On Mon, 30 Oct 2023 at 16:16, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> * Use a better hashing function for the SELinux role tansition hash
> table.
Bah.
While the old hash function was garbage, the new one is quite expensive.
Maybe it's worth it.
But generally, if you find that "oh, just doing a modulus with a power
of two drops all high bits", the first thing to try is probably to
just do "hash_long(x, N)" to get N bits instead.
Assuming the input is somewhat ok in one word, it does a fairly good
job of mixing the bits with a simple multiply-and-shift.
Yes, yes, jhash is a fine hash, but it does a quite *lot* of (simple)
ALU ops. While "hash_long()" is often small enough to be inlined.
I also note that filenametr_hash() does the old "one byte at a time"
hash and partial_name_hash(). Is there any reason that code doesn't
use the "full_name_hash()" which does things a word at a time?
Probably doesn't matter, but since I looked at this to see what the
new hashing was, I noticed...
Linus
