[STABLE,v5.15.y] mm/migrate: set swap entry values of THP tail pages properly.

Commit Message

Zi Yan March 6, 2024, 3:51 p.m. UTC

From: Zi Yan <[e-mail]>

The tail pages in a THP can have swap entry information stored in their
private field. When migrating to a new page, all tail pages of the new
page need to update ->private to avoid future data corruption.

This fix is stable-only, since after commit 07e09c483cbe ("mm/huge_memory:
work on folio->swap instead of page->private when splitting folio"),
subpages of a swapcached THP no longer requires the maintenance.

Adding THPs to the swapcache was introduced in commit
38d8b4e6bdc87 ("mm, THP, swap: delay splitting THP during swap out"),
where each subpage of a THP added to the swapcache had its own swapcache
entry and required the ->private field to point to the correct swapcache
entry. Later, when THP migration functionality was implemented in commit
616b8371539a6 ("mm: thp: enable thp migration in generic path"),
it initially did not handle the subpages of swapcached THPs, failing to
update their ->private fields or replace the subpage pointers in the
swapcache. Subsequently, commit e71769ae5260 ("mm: enable thp migration
for shmem thp") addressed the swapcache update aspect. This patch fixes
the update of subpage ->private fields.

Closes: https://lore.kernel.org/linux-mm/[e-mail]/
Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
Signed-off-by: Zi Yan <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
---
 mm/migrate.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Patch

diff --git a/mm/migrate.c b/mm/migrate.c
index c7d5566623ad..c37af50f312d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -424,8 +424,12 @@  int migrate_page_move_mapping(struct address_space *mapping,
 	if (PageSwapBacked(page)) {
 		__SetPageSwapBacked(newpage);
 		if (PageSwapCache(page)) {
+			int i;
+
 			SetPageSwapCache(newpage);
-			set_page_private(newpage, page_private(page));
+			for (i = 0; i < (1 << compound_order(page)); i++)
+				set_page_private(newpage + i,
+						 page_private(page + i));
 		}
 	} else {
 		VM_BUG_ON_PAGE(PageSwapCache(page), page);