netdev - Re: [PATCH net] neighbor: Call __ipv4_neigh_lookup_noref in neigh

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Date:   Thu, 2 May 2019 10:54:23 +0100 (BST)
From:   Alan Maguire <alan.maguire@...cle.com>
To:     David Ahern <dsahern@...nel.org>
cc:     davem@...emloft.net, netdev@...r.kernel.org,
        alan.maguire@...cle.com, jwestfall@...realistic.net,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH net] neighbor: Call __ipv4_neigh_lookup_noref in
 neigh_xmit

On Wed, 1 May 2019, David Ahern wrote:

> From: David Ahern <dsahern@...il.com>
> 
> Commit cd9ff4de0107 changed the key for IFF_POINTOPOINT devices to
> INADDR_ANY but neigh_xmit which is used for MPLS encapsulations was not
> updated to use the altered key. The result is that every packet Tx does
> a lookup on the gateway address which does not find an entry, a new one
> is created only to find the existing one in the table right before the
> insert since arp_constructor was updated to reset the primary key. This
> is seen in the allocs and destroys counters:
>     ip -s -4 ntable show | head -10 | grep alloc
> 
> which increase for each packet showing the unnecessary overhread.
> 
> Fix by having neigh_xmit use __ipv4_neigh_lookup_noref for NEIGH_ARP_TABLE.
> 
> Fixes: cd9ff4de0107 ("ipv4: Make neigh lookup keys for loopback/point-to-point devices be INADDR_ANY")
> Reported-by: Alan Maguire <alan.maguire@...cle.com>
> Signed-off-by: David Ahern <dsahern@...il.com>
Tested-by: Alan Maguire <alan.maguire@...cle.com>
> ---
>  net/core/neighbour.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index aff051e5521d..9b9da5142613 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -31,6 +31,7 @@
>  #include <linux/times.h>
>  #include <net/net_namespace.h>
>  #include <net/neighbour.h>
> +#include <net/arp.h>
>  #include <net/dst.h>
>  #include <net/sock.h>
>  #include <net/netevent.h>
> @@ -2984,7 +2985,13 @@ int neigh_xmit(int index, struct net_device *dev,
>  		if (!tbl)
>  			goto out;
>  		rcu_read_lock_bh();
> -		neigh = __neigh_lookup_noref(tbl, addr, dev);
> +		if (index == NEIGH_ARP_TABLE) {
> +			u32 key = *((u32 *)addr);
> +
> +			neigh = __ipv4_neigh_lookup_noref(dev, key);
> +		} else {
> +			neigh = __neigh_lookup_noref(tbl, addr, dev);
> +		}
>  		if (!neigh)
>  			neigh = __neigh_create(tbl, addr, dev, false);
>  		err = PTR_ERR(neigh);
> -- 
> 2.11.0
> 
>