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Date: Tue, 6 Sep 2022 16:11:50 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <eric.dumazet@...il.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
<kuni1840@...il.com>, <kuniyu@...zon.com>,
<netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v4 net-next 2/6] tcp: Don't allocate tcp_death_row outside of struct netns_ipv4.
From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 6 Sep 2022 14:26:30 -0700
> On 9/6/22 09:24, Kuniyuki Iwashima wrote:
> > We will soon introduce an optional per-netns ehash and access hash
> > tables via net->ipv4.tcp_death_row->hashinfo instead of &tcp_hashinfo
> > in most places.
> >
> > It could harm the fast path because dereferences of two fields in net
> > and tcp_death_row might incur two extra cache line misses. To save one
> > dereference, let's place tcp_death_row back in netns_ipv4 and fetch
> > hashinfo via net->ipv4.tcp_death_row"."hashinfo.
> >
> > Note tcp_death_row was initially placed in netns_ipv4, and commit
> > fbb8295248e1 ("tcp: allocate tcp_death_row outside of struct netns_ipv4")
> > changed it to a pointer so that we can fire TIME_WAIT timers after freeing
> > net. However, we don't do so after commit 04c494e68a13 ("Revert "tcp/dccp:
> > get rid of inet_twsk_purge()""), so we need not define tcp_death_row as a
> > pointer.
> >
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > ---
> > include/net/netns/ipv4.h | 3 ++-
> > net/dccp/minisocks.c | 2 +-
> > net/ipv4/inet_timewait_sock.c | 4 +---
> > net/ipv4/proc.c | 2 +-
> > net/ipv4/sysctl_net_ipv4.c | 8 ++------
> > net/ipv4/tcp_ipv4.c | 14 +++-----------
> > net/ipv4/tcp_minisocks.c | 2 +-
> > net/ipv6/tcp_ipv6.c | 2 +-
> > 8 files changed, 12 insertions(+), 25 deletions(-)
> >
> > diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
> > index 6320a76cefdc..2c7df93e3403 100644
> > --- a/include/net/netns/ipv4.h
> > +++ b/include/net/netns/ipv4.h
> > @@ -34,6 +34,7 @@ struct inet_hashinfo;
> > struct inet_timewait_death_row {
> > refcount_t tw_refcount;
> >
> > + /* Padding to avoid false sharing, tw_refcount can be often written */
> > struct inet_hashinfo *hashinfo ____cacheline_aligned_in_smp;
> > int sysctl_max_tw_buckets;
> > };
> > @@ -41,7 +42,7 @@ struct inet_timewait_death_row {
> > struct tcp_fastopen_context;
> >
> > struct netns_ipv4 {
> > - struct inet_timewait_death_row *tcp_death_row;
> > + struct inet_timewait_death_row tcp_death_row;
> >
> > #ifdef CONFIG_SYSCTL
> > struct ctl_table_header *forw_hdr;
> > diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
> > index 64d805b27add..39f408d44da5 100644
> > --- a/net/dccp/minisocks.c
> > +++ b/net/dccp/minisocks.c
> > @@ -22,7 +22,7 @@
> > #include "feat.h"
> >
> > struct inet_timewait_death_row dccp_death_row = {
> > - .tw_refcount = REFCOUNT_INIT(1),
> > + .tw_refcount = REFCOUNT_INIT(0),
>
>
> I do not see how this can possibly work.
>
> If the initial (TCP/DCCP) tw_refcount value is 0, then the first attempt
>
> doing a refcount_inc() will trigger a warning/crash.
>
> Have you looked at dmesg/syslog when testing your patch ?
>
> It should contain a loud warning...
Ah.. right... I'll keep the base as 1.
>
>
> > .sysctl_max_tw_buckets = NR_FILE * 2,
> > .hashinfo = &dccp_hashinfo,
> > };
> > diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> > index 47ccc343c9fb..71d3bb0abf6c 100644
> > --- a/net/ipv4/inet_timewait_sock.c
> > +++ b/net/ipv4/inet_timewait_sock.c
> > @@ -59,9 +59,7 @@ static void inet_twsk_kill(struct inet_timewait_sock *tw)
> > inet_twsk_bind_unhash(tw, hashinfo);
> > spin_unlock(&bhead->lock);
> >
> > - if (refcount_dec_and_test(&tw->tw_dr->tw_refcount))
> > - kfree(tw->tw_dr);
> > -
> > + refcount_dec(&tw->tw_dr->tw_refcount);
> > inet_twsk_put(tw);
> > }
> >
> > diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
> > index 0088a4c64d77..37508be97393 100644
> > --- a/net/ipv4/proc.c
> > +++ b/net/ipv4/proc.c
> > @@ -59,7 +59,7 @@ static int sockstat_seq_show(struct seq_file *seq, void *v)
> > socket_seq_show(seq);
> > seq_printf(seq, "TCP: inuse %d orphan %d tw %d alloc %d mem %ld\n",
> > sock_prot_inuse_get(net, &tcp_prot), orphans,
> > - refcount_read(&net->ipv4.tcp_death_row->tw_refcount) - 1,
>
>
> I think your patch changes too many things.
>
> Please leave the refcount base value at 1, not 0 :/
Will do.
