[PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[Jeff Dike]

Commit 58956317c8de ("neighbor: Improve garbage collection")
guarantees neighbour table entries a five-second lifetime.  Processes
which make heavy use of multicast can fill the neighour table with
multicast addresses in five seconds.  At that point, neighbour entries
can't be GC-ed because they aren't five seconds old yet, the kernel
log starts to fill up with "neighbor table overflow!" messages, and
sends start to fail.

This patch allows multicast addresses to be thrown out before they've
lived out their five seconds.  This makes room for non-multicast
addresses and makes messages to all addresses more reliable in these
circumstances.

Signed-off-by: Jeff Dike <jdike@akamai.com>
---
 include/net/neighbour.h | 1 +
 net/core/neighbour.c    | 2 ++
 net/ipv4/arp.c          | 6 ++++++
 net/ipv6/ndisc.c        | 7 +++++++
 4 files changed, 16 insertions(+)

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 81ee17594c32..22ced1381ede 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -204,6 +204,7 @@ struct neigh_table {
 	int			(*pconstructor)(struct pneigh_entry *);
 	void			(*pdestructor)(struct pneigh_entry *);
 	void			(*proxy_redo)(struct sk_buff *skb);
+	int			(*is_multicast)(const void *pkey);
 	bool			(*allow_add)(const struct net_device *dev,
 					     struct netlink_ext_ack *extack);
 	char			*id;
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 8e39e28b0a8d..9500d28a43b0 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -235,6 +235,8 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 
 			write_lock(&n->lock);
 			if ((n->nud_state == NUD_FAILED) ||
+			    (tbl->is_multicast &&
+			     tbl->is_multicast(n->primary_key)) ||
 			    time_after(tref, n->updated))
 				remove = true;
 			write_unlock(&n->lock);
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index 687971d83b4e..b69bd78cfdf6 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -125,6 +125,7 @@ static int arp_constructor(struct neighbour *neigh);
 static void arp_solicit(struct neighbour *neigh, struct sk_buff *skb);
 static void arp_error_report(struct neighbour *neigh, struct sk_buff *skb);
 static void parp_redo(struct sk_buff *skb);
+static int arp_is_multicast(const void *pkey);
 
 static const struct neigh_ops arp_generic_ops = {
 	.family =		AF_INET,
@@ -156,6 +157,7 @@ struct neigh_table arp_tbl = {
 	.key_eq		= arp_key_eq,
 	.constructor	= arp_constructor,
 	.proxy_redo	= parp_redo,
+	.is_multicast   = arp_is_multicast,
 	.id		= "arp_cache",
 	.parms		= {
 		.tbl			= &arp_tbl,
@@ -928,6 +930,10 @@ static void parp_redo(struct sk_buff *skb)
 	arp_process(dev_net(skb->dev), NULL, skb);
 }
 
+static int arp_is_multicast(const void *pkey)
+{
+	return IN_MULTICAST(htonl(*((u32 *)pkey)));
+}
 
 /*
  *	Receive an arp request from the device layer.
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 27f29b957ee7..6aed5536fc5c 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -81,6 +81,7 @@ static void ndisc_error_report(struct neighbour *neigh, struct sk_buff *skb);
 static int pndisc_constructor(struct pneigh_entry *n);
 static void pndisc_destructor(struct pneigh_entry *n);
 static void pndisc_redo(struct sk_buff *skb);
+static int ndisc_is_multicast(const void *pkey);
 
 static const struct neigh_ops ndisc_generic_ops = {
 	.family =		AF_INET6,
@@ -115,6 +116,7 @@ struct neigh_table nd_tbl = {
 	.pconstructor =	pndisc_constructor,
 	.pdestructor =	pndisc_destructor,
 	.proxy_redo =	pndisc_redo,
+	.is_multicast = ndisc_is_multicast,
 	.allow_add  =   ndisc_allow_add,
 	.id =		"ndisc_cache",
 	.parms = {
@@ -1706,6 +1708,11 @@ static void pndisc_redo(struct sk_buff *skb)
 	kfree_skb(skb);
 }
 
+static int ndisc_is_multicast(const void *pkey)
+{
+	return (((struct in6_addr *)pkey)->in6_u.u6_addr8[0] & 0xf0) == 0xf0;
+}
+
 static bool ndisc_suppress_frag_ndisc(struct sk_buff *skb)
 {
 	struct inet6_dev *idev = __in6_dev_get(skb->dev);
-- 
2.17.1

Re: [PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[Jakub Kicinski]

On Sun, 8 Nov 2020 21:50:52 -0500 Jeff Dike wrote:
> Commit 58956317c8de ("neighbor: Improve garbage collection")
> guarantees neighbour table entries a five-second lifetime.  Processes
> which make heavy use of multicast can fill the neighour table with
> multicast addresses in five seconds.  At that point, neighbour entries
> can't be GC-ed because they aren't five seconds old yet, the kernel
> log starts to fill up with "neighbor table overflow!" messages, and
> sends start to fail.
> 
> This patch allows multicast addresses to be thrown out before they've
> lived out their five seconds.  This makes room for non-multicast
> addresses and makes messages to all addresses more reliable in these
> circumstances.
> 
> Signed-off-by: Jeff Dike <jdike@akamai.com>

This makes sense because mcast L2 addr is calculated, not discovered,
and therefore can be recreated at a very low cost, correct?

