linux-kernel - Re: [PATCH net v3 1/2] net: tcp: lookup the best matched listen socket

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Message-ID: <CAL+tcoA9Lvc4Cj9zjWVx1FzEQA=d=OnvZRDWA4nE_1GNbEDaRw@mail.gmail.com>
Date: Sat, 2 Aug 2025 21:05:36 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: edumazet@...gle.com, kuniyu@...gle.com, ncardwell@...gle.com, 
	davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com, 
	horms@...nel.org, shuah@...nel.org, kraig@...gle.com, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net v3 1/2] net: tcp: lookup the best matched listen socket

Hi Menglong,

On Sat, Aug 2, 2025 at 5:28 PM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> For now, the tcp socket lookup will terminate if the socket is reuse port
> in inet_lhash2_lookup(), which makes the socket is not the best match.
>
> For example, we have socket1 and socket2 both listen on "0.0.0.0:1234",
> but socket1 bind on "eth0". We create socket1 first, and then socket2.
> Then, all connections will goto socket2, which is not expected, as socket1
> has higher priority.
>
> This can cause unexpected behavior if TCP MD5 keys is used, as described
> in Documentation/networking/vrf.rst -> Applications.
>
> Therefor, we lookup the best matched socket first, and then do the reuse

s/Therefor/Therefore

> port logic. This can increase some overhead if there are many reuse port
> socket :/
>
> Fixes: c125e80b8868 ("soreuseport: fast reuseport TCP socket selection")
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
> v3:
> * use the approach in V1
> * add the Fixes tag
> ---
>  net/ipv4/inet_hashtables.c  | 13 +++++++------
>  net/ipv6/inet6_hashtables.c | 13 +++++++------
>  2 files changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index ceeeec9b7290..51751337f394 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -389,17 +389,18 @@ static struct sock *inet_lhash2_lookup(const struct net *net,
>         sk_nulls_for_each_rcu(sk, node, &ilb2->nulls_head) {
>                 score = compute_score(sk, net, hnum, daddr, dif, sdif);
>                 if (score > hiscore) {
> -                       result = inet_lookup_reuseport(net, sk, skb, doff,
> -                                                      saddr, sport, daddr, hnum, inet_ehashfn);
> -                       if (result)
> -                               return result;
> -
>                         result = sk;
>                         hiscore = score;
>                 }
>         }
>
> -       return result;
> +       if (!result)
> +               return NULL;
> +
> +       sk = inet_lookup_reuseport(net, result, skb, doff,
> +                                  saddr, sport, daddr, hnum, inet_ehashfn);
> +
> +       return sk ? sk : result;
>  }

IMHO, I don't see it as a bugfix. So can you elaborate on what the exact
side effect you're faced with is when the algorithm finally prefers
socket2 (without
this patch)?

AFAIK, the current approach breaks the initial design and might make
the whole lookup process take a longer time in certain cases like you mentioned.

Thanks,
Jason