[PATCH] mm/migrate: fix shmem xarray update during migration

[Zi Yan]

Pagecache uses multi-index entries for large folio, so does shmem. Only
swap cache still stores multiple entries for a single large folio.
Commit fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
fixed swap cache but got shmem wrong by storing multiple entries for
a large shmem folio. Fix it by storing a single entry for a shmem
folio.

Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
Reported-by: Liu Shixin <liushixin2@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@xxxxxxxxxx/
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 mm/migrate.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 365c6daa8d1b..9db26f5527a8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -44,6 +44,7 @@
 #include <linux/sched/sysctl.h>
 #include <linux/memory-tiers.h>
 #include <linux/pagewalk.h>
+#include <linux/shmem_fs.h>
 
 #include <asm/tlbflush.h>
 
@@ -524,7 +525,11 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 			folio_set_swapcache(newfolio);
 			newfolio->private = folio_get_private(folio);
 		}
-		entries = nr;
+		/* shmem uses high-order entry */
+		if (shmem_mapping(mapping))
+			entries = 1;
+		else
+			entries = nr;
 	} else {
 		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
 		entries = 1;
-- 
2.47.2