Re: [PATCH net-next] tcp: fix the calculation of sysctl_max_tw_buckets in tcp_sk_init()

On Tue, Aug 7, 2018 at 4:41 AM, David Miller [off-list ref] wrote:

From: Yafang Shao <redacted>
Date: Mon,  6 Aug 2018 19:47:00 +0800

quoted

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

quoted

@@ -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.