mm: merging memory blocks resets mempolicy

Using mbind to change the mempolicy to MPOL_BIND on several adjacent
mmapped blocks may result in a reset of the mempolicy to MPOL_DEFAULT in
vma_adjust.

Test code.  Correct result is three lines containing "OK".

#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>
#include <numaif.h>
#include <errno.h>

/* gcc mbind_test.c -lnuma -o mbind_test -Wall */
#define MAXNODE 4096

void allocate()
{
	int ret;
	int len;
	int policy = -1;
	unsigned char *p;
	unsigned long mask[MAXNODE] = { 0 };
	unsigned long retmask[MAXNODE] = { 0 };

	len = getpagesize() * 0x2fc00;
	p = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
		 -1, 0);
	if (p == MAP_FAILED)
		printf("mbind err: %d\n", errno);

	mask[0] = 1;
	ret = mbind(p, len, MPOL_BIND, mask, MAXNODE, 0);
	if (ret < 0)
		printf("mbind err: %d %d\n", ret, errno);
	ret = get_mempolicy(&policy, retmask, MAXNODE, p, MPOL_F_ADDR);
	if (ret < 0)
		printf("get_mempolicy err: %d %d\n", ret, errno);

	if (policy == MPOL_BIND)
		printf("OK\n");
	else
		printf("ERROR: policy is %d\n", policy);
}

int main()
{
	allocate();
	allocate();
	allocate();
	return 0;
}

Signed-off-by: Steven T Hampson <steven.t.hampson@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -829,7 +829,7 @@ again:			remove_next = 1 + (end > next->vm_end);
 		if (next->anon_vma)
 			anon_vma_merge(vma, next);
 		mm->map_count--;
-		mpol_put(vma_policy(next));
+		vma_set_policy(vma, vma_policy(next));
 		kmem_cache_free(vm_area_cachep, next);
 		/*
 		 * In mprotect's case 6 (see comments on vma_merge),