Subject: Re: [PATCH net-next v3] net: ipv4: add support for ECMP hash policy choice

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 15 Mar 2017 12:32:13 +0100

On Tue, Mar 14, 2017 at 03:36 PM GMT, Nikolay Aleksandrov wrote:
> This patch adds support for ECMP hash policy choice via a new sysctl
> called fib_multipath_hash_policy and also adds support for L4 hashes.
> The current values for fib_multipath_hash_policy are:
>  0 - layer 3 (default)
>  1 - layer 4
> If there's an skb hash already set and it matches the chosen policy then it
> will be used instead of being calculated (currently only for L4).
> In L3 mode we always calculate the hash due to the ICMP error special
> case, the flow dissector's field consistentification should handle the
> address order thus we can remove the address reversals.
>
> Signed-off-by: Nikolay Aleksandrov <[email-masked]>
> ---
> v3:
>  - keep the ICMP error special handling and always calc L3 hash
>    Jakub, could you please run your tests with this version ?
>
> v2:
>  - removed the output_key_hash as it's not needed anymore
>  - reverted to my original/internal patch with L3 as default hash
>
>  Documentation/networking/ip-sysctl.txt |  8 +++
>  include/net/ip_fib.h                   | 14 ++----
>  include/net/netns/ipv4.h               |  1 +
>  include/net/route.h                    |  9 +---
>  net/ipv4/fib_semantics.c               | 11 ++--
>  net/ipv4/icmp.c                        | 19 +------
>  net/ipv4/route.c                       | 92 ++++++++++++++++++++++++++--------
>  net/ipv4/sysctl_net_ipv4.c             |  9 ++++
>  8 files changed, 98 insertions(+), 65 deletions(-)
>
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index fc73eeb7b3b8..558e19653106 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -73,6 +73,14 @@ fib_multipath_use_neigh - BOOLEAN
>  	0 - disabled
>  	1 - enabled
>  
> +fib_multipath_hash_policy - INTEGER
> +	Controls which hash policy to use for multipath routes. Only valid
> +	for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.
> +	Default: 0 (Layer 3)
> +	Possible values:
> +	0 - Layer 3
> +	1 - Layer 4
> +
>  route/max_size - INTEGER
>  	Maximum number of routes allowed in the kernel.  Increase
>  	this when using large numbers of interfaces and/or routes.
> diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
> index d9cee9659978..8c608a17bd9a 100644
> --- a/include/net/ip_fib.h
> +++ b/include/net/ip_fib.h
> @@ -383,17 +383,13 @@ int fib_sync_down_dev(struct net_device *dev, unsigned long event, bool force);
>  int fib_sync_down_addr(struct net_device *dev, __be32 local);
>  int fib_sync_up(struct net_device *dev, unsigned int nh_flags);
>  
> -extern u32 fib_multipath_secret __read_mostly;
> -
> -static inline int fib_multipath_hash(__be32 saddr, __be32 daddr)
> -{
> -	return jhash_2words((__force u32)saddr, (__force u32)daddr,
> -			    fib_multipath_secret) >> 1;
> -}
> -
> +#ifdef CONFIG_IP_ROUTE_MULTIPATH
> +int fib_multipath_hash(const struct fib_info *fi, const struct flowi4 *fl4,
> +		       const struct sk_buff *skb);
> +#endif
>  void fib_select_multipath(struct fib_result *res, int hash);
>  void fib_select_path(struct net *net, struct fib_result *res,
> -		     struct flowi4 *fl4, int mp_hash);
> +		     struct flowi4 *fl4);
>  
>  /* Exported by fib_trie.c */
>  void fib_trie_init(void);
> diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
> index 622d2da27135..70a1d4251790 100644
> --- a/include/net/netns/ipv4.h
> +++ b/include/net/netns/ipv4.h
> @@ -152,6 +152,7 @@ struct netns_ipv4 {
>  #endif
>  #ifdef CONFIG_IP_ROUTE_MULTIPATH
