XArray: Update the LRU list in xas_split()

When splitting a value entry, we may need to add the new nodes to the LRU
list and remove the parent node from the LRU list.  The WARN_ON checks
in shadow_lru_isolate() catch this oversight.  This bug was latent
until we stopped splitting folios in shrink_page_list() with commit
820c4e2e ("mm/vmscan: Free non-shmem folios without splitting them").
That allows the creation of large shadow entries, and subsequently when
trying to page in a small page, we will split the large shadow entry
in __filemap_add_folio().

Fixes: 8fc75643 ("XArray: add xas_split")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

lib/xarray.c

@@ -1081,6 +1081,7 @@ void xas_split(struct xa_state *xas, void *entry, unsigned int order)
 					xa_mk_node(child));
 			if (xa_is_value(curr))
 				values--;
+			xas_update(xas, child);
 		} else {
 			unsigned int canon = offset - xas->xa_sibs;
 
@@ -1095,6 +1096,7 @@ void xas_split(struct xa_state *xas, void *entry, unsigned int order)
 	} while (offset-- > xas->xa_offset);
 
 	node->nr_values += values;
+	xas_update(xas, node);
 }
 EXPORT_SYMBOL_GPL(xas_split);
 #endif