[PATCH 15/16] mm/migration: fix possible do_pages_stat_array racing with memory offline

[Miaohe Lin]

When follow_page peeks a page, the page could be reclaimed under heavy
memory pressure and thus be offlined while it's still being used by the
do_pages_stat_array(). Use FOLL_GET to hold the page refcnt to fix this
potential issue.

Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
---
 mm/migrate.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 7b1c0b988234..98a968e6f465 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1788,13 +1788,18 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 			goto set_status;
 
 		/* FOLL_DUMP to ignore special (like zero) pages */
-		page = follow_page(vma, addr, FOLL_DUMP);
+		page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
 
 		err = PTR_ERR(page);
 		if (IS_ERR(page))
 			goto set_status;
 
-		err = page ? page_to_nid(page) : -ENOENT;
+		if (page) {
+			err = page_to_nid(page);
+			put_page(page);
+		} else {
+			err = -ENOENT;
+		}
 set_status:
 		*status = err;
 
-- 
2.23.0