[RFC PATCH net-next 2/4] ipv6: make inet6_addr_lst hash table size configurable

[hawk@kernel.org]

From: Jesper Dangaard Brouer <hawk@kernel.org>

Similar to the IPv4 inet_addr_lst hash table, IPv6 also uses a fixed
256-bucket hash table for address lookups. On hosts with many IPv6
addresses, this results in long hash chains walked under RCU.

The inet6_addr_lst table is used in the route lookup path when no
output interface is specified (oif == 0):

  udpv6_sendmsg
    ip6_sk_dst_lookup_flow
      ip6_dst_lookup_tail
        ip6_route_output -> fib6 lookup
          __rt6_device_match
            ipv6_chk_addr              <-- walks inet6_addr_lst

Add CONFIG_INET6_ADDR_HASH_BUCKETS (default 256, range 64-16384, EXPERT)
so hosts with many addresses can size the table appropriately. The value
is rounded up to the nearest power of 2 at compile time via
order_base_2(). Memory cost is one hlist_head pointer per bucket per net
namespace.

Reported-by: Ivan Babrou <ivan@cloudflare.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
---
 net/ipv6/Kconfig    | 15 +++++++++++++++
 net/ipv6/addrconf.c |  2 +-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/Kconfig b/net/ipv6/Kconfig
index b8f9a8c0302e..963fa0e1a014 100644
--- a/net/ipv6/Kconfig
+++ b/net/ipv6/Kconfig
@@ -238,6 +238,21 @@ config IPV6_FOU_TUNNEL
 	default NET_FOU_IP_TUNNELS && IPV6_FOU
 	select IPV6_TUNNEL
 
+config INET6_ADDR_HASH_BUCKETS
+	int "IPv6 address hash table size" if EXPERT
+	range 64 16384
+	default 256
+	help
+	  Number of hash buckets for looking up local IPv6 addresses,
+	  e.g. in ipv6_chk_addr() and related callers.  Rounded up to
+	  the nearest power of 2.
+
+	  Hosts with many IPv6 addresses benefit from a larger table to
+	  reduce hash chain lengths.
+
+	  The default of 256 is fine for most systems.  A value of 1024
+	  suits hosts with ~500+ addresses.
+
 config IPV6_MULTIPLE_TABLES
 	bool "IPv6: Multiple Routing Tables"
 	select FIB_RULES
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 0e55f139e05d..dad08f0733ef 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -144,7 +144,7 @@ static int ipv6_generate_stable_address(struct in6_addr *addr,
 					u8 dad_count,
 					const struct inet6_dev *idev);
 
-#define IN6_ADDR_HSIZE_SHIFT	8
+#define IN6_ADDR_HSIZE_SHIFT	order_base_2(CONFIG_INET6_ADDR_HASH_BUCKETS)
 #define IN6_ADDR_HSIZE		(1 << IN6_ADDR_HSIZE_SHIFT)
 
 static void addrconf_verify(struct net *net);
-- 
2.43.0