[PATCH 1/1] mm: thp: calculate page_mapcount() correctly for THP pages

[Andrea Arcangeli]

This allows to revert commit 1f25fe20a76af0d960172fb104d4b13697cafa84
and it provides fully accuracy with wrprotect faults so page pinning
will stop causing false positive copy-on-writes.

Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
 mm/util.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index 6cc81e7..a0b9f63 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -383,9 +383,10 @@ struct address_space *page_mapping(struct page *page)
 /* Slow path of page_mapcount() for compound pages */
 int __page_mapcount(struct page *page)
 {
-	int ret;
+	int ret = 0, i;
 
-	ret = atomic_read(&page->_mapcount) + 1;
+	for (i = 0; i < HPAGE_PMD_NR; i++)
+		ret = max(ret, atomic_read(&page->_mapcount) + 1);
 	page = compound_head(page);
 	ret += atomic_read(compound_mapcount_ptr(page)) + 1;
 	if (PageDoubleMap(page))