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Date: Mon, 6 Jan 2003 23:45:55 -0800 (PST)
From: Linus Torvalds <torvalds@transmeta.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
cc: Neil Brown <neilb@cse.unsw.edu.au>,
       Linux Kernel List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Define hash_mem in lib/hash.c to apply hash_long to an
 arbitraty piece of memory.
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On 6 Jan 2003, Jeremy Fitzhardinge wrote:
> 
> I think they have a different set of design requirements.  They're both
> designed to not only generate hashes, but make the hashes
> cryptographically strong (ie, impossible to generate collisions with
> less effort than brute force).  They're naturally slower than a simple
> hash, so you'd only use them if you need the stronger requirements.

The filesystem hashes also have another design criteria: they need to 
reliably give the _same_ hash on different machines.

In particular, the suggested hash_mem() thing is endian-unsafe, meaning
that it will give different answers on an x86 than on a sparc CPU, for
example. Which can be ok if the only thing you care about is some
temporary hash, but is unacceptable for a lot of uses. The filesystem
hashes (well, at least some of them) are also designed to hash out files
on the disk, which means that they _have_ to be the same regardless of
architecture, or you can't move disks between machines.

Quite frankly, I think the suggested hash_mem() is too special-cased to
make any sense as a generic function. The endian problems means that it
_isn't_ really generic anyway, and as such it might as well just be some
internal nfs helper function rather than something in <linux/string.h>

		Linus


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