[..]
> -int folio_migrate_mapping(struct address_space *mapping,
> -		struct folio *newfolio, struct folio *folio, int extra_count)
> +static void __folio_migrate_mapping(struct address_space *mapping,
> +		struct folio *newfolio, struct folio *folio, int expected_cnt)
>   {
>   	XA_STATE(xas, &mapping->i_pages, folio_index(folio));
>   	struct zone *oldzone, *newzone;
> -	int dirty;
> -	int expected_count = folio_expected_refs(mapping, folio) + extra_count;
>   	long nr = folio_nr_pages(folio);
>   	long entries, i;
> +	int dirty;
>   
>   	if (!mapping) {
> -		/* Anonymous page without mapping */

If 'mapping' was NULL, the first line would blow up while dereferencing 
'mapping->i_pages'.

The ordering change was introduced by commit

89eb946a7432b "(mm: Convert page migration to XArray)"

Matthew, please correct me if I'm missing something.

thanks,

-jane