| Message ID | 1533556020-20778-1-git-send-email-laoar.shao@gmail.com |
|---|---|
| State | Changes Requested, archived |
| Delegated to: | David Miller |
| Headers | show |
| Series | [net-next] tcp: fix the calculation of sysctl_max_tw_buckets in tcp_sk_init() | expand |
From: Yafang Shao <laoar.shao@gmail.com> Date: Mon, 6 Aug 2018 19:47:00 +0800 > tcp_hashinfo.ehash_mask is always an odd number, which is set in function > alloc_large_system_hash(). See bellow, > if (_hash_mask) > *_hash_mask = (1 << log2qty) - 1; <<< always odd number > > Hence the local variable 'cnt' is a even number, as a result of that it is > no difference to do the incrementation here. > > Maybe the compiler could also optimize it, but this code is a little ugly. > > Fix: fee83d09 ("ipv4: Namespaceify tcp_max_syn_backlog knob") The correct tag is "Fixes: " > @@ -2543,7 +2543,7 @@ static int __net_init tcp_sk_init(struct net *net) > net->ipv4.sysctl_tcp_tw_reuse = 2; > > cnt = tcp_hashinfo.ehash_mask + 1; > - net->ipv4.tcp_death_row.sysctl_max_tw_buckets = (cnt + 1) / 2; > + net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2; > net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo; This is completely harmless, and does no harm. You aren't "fixing" anything.
On Tue, Aug 7, 2018 at 4:41 AM, David Miller <davem@davemloft.net> wrote: > From: Yafang Shao <laoar.shao@gmail.com> > Date: Mon, 6 Aug 2018 19:47:00 +0800 > >> tcp_hashinfo.ehash_mask is always an odd number, which is set in function >> alloc_large_system_hash(). See bellow, >> if (_hash_mask) >> *_hash_mask = (1 << log2qty) - 1; <<< always odd number >> >> Hence the local variable 'cnt' is a even number, as a result of that it is >> no difference to do the incrementation here. >> >> Maybe the compiler could also optimize it, but this code is a little ugly. >> >> Fix: fee83d09 ("ipv4: Namespaceify tcp_max_syn_backlog knob") > > The correct tag is "Fixes: " OK > >> @@ -2543,7 +2543,7 @@ static int __net_init tcp_sk_init(struct net *net) >> net->ipv4.sysctl_tcp_tw_reuse = 2; >> >> cnt = tcp_hashinfo.ehash_mask + 1; >> - net->ipv4.tcp_death_row.sysctl_max_tw_buckets = (cnt + 1) / 2; >> + net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2; >> net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo; > > This is completely harmless, and does no harm. > It is harmless, meanwhile, it is useless and a little ugly. By the way, why do we init sysctl_max_tw_buckets to (tcp_hashinfo.ehash_mask + 1) / 2 or (tcp_hashinfo.ehash_mask + 2) / 2 ? Per my pespective, this default value is a little huge, sometimes we have to set this value to a lower value with sysctl, otherwise it may easily cause tcp port exhaustion issue. So why not just init it to a lower value ? > You aren't "fixing" anything.
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 9e041fa..a9b7c4b 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -2543,7 +2543,7 @@ static int __net_init tcp_sk_init(struct net *net) net->ipv4.sysctl_tcp_tw_reuse = 2; cnt = tcp_hashinfo.ehash_mask + 1; - net->ipv4.tcp_death_row.sysctl_max_tw_buckets = (cnt + 1) / 2; + 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 / 256);
tcp_hashinfo.ehash_mask is always an odd number, which is set in function alloc_large_system_hash(). See bellow, if (_hash_mask) *_hash_mask = (1 << log2qty) - 1; <<< always odd number Hence the local variable 'cnt' is a even number, as a result of that it is no difference to do the incrementation here. Maybe the compiler could also optimize it, but this code is a little ugly. Fix: fee83d09 ("ipv4: Namespaceify tcp_max_syn_backlog knob") Signed-off-by: Yafang Shao <laoar.shao@gmail.com> --- net/ipv4/tcp_ipv4.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)