netdev - Re: [PATCH 2/4] ipv4: Update pmtu informations on inetpeer only for output routes

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Date:	Wed, 12 Oct 2011 17:08:05 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	steffen.klassert@...unet.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 2/4] ipv4: Update pmtu informations on inetpeer only
 for output routes

From: Steffen Klassert <steffen.klassert@...unet.com>
Date: Tue, 11 Oct 2011 13:10:27 +0200

> @@ -1817,9 +1819,14 @@ static void rt_init_metrics(struct rtable *rt, const struct flowi4 *fl4,
>  		if (inet_metrics_new(peer))
>  			memcpy(peer->metrics, fi->fib_metrics,
>  			       sizeof(u32) * RTAX_MAX);
> -		dst_init_metrics(&rt->dst, peer->metrics, false);
>  
> -		check_peer_pmtu(&rt->dst, peer);
> +		dst_init_metrics(dst, peer->metrics, false);
> +		check_peer_pmtu(dst, peer);
> +
> +		if (rt_is_input_route(rt))
> +			dst_metric_set(dst, RTAX_MTU,
> +				       dst->ops->default_mtu(dst));
> +

You really can't do this, it's going to kill all of the memory savings from
storing metrics in the inetpeer cache.

Every input route is going to have it's metrics COW'd with this change.

The whole idea is to use defaults as heavily as possible, and that's
the entire reason why the dst->ops->default_mtu() method exists, so
that we can just leave the values alone and have read-only copies %99
of the time.

Please rearrange your fix so that these goals are still achieved.

Thanks.
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