netdev - Re: [PATCH net] ipv6: do not overwrite inetpeer metrics prematurely

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Date:	Thu, 6 Mar 2014 21:06:58 +0100
From:	Michal Kubecek <mkubecek@...e.cz>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, kuznet@....inr.ac.ru, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net
Subject: Re: [PATCH net] ipv6: do not overwrite inetpeer metrics prematurely

On Thu, Mar 06, 2014 at 02:24:25PM -0500, David Miller wrote:
> 
> > +static void rt6_metrics_to_peer(struct rt6_info *rt)
> > +{
> > +	struct inet_peer *peer = rt6_has_peer(rt) ? rt6_peer_ptr(rt) : NULL;
> > +	struct dst_entry *dst = &rt->dst;
> > +	unsigned long old = dst->_metrics;
> > +
> > +	if (!(rt->dst.flags & DST_HOST) || dst_metrics_read_only(dst))
> > +		return;
> > +	if (peer && dst_metrics_ptr(dst) == peer->metrics)
> > +		return;
> > +
> > +	dst->ops->cow_metrics(dst, old);
> > +	if (dst->_metrics != old) {
> > +		u32 *old_p = __DST_METRICS_PTR(old);
> > +
> > +		memcpy(dst_metrics_ptr(dst), old_p, RTAX_MAX * sizeof(u32));
> > +		kfree(old_p);
> > +	}
> > +}
> 
> Hmmm...  if inet_metrics_new() is true then ->cow_metrics() will copy the
> metrics from old to new.  So you therefore shouldn't have to do the copy
> explicitly here.

If we are replacing an existing host route with metrics, e.g.

  ip route add fec0::1 dev eth0 rto_min 1000
  ip route change fec0::1 dev eth0 rto_min 1500

then peer will be the existing inetpeer and inet_metrics_new() will be
false. However, we still need to copy the new metrics from the netlink
message over the old ones.

> If inet_metrics_new() is not true, you are overwriting non-new metrics.

The only problem with this is IMHO that if inet_metrics_new() is true,
i.e. when adding a new route with new inetpeer (or old inetpeer whose
metrics were not used before), the memcpy() is done twice, once in
ipv6_cow_metrics() and once in rt6_metrics_to_peer(). We are copying the
same data twice so that the result is correct but it's not efficient and
it's not nice.

The only way I can see to avoid this (except using own metrics always
instead of those in struct inetpeer as we do for non-host routes) would
be not to call ipv6_cow_metrics() at all and write a special function
for this purpose in net/ipv6/route.c which would duplicate the parts of
ipv6_cow_metrics() we really need (and add the free()). Do you think
this is the way to go?

                                                        Michal Kubecek

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