>  	int sysctl_fib_multipath_use_neigh;
> +	int sysctl_fib_multipath_hash_policy;
>  #endif
>  
>  	unsigned int	fib_seq;	/* protected by rtnl_mutex */
> diff --git a/include/net/route.h b/include/net/route.h
> index c0874c87c173..32412c91c222 100644
> --- a/include/net/route.h
> +++ b/include/net/route.h
> @@ -113,14 +113,7 @@ struct in_device;
>  int ip_rt_init(void);
>  void rt_cache_flush(struct net *net);
>  void rt_flush_dev(struct net_device *dev);
> -struct rtable *__ip_route_output_key_hash(struct net *, struct flowi4 *flp,
> -					  int mp_hash);
> -
> -static inline struct rtable *__ip_route_output_key(struct net *net,
> -						   struct flowi4 *flp)
> -{
> -	return __ip_route_output_key_hash(net, flp, -1);
> -}
> +struct rtable *__ip_route_output_key(struct net *net, struct flowi4 *flp);
>  
>  struct rtable *ip_route_output_flow(struct net *, struct flowi4 *flp,
>  				    const struct sock *sk);
> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index 317026a39cfa..6601bd9744c9 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -57,7 +57,6 @@ static unsigned int fib_info_cnt;
>  static struct hlist_head fib_info_devhash[DEVINDEX_HASHSIZE];
>  
>  #ifdef CONFIG_IP_ROUTE_MULTIPATH
> -u32 fib_multipath_secret __read_mostly;
>  
>  #define for_nexthops(fi) {						\
>  	int nhsel; const struct fib_nh *nh;				\
> @@ -576,9 +575,6 @@ static void fib_rebalance(struct fib_info *fi)
>  
>  		atomic_set(&nexthop_nh->nh_upper_bound, upper_bound);
>  	} endfor_nexthops(fi);
> -
> -	net_get_random_once(&fib_multipath_secret,
> -			    sizeof(fib_multipath_secret));
>  }
>  
>  static inline void fib_add_weight(struct fib_info *fi,
> @@ -1641,7 +1637,7 @@ void fib_select_multipath(struct fib_result *res, int hash)
>  #endif
>  
>  void fib_select_path(struct net *net, struct fib_result *res,
> -		     struct flowi4 *fl4, int mp_hash)
> +		     struct flowi4 *fl4)
>  {
>  	bool oif_check;
>  
> @@ -1650,10 +1646,9 @@ void fib_select_path(struct net *net, struct fib_result *res,
>  
>  #ifdef CONFIG_IP_ROUTE_MULTIPATH
>  	if (res->fi->fib_nhs > 1 && oif_check) {
> -		if (mp_hash < 0)
> -			mp_hash = get_hash_from_flowi4(fl4) >> 1;
> +		int h = fib_multipath_hash(res->fi, fl4, NULL);
>  
> -		fib_select_multipath(res, mp_hash);
> +		fib_select_multipath(res, h);
>  	}
>  	else
>  #endif
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index fc310db2708b..46630bc30754 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -464,22 +464,6 @@ static void icmp_reply(struct icmp_bxm *icmp_param, struct sk_buff *skb)
>  	local_bh_enable();
>  }
>  
> -#ifdef CONFIG_IP_ROUTE_MULTIPATH
> -
> -/* Source and destination is swapped. See ip_multipath_icmp_hash */
> -static int icmp_multipath_hash_skb(const struct sk_buff *skb)
> -{
> -	const struct iphdr *iph = ip_hdr(skb);
> -
> -	return fib_multipath_hash(iph->daddr, iph->saddr);
> -}
> -
> -#else
> -
> -#define icmp_multipath_hash_skb(skb) (-1)
> -
> -#endif
> -
>  static struct rtable *icmp_route_lookup(struct net *net,
>  					struct flowi4 *fl4,
>  					struct sk_buff *skb_in,
> @@ -505,8 +489,7 @@ static struct rtable *icmp_route_lookup(struct net *net,
>  	fl4->flowi4_oif = l3mdev_master_ifindex(skb_dst(skb_in)->dev);
>  
>  	security_skb_classify_flow(skb_in, flowi4_to_flowi(fl4));
> -	rt = __ip_route_output_key_hash(net, fl4,
> -					icmp_multipath_hash_skb(skb_in));
> +	rt = __ip_route_output_key(net, fl4);
>  	if (IS_ERR(rt))
>  		return rt;
>  

I'm observing that the above hunk changes things because saddr passed to
icmp_route_lookup doesn't always remain set to ip_hdr(skb_in)->daddr.

