mm/migrate: fix shmem xarray update during migration

commit 60cf233b585cdf1f3c5e52d1225606b86acd08b0 upstream.

A shmem folio can be either in page cache or in swap cache, but not at the
same time.  Namely, once it is in swap cache, folio->mapping should be
NULL, and the folio is no longer in a shmem mapping.

In __folio_migrate_mapping(), to determine the number of xarray entries to
update, folio_test_swapbacked() is used, but that conflates shmem in page
cache case and shmem in swap cache case.  It leads to xarray multi-index
entry corruption, since it turns a sibling entry to a normal entry during
xas_store() (see [1] for a userspace reproduction).  Fix it by only using
folio_test_swapcache() to determine whether xarray is storing swap cache
entries or not to choose the right number of xarray entries to update.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Note:
In __split_huge_page(), folio_test_anon() && folio_test_swapcache() is
used to get swap_cache address space, but that ignores the shmem folio in
swap cache case.  It could lead to NULL pointer dereferencing when a
in-swap-cache shmem folio is split at __xa_store(), since
!folio_test_anon() is true and folio->mapping is NULL.  But fortunately,
its caller split_huge_page_to_list_to_order() bails out early with EBUSY
when folio->mapping is NULL.  So no need to take care of it here.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
Signed-off-by: Zi Yan <[email protected]>
Reported-by: Liu Shixin <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Suggested-by: Hugh Dickins <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Charan Teja Kalla <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Lance Yang <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: x-y-z

mm/migrate.c

@@ -420,19 +420,17 @@ int folio_migrate_mapping(struct address_space *mapping,
 	newfolio->index = folio->index;
 	newfolio->mapping = folio->mapping;
 	folio_ref_add(newfolio, nr); /* add cache reference */
-	if (folio_test_swapbacked(folio)) {
+	if (folio_test_swapbacked(folio))
 		__folio_set_swapbacked(newfolio);
-		if (folio_test_swapcache(folio)) {
-			int i;
+    if (folio_test_swapcache(folio)) {
+        int i;
 
-			folio_set_swapcache(newfolio);
-			for (i = 0; i < nr; i++)
-				set_page_private(folio_page(newfolio, i),
-					page_private(folio_page(folio, i)));
-		}
+        folio_set_swapcache(newfolio);
+        for (i = 0; i < nr; i++)
+            set_page_private(folio_page(newfolio, i),
+                page_private(folio_page(folio, i)));
 		entries = nr;
 	} else {
-		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
 		entries = 1;
 	}