RE: [PATCH 02/16] user_ns: use new hashtable implementation

Yes hash_32 seems reasonable for the uid hash.   With those long hash
chains I wouldn't like to be on a machine with 10,000 processes with
each with a different uid, and a processes calling setuid in the fast
path.

The uid hash that we are playing with is one that I sort of wish that
the hash table could grow in size, so that we could scale up better.

Since uids are likely to be allocated in dense blocks, maybe an
unhashed multi-level lookup scheme might be appropriate.

Index an array with the low 8 (say) bits of the uid.
Each item can be either:  
  1) NULL => free entry.
  2) a pointer to a uid structure (check uid value).
  3) a pointer to an array to index with the next 8 bits.
(2) and (3) can be differentiated by the low address bit.
I think that is updateable with cmpxchg.

Clearly this is a bad algorithm if uids are all multiples of 2^24
but that is true or any hash function.

	David



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