KSM: numa awareness sysfs knob

KSM: numa awareness sysfs knob

Introduces new sysfs boolean knob /sys/kernel/mm/ksm/merge_across_nodes
which control merging pages across different numa nodes.  When it is set
to zero only pages from the same node are merged, otherwise pages from all
nodes can be merged together (default behavior).

Typical use-case could be a lot of KVM guests on NUMA machine and cpus
from more distant nodes would have significant increase of access latency
to the merged ksm page.  Sysfs knob was choosen for higher variability
when some users still prefers higher amount of saved physical memory
regardless of access latency.

Every numa node has its own stable & unstable trees because of faster
searching and inserting.  Changing of merge_nodes value is possible only
when there are not any ksm shared pages in system.

I've tested this patch on numa machines with 2, 4 and 8 nodes and measured
speed of memory access inside of KVM guests with memory pinned to one of
nodes with this benchmark:

http://pholasek.fedorapeople.org/alloc_pg.c

Population standard deviations of access times in percentage of average
were following:

merge_nodes=1
2 nodes 1.4%
4 nodes 1.6%
8 nodes	1.7%

merge_nodes=0
2 nodes	1%
4 nodes	0.32%
8 nodes	0.018%

RFC: https://lkml.org/lkml/2011/11/30/91
v1: https://lkml.org/lkml/2012/1/23/46
v2: https://lkml.org/lkml/2012/6/29/105
v3: https://lkml.org/lkml/2012/9/14/550

Signed-off-by: Petr Holasek <pholasek@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Izik Eidus <izik.eidus@ravellosystems.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Anton Arapov <anton@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>