We recently changed the VSID allocation on PPC64 to use a new scheme
based on a multiplicative hash.  It turns out our choice of multiplier
(the largest 28-bit prime) wasn't so great: with large contiguous
mappings, we can get very poor hash scattering.  In particular earlier
machines (without 16M pages) which had a reasonable about of RAM (>2G
or so) wouldn't boot, because the linear mapping overflowed some hash
buckets.

This patch changes the multiplier to something which seems to work
better (it is, rather arbitrarily, the median of the primes between
2^27 and 2^28).  Some more theory should almost certainly go into the
choice of this constant, to avoid more pathological cases.  But for
now, this choice fixes a serious bug, and seems to do at least as well
at scattering as the old choice on a handful of simple testcases.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>