Re: [PATCH v7 02/28] mm: Introduce struct folio

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Apr 16, 2021 at 04:55:16PM +0100, Matthew Wilcox wrote:
> On Fri, Apr 09, 2021 at 07:50:39PM +0100, Matthew Wilcox (Oracle) wrote:
> > A struct folio is a new abstraction to replace the venerable struct page.
> > A function which takes a struct folio argument declares that it will
> > operate on the entire (possibly compound) page, not just PAGE_SIZE bytes.
> > In return, the caller guarantees that the pointer it is passing does
> > not point to a tail page.
> > +++ b/include/linux/mm_types.h
> [...]
> > +static inline struct folio *page_folio(struct page *page)
> > +{
> > +	unsigned long head = READ_ONCE(page->compound_head);
> > +
> > +	if (unlikely(head & 1))
> > +		return (struct folio *)(head - 1);
> > +	return (struct folio *)page;
> > +}
> 
> I'm looking at changing this for the next revision, and basing it on
> my recent patch to make compound_head() const-preserving:
> 
> +#define page_folio(page)       _Generic((page),                        \
> +       const struct page *:    (const struct folio *)_compound_head(page), \
> +       struct page *:          (struct folio *)_compound_head(page))
> 
> I've also noticed an awkward pattern occurring that I think this makes
> less awkward:
> 
> +/**
> + * folio_page - Return a page from a folio.
> + * @folio: The folio.
> + * @n: The page number to return.
> + *
> + * @n is relative to the start of the folio.  It should be between
> + * 0 and folio_nr_pages(@folio) - 1, but this is not checked for.
> + */
> +#define folio_page(folio, n)   nth_page(&(folio)->page, n)
> 
> That lets me simplify folio_next():
> 
> +static inline struct folio *folio_next(struct folio *folio)
> +{
> +       return (struct folio *)folio_page(folio, folio_nr_pages(folio));
> +}
> 
> (it occurs to me this should also be const-preserving, but it's not clear
> that's needed yet)

Are we risking that we would need to replace inline functions with macros
all the way down? Not sure const-preserving worth it.

-- 
 Kirill A. Shutemov