Re: [PATCH 1/9] mm: Convert ksm_might_need_to_copy() to work on folios

[Matthew Wilcox <willy@infradead.org>]

On Tue, Dec 12, 2023 at 01:32:50PM +0100, David Hildenbrand wrote:
> > +++ b/mm/swapfile.c
> > @@ -1749,11 +1749,13 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
> >   	int ret = 1;
> >   	swapcache = page;
> > -	page = ksm_might_need_to_copy(page, vma, addr);
> > -	if (unlikely(!page))
> > +	folio = ksm_might_need_to_copy(folio, vma, addr);
> > +	if (unlikely(!folio))
> >   		return -ENOMEM;
> > -	else if (unlikely(PTR_ERR(page) == -EHWPOISON))
> > +	else if (unlikely(folio == ERR_PTR(-EHWPOISON)))
> >   		hwpoisoned = true;
> > +	else
> > +		page = folio_file_page(folio, swp_offset(entry));
> 
> Just to double-check:
> 
> assuming ksm_might_need_to_copy() in fact allocated a fresh (order-0) folio,
> folio_file_page() will simply translate to "folio_page(folio, 0)" and should
> be fine.

That's right:

static inline struct page *folio_file_page(struct folio *folio, pgoff_t index)
{
        return folio_page(folio, index & (folio_nr_pages(folio) - 1));
}

so an order-0 folio will return the only page in the folio.