netdev - Re: [PATCH net-next 1/1] net: ipv4: Refine the ipv4_default

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Date:   Tue, 11 Apr 2017 19:03:14 -0700
From:   Joe Perches <joe@...ches.com>
To:     gfree.wind@...mail.com, davem@...emloft.net, kuznet@....inr.ac.ru,
        jmorris@...ei.org, netdev@...r.kernel.org
Cc:     Gao Feng <fgao@...ai8.com>
Subject: Re: [PATCH net-next 1/1] net: ipv4: Refine the ipv4_default_advmss

On Wed, 2017-04-12 at 09:36 +0800, gfree.wind@...mail.com wrote:
> From: Gao Feng <fgao@...ai8.com>
> 
> 1. Don't get the metric RTAX_ADVMSS of dst.
> There are two reasons.
> 1) Its caller dst_metric_advmss has already invoke dst_metric_advmss
> before invoke default_advmss.
> 2) The ipv4_default_advmss is used to get the default mss, it should
> not try to get the metric like ip6_default_advmss.
> 
> 2. Use sizeof(tcphdr)+sizeof(iphdr) instead of literal 40.
> 
> 3. Define one new macro IPV4_MAX_PMTU instead of 65535 according to
> RFC 2675, section 5.1.

trivia:

> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
[]
> @@ -1250,14 +1250,11 @@ static void set_class_tag(struct rtable *rt, u32 tag)
>  
>  static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
>  {
> -	unsigned int advmss = dst_metric_raw(dst, RTAX_ADVMSS);
> +	unsigned int header_size = sizeof(struct tcphdr) + sizeof(struct iphdr);
> +	unsigned int advmss = max_t(unsigned int, dst->dev->mtu - header_size,
> +				    ip_rt_min_advmss);
>  
> -	if (advmss == 0) {
> -		advmss = max_t(unsigned int, dst->dev->mtu - 40,
> -			       ip_rt_min_advmss);
> -		if (advmss > 65535 - 40)
> -			advmss = 65535 - 40;
> -	}
> +	advmss = min_t(unsigned int, advmss, IPV4_MAX_PMTU - header_size);

as all the elements are now unsigned int, the min_t cast
seems unnecessary.