mlock: fix mlock count can not decrease in race condition

Kefeng reported that when running the follow test, the mlock count in
meminfo will increase permanently:

 [1] testcase
 linux:~ # cat test_mlockal
 grep Mlocked /proc/meminfo
  for j in `seq 0 10`
  do
 	for i in `seq 4 15`
 	do
 		./p_mlockall >> log &
 	done
 	sleep 0.2
 done
 # wait some time to let mlock counter decrease and 5s may not enough
 sleep 5
 grep Mlocked /proc/meminfo

 linux:~ # cat p_mlockall.c
 #include <sys/mman.h>
 #include <stdlib.h>
 #include <stdio.h>

 #define SPACE_LEN	4096

 int main(int argc, char ** argv)
 {
	 	int ret;
	 	void *adr = malloc(SPACE_LEN);
	 	if (!adr)
	 		return -1;

	 	ret = mlockall(MCL_CURRENT | MCL_FUTURE);
	 	printf("mlcokall ret = %d\n", ret);

	 	ret = munlockall();
	 	printf("munlcokall ret = %d\n", ret);

	 	free(adr);
	 	return 0;
	 }

In __munlock_pagevec() we should decrement NR_MLOCK for each page where
we clear the PageMlocked flag.  Commit 1ebb7cc6a583 ("mm: munlock: batch
NR_MLOCK zone state updates") has introduced a bug where we don't
decrement NR_MLOCK for pages where we clear the flag, but fail to
isolate them from the lru list (e.g.  when the pages are on some other
cpu's percpu pagevec).  Since PageMlocked stays cleared, the NR_MLOCK
accounting gets permanently disrupted by this.

Fix it by counting the number of page whose PageMlock flag is cleared.

Fixes: 1ebb7cc6a583 (" mm: munlock: batch NR_MLOCK zone state updates")
Link: http://lkml.kernel.org/r/1495678405-54569-1-git-send-email-xieyisheng1@huawei.com
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Reported-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Joern Engel <joern@logfs.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michel Lespinasse <walken@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: zhongjiang <zhongjiang@huawei.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
void munlock_vma_pages_range(struct vm_area_struct *vma,
			     unsigned long start, unsigned long end)
{
	vma->vm_flags &= VM_LOCKED_CLEAR_MASK;

	while (start < end) {
		struct page *page;
		unsigned int page_mask = 0;
		unsigned long page_increm;
		struct pagevec pvec;
		struct zone *zone;
		int zoneid;

		pagevec_init(&pvec, 0);
		/*
		 * Although FOLL_DUMP is intended for get_dump_page(),
		 * it just so happens that its special treatment of the
		 * ZERO_PAGE (returning an error instead of doing get_page)
		 * suits munlock very well (and if somehow an abnormal page
		 * has sneaked into the range, we won't oops here: great).
		 */
		page = follow_page(vma, start, FOLL_GET | FOLL_DUMP);

		if (page && !IS_ERR(page)) {
			if (PageTransTail(page)) {
				VM_BUG_ON_PAGE(PageMlocked(page), page);
				put_page(page); /* follow_page_mask() */
			} else if (PageTransHuge(page)) {
				lock_page(page);
				/*
				 * Any THP page found by follow_page_mask() may
				 * have gotten split before reaching
				 * munlock_vma_page(), so we need to compute
				 * the page_mask here instead.
				 */
				page_mask = munlock_vma_page(page);
				unlock_page(page);
				put_page(page); /* follow_page_mask() */
			} else {
				/*
				 * Non-huge pages are handled in batches via
				 * pagevec. The pin from follow_page_mask()
				 * prevents them from collapsing by THP.
				 */
				pagevec_add(&pvec, page);
				zone = page_zone(page);
				zoneid = page_zone_id(page);

				/*
				 * Try to fill the rest of pagevec using fast
				 * pte walk. This will also update start to
				 * the next page to process. Then munlock the
				 * pagevec.
				 */
				start = __munlock_pagevec_fill(&pvec, vma,
						zoneid, start, end);
				__munlock_pagevec(&pvec, zone);
				goto next;
			}
		}
		page_increm = 1 + page_mask;
		start += page_increm * PAGE_SIZE;
next:
		cond_resched();
	}
}

void munlock_vma_pages_range(struct vm_area_struct *vma,
			     unsigned long start, unsigned long end)
{
	vma->vm_flags &= VM_LOCKED_CLEAR_MASK;

	while (start < end) {
		struct page *page;
		unsigned int page_mask = 0;
		unsigned long page_increm;
		struct pagevec pvec;
		struct zone *zone;
		int zoneid;

		pagevec_init(&pvec, 0);
		/*
		 * Although FOLL_DUMP is intended for get_dump_page(),
		 * it just so happens that its special treatment of the
		 * ZERO_PAGE (returning an error instead of doing get_page)
		 * suits munlock very well (and if somehow an abnormal page
		 * has sneaked into the range, we won't oops here: great).
		 */
		page = follow_page(vma, start, FOLL_GET | FOLL_DUMP);

		if (page && !IS_ERR(page)) {
			if (PageTransTail(page)) {
				VM_BUG_ON_PAGE(PageMlocked(page), page);
				put_page(page); /* follow_page_mask() */
			} else if (PageTransHuge(page)) {
				lock_page(page);
				/*
				 * Any THP page found by follow_page_mask() may
				 * have gotten split before reaching
				 * munlock_vma_page(), so we need to compute
				 * the page_mask here instead.
				 */
				page_mask = munlock_vma_page(page);
				unlock_page(page);
				put_page(page); /* follow_page_mask() */
			} else {
				/*
				 * Non-huge pages are handled in batches via
				 * pagevec. The pin from follow_page_mask()
				 * prevents them from collapsing by THP.
				 */
				pagevec_add(&pvec, page);
				zone = page_zone(page);
				zoneid = page_zone_id(page);

				/*
				 * Try to fill the rest of pagevec using fast
				 * pte walk. This will also update start to
				 * the next page to process. Then munlock the
				 * pagevec.
				 */
				start = __munlock_pagevec_fill(&pvec, vma,
						zoneid, start, end);
				__munlock_pagevec(&pvec, zone);
				goto next;
			}
		}
		page_increm = 1 + page_mask;
		start += page_increm * PAGE_SIZE;
next:
		cond_resched();
	}
}
