Re: [dm-devel] [PATCH 02/16] user_ns: use new hashtable implementation

[ebiederm xmission com (Eric W. Biederman)]

Sasha Levin <levinsasha928 gmail com> writes:

> On 08/15/2012 01:52 AM, Eric W. Biederman wrote:
>> Sasha Levin <levinsasha928 gmail com> 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
uids.

> 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.

Eric