netdev - Re: [PATCH net-next v2 1/1] net: ipv4: Refine the ipv4_default

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Message-ID: <1491969444.17839.30.camel@perches.com>
Date:   Tue, 11 Apr 2017 20:57:24 -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 v2 1/1] net: ipv4: Refine the
 ipv4_default_advmss

On Wed, 2017-04-12 at 10:32 +0800, gfree.wind@...mail.com wrote:
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c

more trivia:

> @@ -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(advmss, IPV4_MAX_PMTU - header_size);
>  	return advmss;
>  }

This would probably be simpler to read as:

	return min(advmss, IPV4_MAX_PMTU - header_size);

though it's almost certain that the compiler emits identical
object code.