[SRU][J][PATCH] netfilter: conntrack: adopt safer max chain length

[Khalid Elmously]

From: Eric Dumazet <edumazet at google.com>

BugLink: https://bugs.launchpad.net/bug/2011616

Customers using GKE 1.25 and 1.26 are facing conntrack issues
root caused to commit c9c3b6811f74 ("netfilter: conntrack: make
max chain length random").

Even if we assume Uniform Hashing, a bucket often reachs 8 chained
items while the load factor of the hash table is smaller than 0.5

With a limit of 16, we reach load factors of 3.
With a limit of 32, we reach load factors of 11.
With a limit of 40, we reach load factors of 15.
With a limit of 50, we reach load factors of 24.

This patch changes MIN_CHAINLEN to 50, to minimize risks.

Ideally, we could in the future add a cushion based on expected
load factor (2 * nf_conntrack_max / nf_conntrack_buckets),
because some setups might expect unusual values.

Fixes: c9c3b6811f74 ("netfilter: conntrack: make max chain length random")
Signed-off-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: Pablo Neira Ayuso <pablo at netfilter.org>
Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
---
 net/netfilter/nf_conntrack_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 43ea8cfd374bb..7ff0da5f998a0 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -96,8 +96,8 @@ static DEFINE_MUTEX(nf_conntrack_mutex);
 #define GC_SCAN_MAX_DURATION	msecs_to_jiffies(10)
 #define GC_SCAN_EXPIRED_MAX	(64000u / HZ)
 
-#define MIN_CHAINLEN	8u
-#define MAX_CHAINLEN	(32u - MIN_CHAINLEN)
+#define MIN_CHAINLEN	50u
+#define MAX_CHAINLEN	(80u - MIN_CHAINLEN)
 
 static struct conntrack_gc_work conntrack_gc_work;
 
-- 
2.34.1