> > + refcount_read(&net->ipv4.tcp_death_row.tw_refcount),
> > sockets, proto_memory_allocated(&tcp_prot));
> > seq_printf(seq, "UDP: inuse %d mem %ld\n",
> > sock_prot_inuse_get(net, &udp_prot),
> > diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> > index 5490c285668b..4d7c110c772f 100644
> > --- a/net/ipv4/sysctl_net_ipv4.c
> > +++ b/net/ipv4/sysctl_net_ipv4.c
> > @@ -530,10 +530,9 @@ static struct ctl_table ipv4_table[] = {
> > };
> >
> > static struct ctl_table ipv4_net_table[] = {
> > - /* tcp_max_tw_buckets must be first in this table. */
> > {
> > .procname = "tcp_max_tw_buckets",
> > -/* .data = &init_net.ipv4.tcp_death_row.sysctl_max_tw_buckets, */
> > + .data = &init_net.ipv4.tcp_death_row.sysctl_max_tw_buckets,
> > .maxlen = sizeof(int),
> > .mode = 0644,
> > .proc_handler = proc_dointvec
> > @@ -1361,8 +1360,7 @@ static __net_init int ipv4_sysctl_init_net(struct net *net)
> > if (!table)
> > goto err_alloc;
> >
> > - /* skip first entry (sysctl_max_tw_buckets) */
> > - for (i = 1; i < ARRAY_SIZE(ipv4_net_table) - 1; i++) {
> > + for (i = 0; i < ARRAY_SIZE(ipv4_net_table) - 1; i++) {
> > if (table[i].data) {
> > /* Update the variables to point into
> > * the current struct net
> > @@ -1377,8 +1375,6 @@ static __net_init int ipv4_sysctl_init_net(struct net *net)
> > }
> > }
> >
> > - table[0].data = &net->ipv4.tcp_death_row->sysctl_max_tw_buckets;
> > -
> > net->ipv4.ipv4_hdr = register_net_sysctl(net, "net/ipv4", table);
> > if (!net->ipv4.ipv4_hdr)
> > goto err_reg;
> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index a07243f66d4c..e2a6511218f8 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
> > @@ -292,7 +292,7 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
> > * complete initialization after this.
> > */
> > tcp_set_state(sk, TCP_SYN_SENT);
> > - tcp_death_row = net->ipv4.tcp_death_row;
> > + tcp_death_row = &net->ipv4.tcp_death_row;
> > err = inet_hash_connect(tcp_death_row, sk);
> > if (err)
> > goto failure;
> > @@ -3091,13 +3091,9 @@ EXPORT_SYMBOL(tcp_prot);
> >
> > static void __net_exit tcp_sk_exit(struct net *net)
> > {
> > - struct inet_timewait_death_row *tcp_death_row = net->ipv4.tcp_death_row;
> > -
> > 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))
> > - kfree(tcp_death_row);
>
> We might add a debug check about tw_refcount being 1 at this point.
>
> WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount)));
I'll add it.
Thank you!
> > }
> >
> > static int __net_init tcp_sk_init(struct net *net)
> > @@ -3129,13 +3125,9 @@ static int __net_init tcp_sk_init(struct net *net)
> > net->ipv4.sysctl_tcp_tw_reuse = 2;
> > net->ipv4.sysctl_tcp_no_ssthresh_metrics_save = 1;
> >
> > - net->ipv4.tcp_death_row = kzalloc(sizeof(struct inet_timewait_death_row), GFP_KERNEL);
> > - if (!net->ipv4.tcp_death_row)
> > - return -ENOMEM;
> > - refcount_set(&net->ipv4.tcp_death_row->tw_refcount, 1);
> > cnt = tcp_hashinfo.ehash_mask + 1;
> > - net->ipv4.tcp_death_row->sysctl_max_tw_buckets = cnt / 2;
> > - net->ipv4.tcp_death_row->hashinfo = &tcp_hashinfo;
> > + net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2;
> > + net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo;
> >
> > net->ipv4.sysctl_max_syn_backlog = max(128, cnt / 128);
> > net->ipv4.sysctl_tcp_sack = 1;
> > diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> > index 80ce27f8f77e..8bddb2a78b21 100644
> > --- a/net/ipv4/tcp_minisocks.c
> > +++ b/net/ipv4/tcp_minisocks.c
> > @@ -250,7 +250,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
> > struct net *net = sock_net(sk);
> > struct inet_timewait_sock *tw;
> >
> > - tw = inet_twsk_alloc(sk, net->ipv4.tcp_death_row, state);
> > + tw = inet_twsk_alloc(sk, &net->ipv4.tcp_death_row, state);
> >
> > if (tw) {
> > struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
> > diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> > index 5c562d69fddf..eb1da7a63fbb 100644
> > --- a/net/ipv6/tcp_ipv6.c
> > +++ b/net/ipv6/tcp_ipv6.c
> > @@ -325,7 +325,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
> > inet->inet_dport = usin->sin6_port;
> >
> > tcp_set_state(sk, TCP_SYN_SENT);
> > - tcp_death_row = net->ipv4.tcp_death_row;
> > + tcp_death_row = &net->ipv4.tcp_death_row;
> > err = inet6_hash_connect(tcp_death_row, sk);
> > if (err)
> > goto late_failure;
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