr)) {
 		skb_dst_drop(skb);
-		skb_dst_set(skb, dst);
+		skb_dst_set(skb, cache_dst);
-
 		return dst_output(net, sk, skb);
 	}
 out: