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    SubjectRe: [kernel-hardening] Re: [PATCH v7 3/6] random: use SipHash in place of MD5
    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

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