From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: Re: [PATCH v7 3/6] random: use SipHash in
 place of MD5
Date: Thu, 22 Dec 2016 10:54:47 -0500
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On Thu, Dec 22, 2016 at 02:10:33PM +0100, Jason A. Donenfeld wrote:
> On Thu, Dec 22, 2016 at 1:47 PM, Hannes Frederic Sowa
> <hannes@stressinduktion.org> wrote:
> > following up on what appears to be a random subject: ;)
> >
> > IIRC, ext4 code by default still uses half_md4 for hashing of filenames
> > in the htree. siphash seems to fit this use case pretty good.
> 
> I saw this too. I'll try to address it in v8 of this series.

This is a separate issue, and this series is getting a bit too
complex.  So I'd suggest pushing this off to a separate change.

Changing the htree hash algorithm is an on-disk format change, and so
we couldn't roll it out until e2fsprogs gets updated and rolled out
pretty broadley.  In fact George sent me patches to add siphash as a
hash algorithm for htree a while back (for both the kernel and
e2fsprogs), but I never got around to testing and applying them,
mainly because while it's technically faster, I had other higher
priority issues to work on --- and see previous comments regarding
pixel peeping.  Improving the hash algorithm by tens or even hundreds
of nanoseconds isn't really going to matter since we only do a htree
lookup on a file creation or cold cache lookup, and the SSD or HDD I/O
times will dominate.  And from the power perspective, saving
microwatts of CPU power isn't going to matter if you're going to be
spinning up the storage device....

						- Ted