linux-kernel - [PATCH net 2/2] net: ioam6: mitigate the two reallocations problem

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Date: Tue,  2 Jul 2024 19:44:51 +0200
From: Justin Iurman <justin.iurman@...ege.be>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	linux-kernel@...r.kernel.org,
	justin.iurman@...ege.be
Subject: [PATCH net 2/2] net: ioam6: mitigate the two reallocations problem

Get the cache _before_ adding bytes. This way, we provide the dst entry
to skb_cow_head(), so that we call LL_RESERVED_SPACE() on it and avoid
two reallocations in some specific cases. We cannot do much when the dst
entry is empty (cache is empty, this is the first time): in that case,
we use skb->mac_len by default and two reallocations will happen in
those specific cases. However, it will only happen once, not every
single time.

Fixes: 8cb3bf8bff3c ("ipv6: ioam: Add support for the ip6ip6 encapsulation")
Signed-off-by: Justin Iurman <justin.iurman@...ege.be>
---
 net/ipv6/ioam6_iptunnel.c | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/net/ipv6/ioam6_iptunnel.c b/net/ipv6/ioam6_iptunnel.c
index b08c13550144..e5a7e7472b71 100644
--- a/net/ipv6/ioam6_iptunnel.c
+++ b/net/ipv6/ioam6_iptunnel.c
@@ -220,14 +220,16 @@ 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 *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 ? skb->mac_len
+					       : LL_RESERVED_SPACE(dst->dev)));
 	if (unlikely(err))
 		return err;
 
@@ -256,16 +258,17 @@ static int ioam6_do_inline(struct net *net, struct sk_buff *skb,
 
 static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
 			  struct ioam6_lwt_encap *tuninfo,
-			  struct in6_addr *tundst)
+			  struct in6_addr *tundst,
+			  struct dst_entry *dst)
 {
-	struct dst_entry *dst = skb_dst(skb);
 	struct ipv6hdr *hdr, *inner_hdr;
 	int hdrlen, len, err;
 
 	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 ? skb->mac_len
+					    : LL_RESERVED_SPACE(dst->dev)));
 	if (unlikely(err))
 		return err;
 
@@ -285,7 +288,7 @@ static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
 	hdr->nexthdr = NEXTHDR_HOP;
 	hdr->payload_len = cpu_to_be16(skb->len - sizeof(*hdr));
 	hdr->daddr = *tundst;
-	ipv6_dev_get_saddr(net, dst->dev, &hdr->daddr,
+	ipv6_dev_get_saddr(net, skb_dst(skb)->dev, &hdr->daddr,
 			   IPV6_PREFER_SRC_PUBLIC, &hdr->saddr);
 
 	skb_postpush_rcsum(skb, hdr, len);
@@ -313,6 +316,10 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 	orig_daddr = ipv6_hdr(skb)->daddr;
 
+	local_bh_disable();
+	dst = dst_cache_get(&ilwt->cache);
+	local_bh_enable();
+
 	switch (ilwt->mode) {
 	case IOAM6_IPTUNNEL_MODE_INLINE:
 do_inline:
@@ -320,7 +327,7 @@ 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, dst);
 		if (unlikely(err))
 			goto drop;
 
@@ -328,7 +335,8 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 	case IOAM6_IPTUNNEL_MODE_ENCAP:
 do_encap:
 		/* Encapsulation (ip6ip6) */
-		err = ioam6_do_encap(net, skb, &ilwt->tuninfo, &ilwt->tundst);
+		err = ioam6_do_encap(net, skb,
+				     &ilwt->tuninfo, &ilwt->tundst, dst);
 		if (unlikely(err))
 			goto drop;
 
@@ -346,10 +354,6 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		goto drop;
 	}
 
-	local_bh_disable();
-	dst = dst_cache_get(&ilwt->cache);
-	local_bh_enable();
-
 	if (unlikely(!dst)) {
 		struct ipv6hdr *hdr = ipv6_hdr(skb);
 		struct flowi6 fl6;
@@ -371,15 +375,15 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		local_bh_disable();
 		dst_cache_set_ip6(&ilwt->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;
-
 	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr))
 		return dst_output(net, sk, skb);
 out:
-- 
2.34.1