mm: memmap_init_zone() performance improvement

mm: memmap_init_zone() performance improvement

We have what we call an "architectural simulator".  It is a computer
program that pretends that it is a computer system.  We use it to test the
firmware before real hardware is available.  We have booted Linux on our
simulator.  As you would expect it takes longer to boot on the simulator
than it does on real hardware.

With my patch - boot time 41 minutes
Without patch - boot time 94 minutes

These numbers do not scale linearly to real hardware.  But indicate to me
a place where Linux can be improved.

memmap_init_zone() loops through every Page Frame Number (pfn), including
pfn values that are within the gaps between existing memory sections.  The
unneeded looping will become a boot performance issue when machines
configure larger memory ranges that will contain larger and more numerous
gaps.

The code will skip across invalid pfn values to reduce the number of loops
executed.

Signed-off-by: Mike Yoknis <mike.yoknis@hp.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>