From: Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH kNFSd ] Fix hash function for IP addresses on 64bit
 little-endian machines.
Date: Thu, 15 Dec 2005 20:47:48 -0800
Message-ID: <20051215204748.24f5abe6.akpm@osdl.org>
References: <20051216152056.4989.patches@notabene>
	<1051216042204.5009@cse.unsw.edu.au>
Mime-Version: 1.0
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Cc: capps@iozone.org, nfs@lists.sourceforge.net,
	William Lee Irwin III <wli@holomorphy.com>
To: NeilBrown <neilb@cse.unsw.edu.au>
In-Reply-To: <1051216042204.5009@cse.unsw.edu.au>
Sender: nfs-admin@lists.sourceforge.net
Errors-To: nfs-admin@lists.sourceforge.net

NeilBrown <neilb@cse.unsw.edu.au> wrote:
>

(add Bill to cc)

> This patch against 2.6.15-rc5-mm2 is suitable, but not critical, for 2.6.15.
> 
> --
> The hash.h hash_long function, when used on a 64 bit machine, ignores
> many of the middle-order bits. (The prime chosen it too bit-sparse).
> 
> IP addresses for clients of an NFS server are very likely to differ
> only in the low-order bits.  As addresses are stored in
> network-byte-order, these bits become middle-order bits in a
> little-endian 64bit 'long', and so do not contribute to the hash.
> Thus you can have the situation where all clients appear on one hash chain.
> 
> So, until hash_long is fixed (or maybe forever), us a hash function
> that works well on IP addresses - xor the bytes together.
> 
> Thanks to "Iozone" <capps@iozone.org> for identifying this problem.
> 
> Cc: "Iozone" <capps@iozone.org>
> 
> Signed-off-by: Neil Brown <neilb@suse.de>
> 
> ### Diffstat output
>  ./net/sunrpc/svcauth_unix.c |   14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff ./net/sunrpc/svcauth_unix.c~current~ ./net/sunrpc/svcauth_unix.c
> --- ./net/sunrpc/svcauth_unix.c~current~	2005-12-16 15:10:12.000000000 +1100
> +++ ./net/sunrpc/svcauth_unix.c	2005-12-16 15:12:00.000000000 +1100
> @@ -101,10 +101,22 @@ static void ip_map_put(struct cache_head
>  	}
>  }
>  
> +#if IP_HASHBITS == 8
> +/* hash_long on a 64 bit machine is currently REALLY BAD for
> + * IP addresses in reverse-endian (i.e. on a little-endian machine).
> + * So use a trivial but reliable hash instead
> + */
> +static inline int hash_ip(unsigned long ip)
> +{
> +	int hash = ip ^ (ip>>16);
> +	return (hash ^ (hash>>8)) & 0xff;
> +}
> +#endif
> +
>  static inline int ip_map_hash(struct ip_map *item)
>  {
>  	return hash_str(item->m_class, IP_HASHBITS) ^ 
> -		hash_long((unsigned long)item->m_addr.s_addr, IP_HASHBITS);
> +		hash_ip((unsigned long)item->m_addr.s_addr);
>  }
>  static inline int ip_map_match(struct ip_map *item, struct ip_map *tmp)
>  {


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