Re: [PATCH v2 2/5] mmzone: Introduce folio_migratetype()

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 14, 2023 at 01:13:05PM -0700, Andrew Morton wrote:
> On Tue, 13 Jun 2023 19:13:09 -0700 "Vishal Moola (Oracle)" <vishal.moola@gmail.com> wrote:
> 
> > Introduce folio_migratetype() as a folio equivalent for
> > get_pageblock_migratetype(). This function intends to return the
> > migratetype the folio is located in, hence the name choice.
> > 
> > ...
> >
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -105,6 +105,9 @@ extern int page_group_by_mobility_disabled;
> >  #define get_pageblock_migratetype(page)					\
> >  	get_pfnblock_flags_mask(page, page_to_pfn(page), MIGRATETYPE_MASK)
> >  
> > +#define folio_migratetype(folio)				\
> > +	get_pfnblock_flags_mask(&folio->page, folio_pfn(folio),		\
> > +			MIGRATETYPE_MASK)
> 
> Theoretically this is risky because it evaluates its argument more than
> once.  Although folio_migratetype(folio++) seems an unlikely thing to do.

folio++ is always an unsafe thing to do.  folios are not consecutive
in memory (unless we know they're order-0).

> An inlined C function is always preferable.  My quick attempt at that
> reveals that the header files are All Messed Up As Usual.

The page-equivalent of this also evaluates its arguments more than once,
so it doesn't see too risky for now?