mm/migrate: set swap entry values of THP tail pages properly.

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 07e09c483c ("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
38d8b4e6bd ("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
616b837153 ("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 e71769ae52 ("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/1707814102-22682-1-git-send-email-quic_charante@quicinc.com/
Fixes: 616b837153 ("mm: thp: enable thp migration in generic path")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reported-and-tested-by: Charan Teja Kalla <quic_charante@quicinc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/migrate.c

@@ -423,8 +423,12 @@ int folio_migrate_mapping(struct address_space *mapping,
 	if (folio_test_swapbacked(folio)) {
 		__folio_set_swapbacked(newfolio);
 		if (folio_test_swapcache(folio)) {
+			int i;
+
 			folio_set_swapcache(newfolio);
-			newfolio->private = folio_get_private(folio);
+			for (i = 0; i < nr; i++)
+				set_page_private(folio_page(newfolio, i),
+					page_private(folio_page(folio, i)));
 		}
 		entries = nr;
 	} else {