ipv6: remove max_size check inline with ipv4

In ip6_dst_gc() replace:

  if (entries > gc_thresh)

With:

  if (entries > ops->gc_thresh)

Sending Ipv6 packets in a loop via a raw socket triggers an issue where a
route is cloned by ip6_rt_cache_alloc() for each packet sent. This quickly
consumes the Ipv6 max_size threshold which defaults to 4096 resulting in
these warnings:

[1]   99.187805] dst_alloc: 7728 callbacks suppressed
[2] Route cache is full: consider increasing sysctl net.ipv6.route.max_size.
.
.
[300] Route cache is full: consider increasing sysctl net.ipv6.route.max_size.

When this happens the packet is dropped and sendto() gets a network is
unreachable error:

remaining pkt 200557 errno 101
remaining pkt 196462 errno 101
.
.
remaining pkt 126821 errno 101

Implement David Aherns suggestion to remove max_size check seeing that Ipv6
has a GC to manage memory usage. Ipv4 already does not check max_size.

Here are some memory comparisons for Ipv4 vs Ipv6 with the patch:

Test by running 5 instances of a program that sends UDP packets to a raw
socket 5000000 times. Compare Ipv4 and Ipv6 performance with a similar
program.

Ipv4:

Before test:

MemFree:        29427108 kB
Slab:             237612 kB

ip6_dst_cache       1912   2528    256   32    2 : tunables    0    0    0
xfrm_dst_cache         0      0    320   25    2 : tunables    0    0    0
ip_dst_cache        2881   3990    192   42    2 : tunables    0    0    0

During test:

MemFree:        29417608 kB
Slab:             247712 kB

ip6_dst_cache       1912   2528    256   32    2 : tunables    0    0    0
xfrm_dst_cache         0      0    320   25    2 : tunables    0    0    0
ip_dst_cache       44394  44394    192   42    2 : tunables    0    0    0

After test:

MemFree:        29422308 kB
Slab:             238104 kB

ip6_dst_cache       1912   2528    256   32    2 : tunables    0    0    0
xfrm_dst_cache         0      0    320   25    2 : tunables    0    0    0
ip_dst_cache        3048   4116    192   42    2 : tunables    0    0    0

Ipv6 with patch:

Errno 101 errors are not observed anymore with the patch.

Before test:

MemFree:        29422308 kB
Slab:             238104 kB

ip6_dst_cache       1912   2528    256   32    2 : tunables    0    0    0
xfrm_dst_cache         0      0    320   25    2 : tunables    0    0    0
ip_dst_cache        3048   4116    192   42    2 : tunables    0    0    0

During Test:

MemFree:        29431516 kB
Slab:             240940 kB

ip6_dst_cache      11980  12064    256   32    2 : tunables    0    0    0
xfrm_dst_cache         0      0    320   25    2 : tunables    0    0    0
ip_dst_cache        3048   4116    192   42    2 : tunables    0    0    0

After Test:

MemFree:        29441816 kB
Slab:             238132 kB

ip6_dst_cache       1902   2432    256   32    2 : tunables    0    0    0
xfrm_dst_cache         0      0    320   25    2 : tunables    0    0    0
ip_dst_cache        3048   4116    192   42    2 : tunables    0    0    0

Tested-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: Jon Maxwell <jmaxwell37@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230112012532.311021-1-jmaxwell37@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Jon Maxwell

include/net/dst_ops.h

@@ -16,7 +16,7 @@ struct dst_ops {
 	unsigned short		family;
 	unsigned int		gc_thresh;
 
-	int			(*gc)(struct dst_ops *ops);
+	void			(*gc)(struct dst_ops *ops);
 	struct dst_entry *	(*check)(struct dst_entry *, __u32 cookie);
 	unsigned int		(*default_advmss)(const struct dst_entry *);
 	unsigned int		(*mtu)(const struct dst_entry *);

net/core/dst.c

@@ -82,12 +82,8 @@ void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
 
 	if (ops->gc &&
 	    !(flags & DST_NOCOUNT) &&
-	    dst_entries_get_fast(ops) > ops->gc_thresh) {
-		if (ops->gc(ops)) {
-			pr_notice_ratelimited("Route cache is full: consider increasing sysctl net.ipv6.route.max_size.\n");
-			return NULL;
-		}
-	}
+	    dst_entries_get_fast(ops) > ops->gc_thresh)
+		ops->gc(ops);
 
 	dst = kmem_cache_alloc(ops->kmem_cachep, GFP_ATOMIC);
 	if (!dst)

net/ipv6/route.c

@@ -91,7 +91,7 @@ static struct dst_entry *ip6_negative_advice(struct dst_entry *);
 static void		ip6_dst_destroy(struct dst_entry *);
 static void		ip6_dst_ifdown(struct dst_entry *,
 				       struct net_device *dev, int how);
-static int		 ip6_dst_gc(struct dst_ops *ops);
+static void		 ip6_dst_gc(struct dst_ops *ops);
 
 static int		ip6_pkt_discard(struct sk_buff *skb);
 static int		ip6_pkt_discard_out(struct net *net, struct sock *sk, struct sk_buff *skb);
@@ -3284,23 +3284,21 @@ struct dst_entry *icmp6_dst_alloc(struct net_device *dev,
 	return dst;
 }
 
-static int ip6_dst_gc(struct dst_ops *ops)
+static void ip6_dst_gc(struct dst_ops *ops)
 {
 	struct net *net = container_of(ops, struct net, ipv6.ip6_dst_ops);
 	int rt_min_interval = net->ipv6.sysctl.ip6_rt_gc_min_interval;
-	int rt_max_size = net->ipv6.sysctl.ip6_rt_max_size;
 	int rt_elasticity = net->ipv6.sysctl.ip6_rt_gc_elasticity;
 	int rt_gc_timeout = net->ipv6.sysctl.ip6_rt_gc_timeout;
 	unsigned long rt_last_gc = net->ipv6.ip6_rt_last_gc;
 	unsigned int val;
 	int entries;
 
 	entries = dst_entries_get_fast(ops);
-	if (entries > rt_max_size)
+	if (entries > ops->gc_thresh)
 		entries = dst_entries_get_slow(ops);
 
-	if (time_after(rt_last_gc + rt_min_interval, jiffies) &&
-	    entries <= rt_max_size)
+	if (time_after(rt_last_gc + rt_min_interval, jiffies))
 		goto out;
 
 	fib6_run_gc(atomic_inc_return(&net->ipv6.ip6_rt_gc_expire), net, true);
@@ -3310,7 +3308,6 @@ static int ip6_dst_gc(struct dst_ops *ops)
 out:
 	val = atomic_read(&net->ipv6.ip6_rt_gc_expire);
 	atomic_set(&net->ipv6.ip6_rt_gc_expire, val - (val >> rt_elasticity));
-	return entries > rt_max_size;
 }
 
 static int ip6_nh_lookup_table(struct net *net, struct fib6_config *cfg,
@@ -6512,7 +6509,7 @@ static int __net_init ip6_route_net_init(struct net *net)
 #endif
 
 	net->ipv6.sysctl.flush_delay = 0;
-	net->ipv6.sysctl.ip6_rt_max_size = 4096;
+	net->ipv6.sysctl.ip6_rt_max_size = INT_MAX;
 	net->ipv6.sysctl.ip6_rt_gc_min_interval = HZ / 2;
 	net->ipv6.sysctl.ip6_rt_gc_timeout = 60*HZ;
 	net->ipv6.sysctl.ip6_rt_gc_interval = 30*HZ;