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Re: [dm-devel] [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.


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