[PATCH v4 1/2] mm: migration: fix the FOLL_GET failure on following huge page

[Haiyue Wang]

Not all huge page APIs support FOLL_GET option, so the __NR_move_pages
will fail to get the page node information for huge page.

This is an temporary solution to mitigate the racing fix.

After supporting follow huge page by FOLL_GET is done, this fix can be
reverted safely.

Fixes: 4cd614841c06 ("mm: migration: fix possible do_pages_stat_array racing with memory offline")
Signed-off-by: Haiyue Wang <haiyue.wang@xxxxxxxxx>
---
 mm/migrate.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 6a1597c92261..581dfaad9257 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1848,6 +1848,7 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 
 	for (i = 0; i < nr_pages; i++) {
 		unsigned long addr = (unsigned long)(*pages);
+		unsigned int foll_flags = FOLL_DUMP;
 		struct vm_area_struct *vma;
 		struct page *page;
 		int err = -EFAULT;
@@ -1856,8 +1857,12 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 		if (!vma)
 			goto set_status;
 
+		/* Not all huge page follow APIs support 'FOLL_GET' */
+		if (!is_vm_hugetlb_page(vma))
+			foll_flags |= FOLL_GET;
+
 		/* FOLL_DUMP to ignore special (like zero) pages */
-		page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
+		page = follow_page(vma, addr, foll_flags);
 
 		err = PTR_ERR(page);
 		if (IS_ERR(page))
@@ -1865,7 +1870,8 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 
 		if (page && !is_zone_device_page(page)) {
 			err = page_to_nid(page);
-			put_page(page);
+			if (foll_flags & FOLL_GET)
+				put_page(page);
 		} else {
 			err = -ENOENT;
 		}
-- 
2.37.2