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

[Andrew Morton]

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.