netdev - [PATCH net] netfilter: conntrack:

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Date:   Tue,  7 Mar 2023 05:22:54 +0000
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jozsef Kadlecsik <kadlec@...filter.org>
Cc:     netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, eric.dumazet@...il.com,
        Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net] netfilter: conntrack:

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@...gle.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 7250082e7de56c77298b0d3b62c4f0dff95b77cc..c6a6a6099b4e2200951367ccd90c12c0ac800a6a 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;
 
-- 
2.40.0.rc0.216.gc4246ad0f0-goog