mm: thp: khugepaged: add policy for finding target node

mm: thp: khugepaged: add policy for finding target node

Khugepaged will scan/free HPAGE_PMD_NR normal pages and replace with a
hugepage which is allocated from the node of the first scanned normal
page, but this policy is too rough and may end with unexpected result to
upper users.

The problem is the original page-balancing among all nodes will be broken
after hugepaged started.  Thinking about the case if the first scanned
normal page is allocated from node A, most of other scanned normal pages
are allocated from node B or C..  But hugepaged will always allocate
hugepage from node A which will cause extra memory pressure on node A
which is not the situation before khugepaged started.

This patch try to fix this problem by making khugepaged allocate hugepage
from the node which have max record of scaned normal pages hit, so that
the effect to original page-balancing can be minimized.

The other problem is if normal scanned pages are equally allocated from
Node A,B and C, after khugepaged started Node A will still suffer extra
memory pressure.

Andrew Davidoff reported a related issue several days ago.  He wanted his
application interleaving among all nodes and "numactl --interleave=all
./test" was used to run the testcase, but the result wasn't not as
expected.

cat /proc/2814/numa_maps:
7f50bd440000 interleave:0-3 anon=51403 dirty=51403 N0=435 N1=435 N2=435
N3=50098
The end result showed that most pages are from Node3 instead of interleave
among node0-3 which was unreasonable.

This patch also fix this issue by allocating hugepage round robin from all
nodes have the same record, after this patch the result was as expected:
7f78399c0000 interleave:0-3 anon=51403 dirty=51403 N0=12723 N1=12723
N2=13235 N3=12722

The simple testcase is like this:

int main() {
	char *p;
	int i;
	int j;

	for (i=0; i < 200; i++) {
		p = (char *)malloc(1048576);
		printf("malloc done\n");

		if (p == 0) {
			printf("Out of memory\n");
			return 1;
		}
		for (j=0; j < 1048576; j++) {
			p[j] = 'A';
		}
		printf("touched memory\n");

		sleep(1);
	}
	printf("enter sleep\n");
	while(1) {
		sleep(100);
	}
}

Reported-by: Andrew Davidoff <davidoff@qedmf.net>
Tested-by: Andrew Davidoff <davidoff@qedmf.net>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>