netdev - Re: [PATCH v2 net-next 4/5] tcp: Save unnecessary inet_twsk

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Date:   Mon, 29 Aug 2022 16:11:57 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 4/5] tcp: Save unnecessary inet_twsk_purge() calls.

On Mon, Aug 29, 2022 at 9:21 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> While destroying netns, we call inet_twsk_purge() in tcp_sk_exit_batch()
> and tcpv6_net_exit_batch() for AF_INET and AF_INET6.  These commands
> trigger the kernel to walk through the potentially big ehash twice even
> though the netns has no TIME_WAIT sockets.
>
>   # ip netns add test
>   # ip netns del test
>
> AF_INET6 uses module_init() to be loaded after AF_INET which uses
> fs_initcall(), so tcpv6_net_ops is always registered after tcp_sk_ops.
> Also, we clean up netns in the reverse order, so tcpv6_net_exit_batch()
> is always called before tcp_sk_exit_batch().
>
> The characteristic enables us to save such unneeded iterations.  This
> change eliminates the tax by the additional unshare() described in the
> next patch to guarantee the per-netns ehash size.

Patch seems wrong to me, or not complete at least...

It seems you missed an existing check in inet_twsk_purge():

if ((tw->tw_family != family) ||
    refcount_read(&twsk_net(tw)->ns.count))
    continue;

To get rid of both IPv6 and IPv6 tw sockets, we currently need to call
inet_twsk_purge() twice,
with AF_INET and AF_INET6.


>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>  include/net/tcp.h        |  1 +
>  net/ipv4/tcp_ipv4.c      |  6 ++++--
>  net/ipv4/tcp_minisocks.c | 24 ++++++++++++++++++++++++
>  net/ipv6/tcp_ipv6.c      |  2 +-
>  4 files changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index d10962b9f0d0..f60996c1d7b3 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -346,6 +346,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb);
>  void tcp_rcv_space_adjust(struct sock *sk);
>  int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp);
>  void tcp_twsk_destructor(struct sock *sk);
> +void tcp_twsk_purge(struct list_head *net_exit_list, int family);
>  ssize_t tcp_splice_read(struct socket *sk, loff_t *ppos,
>                         struct pipe_inode_info *pipe, size_t len,
>                         unsigned int flags);
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index b07930643b11..f4a502d57d45 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -3109,8 +3109,10 @@ static void __net_exit tcp_sk_exit(struct net *net)
>         if (net->ipv4.tcp_congestion_control)
>                 bpf_module_put(net->ipv4.tcp_congestion_control,
>                                net->ipv4.tcp_congestion_control->owner);
> -       if (refcount_dec_and_test(&tcp_death_row->tw_refcount))
> +       if (refcount_dec_and_test(&tcp_death_row->tw_refcount)) {
>                 kfree(tcp_death_row);
> +               net->ipv4.tcp_death_row = NULL;
> +       }
>  }
>
>  static int __net_init tcp_sk_init(struct net *net)
> @@ -3210,7 +3212,7 @@ static void __net_exit tcp_sk_exit_batch(struct list_head *net_exit_list)
>  {
>         struct net *net;
>
> -       inet_twsk_purge(&tcp_hashinfo, AF_INET);
> +       tcp_twsk_purge(net_exit_list, AF_INET);
>
>         list_for_each_entry(net, net_exit_list, exit_list)
>                 tcp_fastopen_ctx_destroy(net);
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index 361aad67c6d6..9168c5a33344 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -347,6 +347,30 @@ void tcp_twsk_destructor(struct sock *sk)
>  }
>  EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
>
> +void tcp_twsk_purge(struct list_head *net_exit_list, int family)
> +{
> +       struct net *net;
> +
> +       list_for_each_entry(net, net_exit_list, exit_list) {
> +               if (!net->ipv4.tcp_death_row)
> +                       continue;
> +
> +               /* AF_INET6 using module_init() is always registered after
> +                * AF_INET using fs_initcall() and cleaned up in the reverse
> +                * order.
> +                *
> +                * The last refcount is decremented later in tcp_sk_exit().
> +                */
> +               if (IS_ENABLED(CONFIG_IPV6) && family == AF_INET6 &&
> +                   refcount_read(&net->ipv4.tcp_death_row->tw_refcount) == 1)
> +                       continue;
> +
> +               inet_twsk_purge(&tcp_hashinfo, family);
> +               break;
> +       }
> +}
> +EXPORT_SYMBOL_GPL(tcp_twsk_purge);
> +
>  /* Warning : This function is called without sk_listener being locked.
>   * Be sure to read socket fields once, as their value could change under us.
>   */
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 27b2fd98a2c4..9cbc7f0d7149 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -2229,7 +2229,7 @@ static void __net_exit tcpv6_net_exit(struct net *net)
>
>  static void __net_exit tcpv6_net_exit_batch(struct list_head *net_exit_list)
>  {
> -       inet_twsk_purge(&tcp_hashinfo, AF_INET6);
> +       tcp_twsk_purge(net_exit_list, AF_INET6);
>  }
>
>  static struct pernet_operations tcpv6_net_ops = {
> --
> 2.30.2
>