This happens when generating an ICMP error in response to a datagram
which destination address is not a local one, that is from ip_forward.

-Jakub

> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 8471dd116771..3d7f99747285 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -1734,11 +1734,11 @@ static int __mkroute_input(struct sk_buff *skb,
>  }
>  
>  #ifdef CONFIG_IP_ROUTE_MULTIPATH
> -
>  /* To make ICMP packets follow the right flow, the multipath hash is
> - * calculated from the inner IP addresses in reverse order.
> + * calculated from the inner IP addresses.
>   */
> -static int ip_multipath_icmp_hash(struct sk_buff *skb)
> +static void ip_multipath_icmp_hash(const struct sk_buff *skb,
> +				   struct flowi4 *fl4)
>  {
>  	const struct iphdr *outer_iph = ip_hdr(skb);
>  	struct icmphdr _icmph;
> @@ -1746,33 +1746,85 @@ static int ip_multipath_icmp_hash(struct sk_buff *skb)
>  	struct iphdr _inner_iph;
>  	const struct iphdr *inner_iph;
>  
> +	fl4->saddr = outer_iph->saddr;
> +	fl4->daddr = outer_iph->daddr;
>  	if (unlikely((outer_iph->frag_off & htons(IP_OFFSET)) != 0))
> -		goto standard_hash;
> +		return;
>  
>  	icmph = skb_header_pointer(skb, outer_iph->ihl * 4, sizeof(_icmph),
>  				   &_icmph);
>  	if (!icmph)
> -		goto standard_hash;
> +		return;
>  
>  	if (icmph->type != ICMP_DEST_UNREACH &&
>  	    icmph->type != ICMP_REDIRECT &&
>  	    icmph->type != ICMP_TIME_EXCEEDED &&
> -	    icmph->type != ICMP_PARAMETERPROB) {
> -		goto standard_hash;
> -	}
> +	    icmph->type != ICMP_PARAMETERPROB)
> +		return;
>  
>  	inner_iph = skb_header_pointer(skb,
>  				       outer_iph->ihl * 4 + sizeof(_icmph),
>  				       sizeof(_inner_iph), &_inner_iph);
>  	if (!inner_iph)
> -		goto standard_hash;
> +		return;
> +	fl4->saddr = inner_iph->saddr;
> +	fl4->daddr = inner_iph->daddr;
> +}
>  
> -	return fib_multipath_hash(inner_iph->daddr, inner_iph->saddr);
> +int fib_multipath_hash(const struct fib_info *fi, const struct flowi4 *fl4,
> +		       const struct sk_buff *skb)
> +{
> +	struct net *net = fi->fib_net;
> +	struct flow_keys hash_keys;
> +	u32 mhash;
>  
> -standard_hash:
> -	return fib_multipath_hash(outer_iph->saddr, outer_iph->daddr);
> -}
> +	switch (net->ipv4.sysctl_fib_multipath_hash_policy) {
> +	case 0:
> +		memset(&hash_keys, 0, sizeof(hash_keys));
> +		hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
> +		if (skb && ip_hdr(skb)->protocol == IPPROTO_ICMP) {
> +			struct flowi4 _fl4;
>  
> +			ip_multipath_icmp_hash(skb, &_fl4);
> +			hash_keys.addrs.v4addrs.src = _fl4.saddr;
> +			hash_keys.addrs.v4addrs.dst = _fl4.daddr;
> +		} else {
> +			hash_keys.addrs.v4addrs.src = fl4->saddr;
> +			hash_keys.addrs.v4addrs.dst = fl4->daddr;
> +		}
> +		break;
> +	case 1:
> +		/* skb is currently provided only when forwarding */
> +		if (skb) {
> +			unsigned int flag = FLOW_DISSECTOR_F_STOP_AT_ENCAP;
> +			struct flow_keys keys;
> +
> +			/* short-circuit if we already have L4 hash present */
> +			if (skb->l4_hash)
