linux-kernel - Re: [PATCH net-next] tcp: fix the calculation of sysctl_max_tw_buckets in tcp_sk

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Message-Id: <20180806.134116.1016660959593702560.davem@davemloft.net>
Date:   Mon, 06 Aug 2018 13:41:16 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     laoar.shao@...il.com
Cc:     edumazet@...gle.com, yanhaishuang@...s.chinamobile.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: fix the calculation of
 sysctl_max_tw_buckets in tcp_sk_init()

From: Yafang Shao <laoar.shao@...il.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.