ipv6: elide flowlabel check if no exclusive leases exist

Processes can request ipv6 flowlabels with cmsg IPV6_FLOWINFO.
If not set, by default an autogenerated flowlabel is selected.

Explicit flowlabels require a control operation per label plus a
datapath check on every connection (every datagram if unconnected).
This is particularly expensive on unconnected sockets multiplexing
many flows, such as QUIC.

In the common case, where no lease is exclusive, the check can be
safely elided, as both lease request and check trivially succeed.
Indeed, autoflowlabel does the same even with exclusive leases.

Elide the check if no process has requested an exclusive lease.

fl6_sock_lookup previously returns either a reference to a lease or
NULL to denote failure. Modify to return a real error and update
all callers. On return NULL, they can use the label and will elide
the atomic_dec in fl6_sock_release.

This is an optimization. Robust applications still have to revert to
requesting leases if the fast path fails due to an exclusive lease.

Changes RFC->v1:
  - use static_key_false_deferred to rate limit jump label operations
    - call static_key_deferred_flush to stop timers on exit
  - move decrement out of RCU context
  - defer optimization also if opt data is associated with a lease
  - updated all fp6_sock_lookup callers, not just udp

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 23 insertions, 4 deletions

diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
index 545e339b8c4f..ad284b1fd308 100644
--- a/net/ipv6/ip6_flowlabel.c
+++ b/net/ipv6/ip6_flowlabel.c
@@ -17,6 +17,7 @@
 #include <linux/slab.h>
 #include <linux/export.h>
 #include <linux/pid_namespace.h>
+#include <linux/jump_label_ratelimit.h>
 
 #include <net/net_namespace.h>
 #include <net/sock.h>
@@ -53,6 +54,9 @@ static DEFINE_SPINLOCK(ip6_fl_lock);
 
 static DEFINE_SPINLOCK(ip6_sk_fl_lock);
 
+DEFINE_STATIC_KEY_DEFERRED_FALSE(ipv6_flowlabel_exclusive, HZ);
+EXPORT_SYMBOL(ipv6_flowlabel_exclusive);
+
 #define for_each_fl_rcu(hash, fl)				\
 	for (fl = rcu_dereference_bh(fl_ht[(hash)]);		\
 	     fl != NULL;					\
@@ -90,6 +94,13 @@ static struct ip6_flowlabel *fl_lookup(struct net *net, __be32 label)
 	return fl;
 }
 
+static bool fl_shared_exclusive(struct ip6_flowlabel *fl)
+{
+	return fl->share == IPV6_FL_S_EXCL ||
+	       fl->share == IPV6_FL_S_PROCESS ||
+	       fl->share == IPV6_FL_S_USER;
+}
+
 static void fl_free_rcu(struct rcu_head *head)
 {
 	struct ip6_flowlabel *fl = container_of(head, struct ip6_flowlabel, rcu);
@@ -103,8 +114,13 @@ static void fl_free_rcu(struct rcu_head *head)
 
 static void fl_free(struct ip6_flowlabel *fl)
 {
-	if (fl)
-		call_rcu(&fl->rcu, fl_free_rcu);
+	if (!fl)
+		return;
+
+	if (fl_shared_exclusive(fl) || fl->opt)
+		static_branch_slow_dec_deferred(&ipv6_flowlabel_exclusive);
+
+	call_rcu(&fl->rcu, fl_free_rcu);
 }
 
 static void fl_release(struct ip6_flowlabel *fl)
@@ -240,7 +256,7 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 
 /* Socket flowlabel lists */
 
-struct ip6_flowlabel *fl6_sock_lookup(struct sock *sk, __be32 label)
+struct ip6_flowlabel *__fl6_sock_lookup(struct sock *sk, __be32 label)
 {
 	struct ipv6_fl_socklist *sfl;
 	struct ipv6_pinfo *np = inet6_sk(sk);
@@ -260,7 +276,7 @@ struct ip6_flowlabel *fl6_sock_lookup(struct sock *sk, __be32 label)
 	rcu_read_unlock_bh();
 	return NULL;
 }
-EXPORT_SYMBOL_GPL(fl6_sock_lookup);
+EXPORT_SYMBOL_GPL(__fl6_sock_lookup);
 
 void fl6_free_socklist(struct sock *sk)
 {
@@ -419,6 +435,8 @@ fl_create(struct net *net, struct sock *sk, struct in6_flowlabel_req *freq,
 	}
 	fl->dst = freq->flr_dst;
 	atomic_set(&fl->users, 1);
+	if (fl_shared_exclusive(fl) || fl->opt)
+		static_branch_deferred_inc(&ipv6_flowlabel_exclusive);
 	switch (fl->share) {
 	case IPV6_FL_S_EXCL:
 	case IPV6_FL_S_ANY:
@@ -854,6 +872,7 @@ int ip6_flowlabel_init(void)
 
 void ip6_flowlabel_cleanup(void)
 {
+	static_key_deferred_flush(&ipv6_flowlabel_exclusive);
 	del_timer(&ip6_fl_gc_timer);
 	unregister_pernet_subsys(&ip6_flowlabel_net_ops);
 }