On Wed, 14 Jun 2023 21:17:34 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > 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@xxxxxxxxx> 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). OK, folio_migratetype(expensive_function_which_returns_a_folio()) or folio_migratetype(function_with_side_effects_which_returns_a_folio()). There are many failure modes. > > 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? We got lucky. It's just bad practice.
