[PATCH v2 net-next] tcp: improve inet6_ehashfn() entropy

[Eric Dumazet <edumazet@google.com>]

Instead of only using the 32 low order bits of the local address,
use all of them.

Xor net_hash_mix(net) with the 32 high order bits of the local address
so that we can use __jhash_mix() three times.

If we were hashing 4 extra bytes, we would need one __jhash_final()
which is a bit expensive.

Using net_hash_mix() at the beginning allows beter register allocation.

We no longer use a cascade of two jhash and inet6_ehash_secret,
this was dubious/weak.

Add a comment explaining why @lport is not part of the jhash computation.

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux.1
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-24 (-24)
Function                                     old     new   delta
inet6_ehashfn                                330     306     -24
Total: Before=24855958, After=24855934, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
v2: do not include @lport in the jhash computation.
    Add a comment explaining why we need @lport being added.

 net/ipv6/inet6_hashtables.c | 40 ++++++++++++++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/inet6_hashtables.c b/net/ipv6/inet6_hashtables.c
index 72bc68fef48abc2c423c688b47154256828555e4..109fbf620ad7cde54300b937db58ada0d5c80664 100644
--- a/net/ipv6/inet6_hashtables.c
+++ b/net/ipv6/inet6_hashtables.c
@@ -35,13 +35,43 @@ u32 inet6_ehashfn(const struct net *net,
 		  const struct in6_addr *laddr, const u16 lport,
 		  const struct in6_addr *faddr, const __be16 fport)
 {
-	u32 lhash, fhash;
+	u32 a, b, c;
+
+	/*
+	 * Please look at jhash() implementation for reference.
+	 * Hash laddr + faddr + lport/fport + net_hash_mix.
+	 * Notes:
+	 * We combine laddr[0] (high order 32 bits of local address)
+	 * with net_hash_mix() to avoid using __jhash_final(a, b, c).
+	 *
+	 * We do not include JHASH_INITVAL + 36 contribution
+	 * to initial values of a, b, c.
+	 */
+
+	a = b = c = tcp_ipv6_hash_secret;
+
+	a += (__force u32)laddr->s6_addr32[0] ^ net_hash_mix(net);
+	b += (__force u32)laddr->s6_addr32[1];
+	c += (__force u32)laddr->s6_addr32[2];
+	__jhash_mix(a, b, c);
 
-	lhash = (__force u32)laddr->s6_addr32[3];
-	fhash = __ipv6_addr_jhash(faddr, tcp_ipv6_hash_secret);
+	a += (__force u32)laddr->s6_addr32[3];
+	b += (__force u32)faddr->s6_addr32[0];
+	c += (__force u32)faddr->s6_addr32[1];
+	__jhash_mix(a, b, c);
 
-	return lport + __inet6_ehashfn(lhash, 0, fhash, fport,
-				       inet6_ehash_secret + net_hash_mix(net));
+	a += (__force u32)faddr->s6_addr32[2];
+	b += (__force u32)faddr->s6_addr32[3];
+	c += (__force u32)fport;
+	__jhash_mix(a, b, c);
+
+	/* Note: We need to add @lport instead of fully hashing it.
+	 * See commits 9544d60a2605 ("inet: change lport contribution
+	 * to inet_ehashfn() and inet6_ehashfn()") and d4438ce68bf1
+	 * ("inet: call inet6_ehashfn() once from inet6_hash_connect()")
+	 * for references.
+	 */
+	return lport + c;
 }
 EXPORT_SYMBOL_GPL(inet6_ehashfn);
 
-- 
2.53.0.880.g73c4285caa-goog