netdev - RE: [PATCH 3/7] netfilter: xt_HMARK: modulus is expensive for hash calculation

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B6F10@saturn3.aculab.com>
Date:	Thu, 17 May 2012 09:16:11 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	<pablo@...filter.org>, <netfilter-devel@...r.kernel.org>
Cc:	<davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: RE: [PATCH 3/7] netfilter: xt_HMARK: modulus is expensive for hash calculation

 
> From: Pablo Neira Ayuso <pablo@...filter.org>
> 
> Use:
> 
> ((u64)(HASH_VAL * HASH_SIZE)) >> 32
> 
> as suggested by David S. Miller.

That (u64) cast is very unlikely to have any effect.
If you want a 64 bit result from the product of two
32 bit values, you have to cast one of the 32 bit values
prior to the multiply - as in the patch below.

	David

> 
> Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
> ---
>  net/netfilter/xt_HMARK.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/xt_HMARK.c b/net/netfilter/xt_HMARK.c
> index 5817d03..0a96a43 100644
> --- a/net/netfilter/xt_HMARK.c
> +++ b/net/netfilter/xt_HMARK.c
> @@ -109,7 +109,7 @@ hmark_hash(struct hmark_tuple *t, const 
> struct xt_hmark_info *info)
>  	hash = jhash_3words(t->src, t->dst, t->uports.v32, 
> info->hashrnd);
>  	hash = hash ^ (t->proto & info->proto_mask);
>  
> -	return (hash % info->hmodulus) + info->hoffset;
> +	return (((u64)hash * info->hmodulus) >> 32) + info->hoffset;
>  }
>  
>  static void
> -- 
> 1.7.10
> 
> --
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