Subject: Re: [PATCH net-next] ipv6: implement rt_genid_bump_ipv6 with fn_sernum and remove rt6i_genid

From: Hannes Frederic Sowa <[email-masked]>
Date: Thu, 11 Sep 2014 16:32:39 +0200

On Do, 2014-09-11 at 10:19 -0400, Vlad Yasevich wrote:
> On 09/11/2014 08:05 AM, Hannes Frederic Sowa wrote:
> > On Mi, 2014-09-10 at 13:09 -0700, David Miller wrote:
> >> From: Hannes Frederic Sowa <[email-masked]>
> >> Date: Wed, 10 Sep 2014 11:31:28 +0200
> >>
> >>> In case we need to force the sockets to relookup the routes we now
> >>> increase the fn_sernum on all fibnodes in the routing tree. This is a
> >>> costly operation but should only happen if we have major routing/policy
> >>> changes in the kernel (e.g. manual route adding/removal, xfrm policy
> >>> changes).
> >>
> >> Core routers can update thousands of route updates per second, and they
> >> do this via what you refer to as "manual route adding/removal".
> >>
> >> I don't think we want to put such a scalability problem into the tree.
> >>
> >> There has to be a lightweight way to address this.
> > 
> > An alternative approach without traversing the routing table, but each
> > newly inserted route (even only cached ones) might bump all other routes
> > out of the per-socket caches:
> > 
> > diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
> > index 9bcb220..a7e45b9 100644
> > --- a/include/net/ip6_fib.h
> > +++ b/include/net/ip6_fib.h
> > @@ -119,8 +119,6 @@ struct rt6_info {
> >  	struct inet6_dev		*rt6i_idev;
> >  	unsigned long			_rt6i_peer;
> >  
> > -	u32				rt6i_genid;
> > -
> >  	/* more non-fragment space at head required */
> >  	unsigned short			rt6i_nfheader_len;
> >  
> > diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
> > index 361d260..428fdcb 100644
> > --- a/include/net/net_namespace.h
> > +++ b/include/net/net_namespace.h
> > @@ -358,18 +358,28 @@ static inline int rt_genid_ipv6(struct net *net)
> >  	return atomic_read(&net->ipv6.rt_genid);
> >  }
> >  
> > -static inline void rt_genid_bump_ipv6(struct net *net)
> > +static inline int rt_genid_bump_ipv6(struct net *net)
> >  {
> > -	atomic_inc(&net->ipv6.rt_genid);
> > +	int new, old;
> > +
> > +	do {
> > +		old = atomic_read(&net->ipv6.rt_genid);
> > +		new = old + 1;
> > +		if (new <= 0)
> > +			new = 1;
> > +	} while (atomic_cmpxchg(&net->ipv6.rt_genid, old, new) != old);
> > +	return new;
> > +
> >  }
> >  #else
> >  static inline int rt_genid_ipv6(struct net *net)
> >  {
> > -	return 0;
> > +	return 1;
> >  }
> >  
> > -static inline void rt_genid_bump_ipv6(struct net *net)
> > +static inline int rt_genid_bump_ipv6(struct net *net)
> >  {
> > +	return 1;
> >  }
> >  #endif
> >  
> > diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> > index 76b7f5e..4a2f130 100644
> > --- a/net/ipv6/ip6_fib.c
> > +++ b/net/ipv6/ip6_fib.c
> > @@ -84,7 +84,10 @@ static int fib6_walk_continue(struct fib6_walker_t *w);
> >   *	result of redirects, path MTU changes, etc.
> >   */
> >  
> > -static __u32 rt_sernum;
> > +static int fib6_new_sernum(struct net *net)
> > +{
> > +	return rt_genid_bump_ipv6(net);
> > +}
> >  
> >  static void fib6_gc_timer_cb(unsigned long arg);
> >  
> > @@ -104,13 +107,6 @@ static inline void fib6_walker_unlink(struct fib6_walker_t *w)
> >  	list_del(&w->lh);
> >  	write_unlock_bh(&fib6_walker_lock);
> >  }
> > -static __inline__ u32 fib6_new_sernum(void)
> > -{
> > -	u32 n = ++rt_sernum;
> > -	if ((__s32)n <= 0)
> > -		rt_sernum = n = 1;
> > -	return n;
> > -}
> >  
> >  /*
> >   *	Auxiliary address test functions for the radix tree.
> > @@ -421,16 +417,15 @@ out:
> >   */
> >  
> >  static struct fib6_node *fib6_add_1(struct fib6_node *root,
