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    SubjectRe: [PATCH 02/16] user_ns: use new hashtable implementation
    Sasha Levin <> writes:

    > On 08/15/2012 01:52 AM, Eric W. Biederman wrote:
    >> Sasha Levin <> writes:
    >>> Switch user_ns to use the new hashtable implementation. This reduces the amount of
    >>> generic unrelated code in user_ns.
    >> Two concerns here.
    >> 1) When adding a new entry you recompute the hash where previously that
    >> was not done. I believe that will slow down adding of new entries.
    > I figured that the price for the extra hashing isn't significant since hash_32
    > is just a multiplication and a shift.
    > I'll modify the code to calculate the key just once.

    Honestly I don't know either way, but it seemed a shame to give up a
    common and trivial optimization.

    >> 2) Using hash_32 for uids is an interesting choice. hash_32 discards
    >> the low bits. Last I checked for uids the low bits were the bits
    >> that were most likely to be different and had the most entropy.
    >> I'm not certain how multiplying by the GOLDEN_RATION_PRIME_32 will
    >> affect things but I would be surprised if it shifted all of the
    >> randomness from the low bits to the high bits.
    > "Is hash_* good enough for our purpose?" - I was actually surprised that no one
    > raised that question during the RFC and assumed it was because everybody agreed
    > that it's indeed good enough.
    > I can offer the following: I'll write a small module that will hash 1...10000
    > into a hashtable which uses 7 bits (just like user_ns) and post the distribution
    > we'll get.

    That won't hurt. I think 1-100 then 1000-1100 may actually be more
    representative. Not that I would mind seeing the larger range.
    Especially since I am in the process of encouraging the use of more

    > If the results of the above will be satisfactory we can avoid the discussion
    > about which hash function we should really be using. If not, I guess now is a
    > good time for that :)

    Yes. A small emperical test sounds good.


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