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

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Date:	Fri, 7 Mar 2014 22:38:42 +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 Fri, Mar 07, 2014 at 03:52:58PM -0500, David Miller wrote:
> From: Michal Kubecek <mkubecek@...e.cz>
> Date: Thu, 6 Mar 2014 21:06:58 +0100
> 
> > 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?
> 
> Thank you for explaining all of this, I would like to think about this
> some more.
> 
> My initial suspicion is that the something about the test in cow
> metrics might need to be adjusted.
> 
> The conceptual attributes we have built for inetpeer metrics, that of
> "newness" and "read-only", might not be built adequately for the task
> at hand here.

Today I also realized another problem with current code: if we already
have inetpeer with metrics and use "ip route change" with metrics, e.g.

  ip route add fec0::1 dev eth0 rto_min 1000
  ip route change fec0::1 dev eth0 hoplimit 10

only the metrics listed in the netlink message (hoplimit here) are
modified and the rest (rto_min here) is preserved. This is inconsistent
with non-host IPv6 routes and IPv4 routes where the whole set of metrics
is replaced.

                                                         Michal Kubecek

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