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

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Message-ID: <CADxym3bD17eL0U0sQkuSZZgNtg7gmvzt0YAA+CiHf9Lw5-+awA@mail.gmail.com>
Date: Sun, 3 Aug 2025 09:59:35 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Jason Xing <kerneljasonxing@...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

On Sat, Aug 2, 2025 at 9:06 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> 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)?

Hi, Jason. The case is that the user has several NIC,
and there are some sockets that are binded to them,
who listen on TCP port 6666. And a global socket doesn't
bind any NIC and listens on TCP port 6666.

In theory, the connection request from the NIC will goto
the listen socket that is binded on it. When on socket
is binded on the NIC, it goto the global socket. However,
the connection request always goto the global socket, which
is not expected.

What's worse is that when TCP MD5 is used on the socket,
the connection will fail :/

More discussion can be found here:
https://lore.kernel.org/netdev/20250801090949.129941-1-dongml2@chinatelecom.cn/
https://lore.kernel.org/netdev/20250731123309.184496-1-dongml2@chinatelecom.cn/

>
> 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.

Kuniyuki said he will post some patch when the net-next is
open, which reorder the socket in __inet_hash() to make
the reuseport socket lookup keep O(1).

So this patch is more like a temporary solution :/

Thanks!
Menglong Dong

>
> Thanks,
> Jason