Re: [PATCH v4 3/6] mm: migrate: split folio_migrate_mapping()

[Jane Chu <jane.chu@xxxxxxxxxx>]

    [..]

> -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