NACK/Cmnt: [SRU][v2][J][PATCH] netfilter: conntrack: adopt safer max chain length

[Stefan Bader]

On 14.03.23 19:21, Khalid Elmously wrote:
> From: Eric Dumazet <edumazet at google.com>
> 
> BugLink: https://bugs.launchpad.net/bug/2011616

The bug reference leads nowhere for me. But also, this change appears to 
be a v6.3-rc2. So it should not _only_ be applied to Jammy but also to 
Kinetic, Lunar. Maybe OEM kernels as well but I admit its hard to keep 
track of them.

-Stefan

> 
> 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>
> (cherry picked from commit c77737b736ceb50fdf150434347dbd81ec76dbb1)
> 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;
>   

-- 
- Stefan

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