> > -				     struct in6_addr *addr, int plen,
> > -				     int offset, int allow_create,
> > -				     int replace_required)
> > +				    struct in6_addr *addr, int plen,
> > +				    int offset, int allow_create,
> > +				    int replace_required, int sernum)
> >  {
> >  	struct fib6_node *fn, *in, *ln;
> >  	struct fib6_node *pn = NULL;
> >  	struct rt6key *key;
> >  	int	bit;
> >  	__be32	dir = 0;
> > -	__u32	sernum = fib6_new_sernum();
> >  
> >  	RT6_TRACE("fib6_add_1\n");
> >  
> > @@ -844,6 +839,7 @@ void fib6_force_start_gc(struct net *net)
> >  int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >  	     struct nlattr *mx, int mx_len)
> >  {
> > +	struct net *net = dev_net(rt->dst.dev);
> >  	struct fib6_node *fn, *pn = NULL;
> >  	int err = -ENOMEM;
> >  	int allow_create = 1;
> > @@ -860,7 +856,7 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >  
> >  	fn = fib6_add_1(root, &rt->rt6i_dst.addr, rt->rt6i_dst.plen,
> >  			offsetof(struct rt6_info, rt6i_dst), allow_create,
> > -			replace_required);
> > +			replace_required, fib6_new_sernum(net));
> >  	if (IS_ERR(fn)) {
> >  		err = PTR_ERR(fn);
> >  		fn = NULL;
> > @@ -894,14 +890,15 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >  			sfn->leaf = info->nl_net->ipv6.ip6_null_entry;
> >  			atomic_inc(&info->nl_net->ipv6.ip6_null_entry->rt6i_ref);
> >  			sfn->fn_flags = RTN_ROOT;
> > -			sfn->fn_sernum = fib6_new_sernum();
> > +			sfn->fn_sernum = fib6_new_sernum(net);
> >  
> >  			/* Now add the first leaf node to new subtree */
> >  
> >  			sn = fib6_add_1(sfn, &rt->rt6i_src.addr,
> >  					rt->rt6i_src.plen,
> >  					offsetof(struct rt6_info, rt6i_src),
> > -					allow_create, replace_required);
> > +					allow_create, replace_required,
> > +					fib6_new_sernum(net));
> >  
> >  			if (IS_ERR(sn)) {
> >  				/* If it is failed, discard just allocated
> > @@ -920,7 +917,8 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >  			sn = fib6_add_1(fn->subtree, &rt->rt6i_src.addr,
> >  					rt->rt6i_src.plen,
> >  					offsetof(struct rt6_info, rt6i_src),
> > -					allow_create, replace_required);
> > +					allow_create, replace_required,
> > +					fib6_new_sernum(net));
> >  
> >  			if (IS_ERR(sn)) {
> >  				err = PTR_ERR(sn);
> > diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> > index f74b041..54b7d81 100644
> > --- a/net/ipv6/route.c
> > +++ b/net/ipv6/route.c
> > @@ -314,7 +314,6 @@ static inline struct rt6_info *ip6_dst_alloc(struct net *net,
> >  
> >  		memset(dst + 1, 0, sizeof(*rt) - sizeof(*dst));
> >  		rt6_init_peer(rt, table ? &table->tb6_peers : net->ipv6.peers);
> > -		rt->rt6i_genid = rt_genid_ipv6(net);
> >  		INIT_LIST_HEAD(&rt->rt6i_siblings);
> >  	}
> >  	return rt;
> > @@ -1096,10 +1095,7 @@ static struct dst_entry *ip6_dst_check(struct dst_entry *dst, u32 cookie)
> >  	 * DST_OBSOLETE_FORCE_CHK which forces validation calls down
> >  	 * into this function always.
> >  	 */
> > -	if (rt->rt6i_genid != rt_genid_ipv6(dev_net(rt->dst.dev)))
> > -		return NULL;
> > -
> > -	if (!rt->rt6i_node || (rt->rt6i_node->fn_sernum != cookie))
> > +	if (!rt->rt6i_node || rt_genid_ipv6(dev_net(rt->dst.dev)) != cookie)
> >  		return NULL;
> >  
> >  	if (rt6_check_expired(rt))
> > 
> 
> Ok, so now we bump the gen_id every time we add a route and use that as fn_sernum for
> that route.  But this doesn't solve the problem that we are seeing in that a re-lookup
> of the route still gives us an older route with an older gen_id.

Hmm, this patch completely removes rt6i_genid from rt6_info (first
hunk). We decide if we need to do the relookup based on the socket's
cookie and the per-netns serial number of the ipv6 routing tables.

Bye,
Hannes