Re: [RFC net-next 2/2] net: ipv4: Consolidate ipv4_mtu and ip_dst_mtu_maybe_forward

[David Ahern <dsahern@gmail.com>]

On 6/30/21 7:17 PM, Vadim Fedorenko wrote:
> Consolidate IPv4 MTU code the same way it is done in IPv6 to have code
> aligned in both address families
> 
> Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
> ---
>  include/net/ip.h | 22 ++++++++++++++++++----
>  net/ipv4/route.c | 21 +--------------------
>  2 files changed, 19 insertions(+), 24 deletions(-)
> 
> diff --git a/include/net/ip.h b/include/net/ip.h
> index d9683bef8684..ed261f2a40ac 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -436,18 +436,32 @@ static inline bool ip_sk_ignore_df(const struct sock *sk)
>  static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
>  						    bool forwarding)
>  {
> +	const struct rtable *rt = (const struct rtable *)dst;

I realize this a code move from ipv4_mtu, but please use container_of
here; I have been removing the typecasts as code is changed.

>  	struct net *net = dev_net(dst->dev);
>  	unsigned int mtu;
>  
>  	if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
>  	    ip_mtu_locked(dst) ||
> -	    !forwarding)
> -		return dst_mtu(dst);
> +	    !forwarding) {
> +		mtu = rt->rt_pmtu;
> +		if (mtu && time_before(jiffies, rt->dst.expires))
> +			goto out;
> +	}
>  
>  	/* 'forwarding = true' case should always honour route mtu */
>  	mtu = dst_metric_raw(dst, RTAX_MTU);
> -	if (!mtu)
> -		mtu = min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU);
> +	if (mtu)
> +		goto out;
> +
> +	mtu = READ_ONCE(dst->dev->mtu);
> +
> +	if (unlikely(ip_mtu_locked(dst))) {
> +		if (rt->rt_uses_gateway && mtu > 576)
> +			mtu = 576;
> +	}
> +
> +out:
> +	mtu = min_t(unsigned int, mtu, IP_MAX_MTU);
>  
>  	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
>  }
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 99c06944501a..04754d55b3c1 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -1299,26 +1299,7 @@ static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
>  
>  INDIRECT_CALLABLE_SCOPE unsigned int ipv4_mtu(const struct dst_entry *dst)
>  {
> -	const struct rtable *rt = (const struct rtable *)dst;
> -	unsigned int mtu = rt->rt_pmtu;
> -
> -	if (!mtu || time_after_eq(jiffies, rt->dst.expires))
> -		mtu = dst_metric_raw(dst, RTAX_MTU);
> -
> -	if (mtu)
> -		goto out;
> -
> -	mtu = READ_ONCE(dst->dev->mtu);
> -
> -	if (unlikely(ip_mtu_locked(dst))) {
> -		if (rt->rt_uses_gateway && mtu > 576)
> -			mtu = 576;
> -	}
> -
> -out:
> -	mtu = min_t(unsigned int, mtu, IP_MAX_MTU);
> -
> -	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
> +	return ip_dst_mtu_maybe_forward(dst, false);
>  }
>  EXPORT_INDIRECT_CALLABLE(ipv4_mtu);
>  
>