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

[Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>]

On 2024/6/6 8:54, Jane Chu wrote:
> [..]
> > -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'.

0->i_pages is wrong, but &(0->i_pages) is legal, and then xas->xa = NULL.
> 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