mm: migrate high-order folios in swap cache correctly

Large folios occupy N consecutive entries in the swap cache instead of
using multi-index entries like the page cache.  However, if a large folio
is re-added to the LRU list, it can be migrated.  The migration code was
not aware of the difference between the swap cache and the page cache and
assumed that a single xas_store() would be sufficient.

This leaves potentially many stale pointers to the now-migrated folio in
the swap cache, which can lead to almost arbitrary data corruption in the
future.  This can also manifest as infinite loops with the RCU read lock
held.

[[email protected]: modifications to the changelog & tweaked the fix]
Fixes: 3417013 ("mm/migrate: Add folio_migrate_mapping()")
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Charan Teja Kalla <[email protected]>
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reported-by: Charan Teja Kalla <[email protected]>
Closes: https://lkml.kernel.org/r/[email protected]
Cc: David Hildenbrand <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Charan Teja Kalla

mm/migrate.c

@@ -405,6 +405,7 @@ int folio_migrate_mapping(struct address_space *mapping,
 	int dirty;
 	int expected_count = folio_expected_refs(mapping, folio) + extra_count;
 	long nr = folio_nr_pages(folio);
+	long entries, i;
 
 	if (!mapping) {
 		/* Anonymous page without mapping */
@@ -442,8 +443,10 @@ int folio_migrate_mapping(struct address_space *mapping,
 			folio_set_swapcache(newfolio);
 			newfolio->private = folio_get_private(folio);
 		}
+		entries = nr;
 	} else {
 		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
+		entries = 1;
 	}
 
 	/* Move dirty while page refs frozen and newpage not yet exposed */
@@ -453,7 +456,11 @@ int folio_migrate_mapping(struct address_space *mapping,
 		folio_set_dirty(newfolio);
 	}
 
-	xas_store(&xas, newfolio);
+	/* Swap cache still stores N entries instead of a high-order entry */
+	for (i = 0; i < entries; i++) {
+		xas_store(&xas, newfolio);
+		xas_next(&xas);
+	}
 
 	/*
 	 * Drop cache reference from old page by unfreezing