netdev - Re: [PATCH 1/6] hash: Introduce ptr_hash

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Date:	Mon, 06 Aug 2012 13:44:59 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	xemul@...allels.com
Cc:	eric.dumazet@...il.com, ebiederm@...ssion.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/6] hash: Introduce ptr_hash_mix routine

From: Pavel Emelyanov <xemul@...allels.com>
Date: Mon, 06 Aug 2012 18:13:47 +0400

> @@ -67,4 +68,13 @@ static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
>  {
>  	return hash_long((unsigned long)ptr, bits);
>  }
> +
> +static inline u32 ptr_hash_mix(const void *ptr)
> +{
> +#if BITS_PER_LONG == 32
> +	return (u32)(unsigned long)ptr;
> +#else
> +	return (u32)((unsigned long)ptr >> L1_CACHE_SHIFT);
> +#endif
> +}
>  #endif /* _LINUX_HASH_H */

This doesn't make much sense to me.

If the whole 32-bits of the pointer is useful for entropy on 32-bit
why isn't the whole 64-bits useful on 64-bit?

I would, instead, expect something like:

	ptr ^ (ptr >> 32)

for the 64-bit case.

Also, that L1_CACHE_SHIFT is something callers can decide to do.

Only they know the size of their structure, the alignment used to
allocate such objects, and thus what bits are "less relevant" and
therefore profitable to elide from the bottom of the value.
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