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Re: [PATCH] ipvs: Fix reuse connection if RS weight is 0

To: yangxingwu <xingwu.yang@xxxxxxxxx>
Subject: Re: [PATCH] ipvs: Fix reuse connection if RS weight is 0
Cc: Simon Horman <horms@xxxxxxxxxxxx>, pablo@xxxxxxxxxxxxx, kadlec@xxxxxxxxxxxxx, fw@xxxxxxxxx, "David S. Miller" <davem@xxxxxxxxxxxxx>, kuba@xxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, lvs-devel@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, coreteam@xxxxxxxxxxxxx, linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>, linux-doc@xxxxxxxxxxxxxxx, corbet@xxxxxxx
From: Julian Anastasov <ja@xxxxxx>
Date: Mon, 25 Oct 2021 21:12:33 +0300 (EEST)
        Hello,

On Mon, 25 Oct 2021, yangxingwu wrote:

> Since commit dc7b3eb900aa ("ipvs: Fix reuse connection if real server is
> dead"), new connections to dead servers are redistributed immediately to
> new servers.
> 
> Then commit d752c3645717 ("ipvs: allow rescheduling of new connections when
> port reuse is detected") disable expire_nodest_conn if conn_reuse_mode is
> 0. And new connection may be distributed to a real server with weight 0.

        Your change does not look correct to me. At the time
expire_nodest_conn was created, it was not checked when
weight is 0. At different places different terms are used
but in short, we have two independent states for real server:

- inhibited: weight=0 and no new connections should be served,
        packets for existing connections can be routed to server
        if it is still available and packets are not dropped
        by expire_nodest_conn.
        The new feature is that port reuse detection can
        redirect the new TCP connection into a new IPVS conn and
        to expire the existing cp/ct.

- unavailable (!IP_VS_DEST_F_AVAILABLE): server is removed,
        can be temporary, drop traffic for existing connections
        but on expire_nodest_conn we can select different server

        The new conn_reuse_mode flag allows port reuse to
be detected. Only then expire_nodest_conn has the
opportunity with commit dc7b3eb900aa to check weight=0
and to consider the old traffic as finished. If a new
server is selected, any retrans from previous connection
would be considered as part from the new connection. It
is a rapid way to switch server without checking with
is_new_conn_expected() because we can not have many
conns/conntracks to different servers.

> Signed-off-by: yangxingwu <xingwu.yang@xxxxxxxxx>
> ---
>  Documentation/networking/ipvs-sysctl.rst | 3 +--
>  net/netfilter/ipvs/ip_vs_core.c          | 5 +++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/networking/ipvs-sysctl.rst 
> b/Documentation/networking/ipvs-sysctl.rst
> index 2afccc63856e..1cfbf1add2fc 100644
> --- a/Documentation/networking/ipvs-sysctl.rst
> +++ b/Documentation/networking/ipvs-sysctl.rst
> @@ -37,8 +37,7 @@ conn_reuse_mode - INTEGER
>  
>       0: disable any special handling on port reuse. The new
>       connection will be delivered to the same real server that was
> -     servicing the previous connection. This will effectively
> -     disable expire_nodest_conn.
> +     servicing the previous connection.
>  
>       bit 1: enable rescheduling of new connections when it is safe.
>       That is, whenever expire_nodest_conn and for TCP sockets, when
> diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
> index 128690c512df..9279aed69e23 100644
> --- a/net/netfilter/ipvs/ip_vs_core.c
> +++ b/net/netfilter/ipvs/ip_vs_core.c
> @@ -2042,14 +2042,15 @@ ip_vs_in(struct netns_ipvs *ipvs, unsigned int 
> hooknum, struct sk_buff *skb, int
>                            ipvs, af, skb, &iph);
>  
>       conn_reuse_mode = sysctl_conn_reuse_mode(ipvs);
> -     if (conn_reuse_mode && !iph.fragoffs && is_new_conn(skb, &iph) && cp) {
> +     if (!iph.fragoffs && is_new_conn(skb, &iph) && cp) {
>               bool old_ct = false, resched = false;
>  
>               if (unlikely(sysctl_expire_nodest_conn(ipvs)) && cp->dest &&
>                   unlikely(!atomic_read(&cp->dest->weight))) {
>                       resched = true;
>                       old_ct = ip_vs_conn_uses_old_conntrack(cp, skb);
> -             } else if (is_new_conn_expected(cp, conn_reuse_mode)) {
> +             } else if (conn_reuse_mode &&
> +                        is_new_conn_expected(cp, conn_reuse_mode)) {
>                       old_ct = ip_vs_conn_uses_old_conntrack(cp, skb);
>                       if (!atomic_read(&cp->n_control)) {
>                               resched = true;
> -- 
> 2.30.2

Regards

--
Julian Anastasov <ja@xxxxxx>

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