Perhaps it would make sense to widen the API to any "computed" address
rather than implicitly depending on this behavior for mcast?

I'm not an expert tho, maybe others disagree.

> +static int arp_is_multicast(const void *pkey)
> +{
> +	return IN_MULTICAST(htonl(*((u32 *)pkey)));
> +}

net/ipv4/arp.c:935:16: warning: cast from restricted __be32

s/u32/__be32/
s/htonl/ntohl/

Re: [PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[David Ahern]

in addition to Jakub's comments ...

On 11/8/20 7:50 PM, Jeff Dike wrote:
> @@ -1706,6 +1708,11 @@ static void pndisc_redo(struct sk_buff *skb)
>  	kfree_skb(skb);
>  }
>  
> +static int ndisc_is_multicast(const void *pkey)
> +{
> +	return (((struct in6_addr *)pkey)->in6_u.u6_addr8[0] & 0xf0) == 0xf0;

ipv6_addr_type() and IPV6_ADDR_MULTICAST is the better way to code this.



> +}
> +
>  static bool ndisc_suppress_frag_ndisc(struct sk_buff *skb)
>  {
>  	struct inet6_dev *idev = __in6_dev_get(skb->dev);
> 

Re: [PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[Jeff Dike]

Hi Jakub,

On 11/9/20 2:47 PM, Jakub Kicinski wrote:
> This makes sense because mcast L2 addr is calculated, not discovered,
> and therefore can be recreated at a very low cost, correct?

Yes.

> Perhaps it would make sense to widen the API to any "computed" address
> rather than implicitly depending on this behavior for mcast?

I'm happy to do that, but I don't know of any other types of addresses which are computed and end up in the neighbors table.

> I'm not an expert tho, maybe others disagree.
> 
>> +static int arp_is_multicast(const void *pkey)
>> +{
>> +	return IN_MULTICAST(htonl(*((u32 *)pkey)));
>> +}
> 
> net/ipv4/arp.c:935:16: warning: cast from restricted __be32
> 
> s/u32/__be32/
> s/htonl/ntohl/

Thanks, I ran sparse, but must have missed that somehow.

Jeff

Re: [PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[Jeff Dike]

Hi David,

On 11/9/20 10:55 PM, David Ahern wrote:
> ipv6_addr_type() and IPV6_ADDR_MULTICAST is the better way to code this.

Thanks, will fix.

Jeff

Re: [PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[David Ahern]

On 11/10/20 7:21 AM, Jeff Dike wrote:
> Hi Jakub,
> 
> On 11/9/20 2:47 PM, Jakub Kicinski wrote:
>> This makes sense because mcast L2 addr is calculated, not discovered,
>> and therefore can be recreated at a very low cost, correct?
> 
> Yes.
> 
>> Perhaps it would make sense to widen the API to any "computed" address
>> rather than implicitly depending on this behavior for mcast?
> 
> I'm happy to do that, but I don't know of any other types of addresses which are computed and end up in the neighbors table.
> 
>> I'm not an expert tho, maybe others disagree.
>>
>>> +static int arp_is_multicast(const void *pkey)
>>> +{
>>> +	return IN_MULTICAST(htonl(*((u32 *)pkey)));
>>> +}
>>
>> net/ipv4/arp.c:935:16: warning: cast from restricted __be32
>>
>> s/u32/__be32/
>> s/htonl/ntohl/
> 
> Thanks, I ran sparse, but must have missed that somehow.
> 

I missed this yesterday -- ipv4_is_multicast() is more appropriate and
the norm for IPv4 addresses.

Re: [PATCH net V2] Exempt multicast addresses from five-second neighbor lifetime

[Jakub Kicinski]

On Tue, 10 Nov 2020 09:21:53 -0500 Jeff Dike wrote:
> > Perhaps it would make sense to widen the API to any "computed" address
> > rather than implicitly depending on this behavior for mcast?  
> 
> I'm happy to do that, but I don't know of any other types of
> addresses which are computed and end up in the neighbors table.

Fair point, thinking about it again only mcast or local addresses 
could be computed but I never heard of local addresses being used
like that, so you can stick to what you have.