[RFC PATCH 16/31] mm: thp: check compound_mapcount of PMD-mapped PUD THPs at free time.

[Zi Yan]

From: Zi Yan <ziy@xxxxxxxxxx>

PMD mappings on a PUD THP should be zero when the page is freed.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 mm/page_alloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dbcccc022b30..b87a2ca0a97c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1007,8 +1007,10 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
 		/* sub_compound_map_ptr store here */
 		if (compound_order(head_page) == HPAGE_PUD_ORDER &&
 			(page - head_page) % HPAGE_PMD_NR == 3) {
-			if (unlikely(atomic_read(&page->compound_mapcount) != -1))
+			if (unlikely(atomic_read(&page->compound_mapcount) != -1)) {
+				pr_err("sub_compound_mapcount: %d\n", atomic_read(&page->compound_mapcount) + 1);
 				bad_page(page, "nonzero sub_compound_mapcount", 0);
+			}
 			break;
 		}
 		if (page->mapping != TAIL_MAPPING) {
-- 
2.20.1