> +				return skb_get_hash_raw(skb) >> 1;
> +			memset(&hash_keys, 0, sizeof(hash_keys));
> +			skb_flow_dissect_flow_keys(skb, &keys, flag);
> +			hash_keys.addrs.v4addrs.src = keys.addrs.v4addrs.src;
> +			hash_keys.addrs.v4addrs.dst = keys.addrs.v4addrs.dst;
> +			hash_keys.ports.src = keys.ports.src;
> +			hash_keys.ports.dst = keys.ports.dst;
> +			hash_keys.basic.ip_proto = keys.basic.ip_proto;
> +		} else {
> +			memset(&hash_keys, 0, sizeof(hash_keys));
> +			hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
> +			hash_keys.addrs.v4addrs.src = fl4->saddr;
> +			hash_keys.addrs.v4addrs.dst = fl4->daddr;
> +			hash_keys.ports.src = fl4->fl4_sport;
> +			hash_keys.ports.dst = fl4->fl4_dport;
> +			hash_keys.basic.ip_proto = fl4->flowi4_proto;
> +		}
> +		break;
> +	}
> +	mhash = flow_hash_from_keys(&hash_keys);
> +
> +	return mhash >> 1;
> +}
> +EXPORT_SYMBOL_GPL(fib_multipath_hash);
>  #endif /* CONFIG_IP_ROUTE_MULTIPATH */
>  
>  static int ip_mkroute_input(struct sk_buff *skb,
> @@ -1782,12 +1834,9 @@ static int ip_mkroute_input(struct sk_buff *skb,
>  {
>  #ifdef CONFIG_IP_ROUTE_MULTIPATH
>  	if (res->fi && res->fi->fib_nhs > 1) {
> -		int h;
> +		struct flowi4 fl4 = { .daddr = daddr, .saddr = saddr };
> +		int h = fib_multipath_hash(res->fi, &fl4, skb);
>  
> -		if (unlikely(ip_hdr(skb)->protocol == IPPROTO_ICMP))
> -			h = ip_multipath_icmp_hash(skb);
> -		else
> -			h = fib_multipath_hash(saddr, daddr);
>  		fib_select_multipath(res, h);
>  	}
>  #endif
> @@ -2202,8 +2251,7 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
>   * Major route resolver routine.
>   */
>  
> -struct rtable *__ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
> -					  int mp_hash)
> +struct rtable *__ip_route_output_key(struct net *net, struct flowi4 *fl4)
>  {
>  	struct net_device *dev_out = NULL;
>  	__u8 tos = RT_FL_TOS(fl4);
> @@ -2365,7 +2413,7 @@ struct rtable *__ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
>  		goto make_route;
>  	}
>  
> -	fib_select_path(net, &res, fl4, mp_hash);
> +	fib_select_path(net, &res, fl4);
>  
>  	dev_out = FIB_RES_DEV(res);
>  	fl4->flowi4_oif = dev_out->ifindex;
> @@ -2378,7 +2426,7 @@ struct rtable *__ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
>  	rcu_read_unlock();
>  	return rth;
>  }
> -EXPORT_SYMBOL_GPL(__ip_route_output_key_hash);
> +EXPORT_SYMBOL_GPL(__ip_route_output_key);
>  
>  static struct dst_entry *ipv4_blackhole_dst_check(struct dst_entry *dst, u32 cookie)
>  {
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index d6880a6149ee..62c4f94923e5 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -1004,6 +1004,15 @@ static struct ctl_table ipv4_net_table[] = {
>  		.extra1		= &zero,
>  		.extra2		= &one,
>  	},
> +	{
> +		.procname	= "fib_multipath_hash_policy",
> +		.data		= &init_net.ipv4.sysctl_fib_multipath_hash_policy,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &zero,
> +		.extra2		= &one,
> +	},
>  #endif
>  	{
>  		.procname	= "ip_unprivileged_port_start",