Re: [PATCH 1/1] mm/rmap: add CONFIG_MM_ID guard for folio_test_large_maybe_mapped_shared()

[David Hildenbrand]

On 18.04.25 09:26, Lance Yang wrote:
> Hi Andrew,
>
> Thanks for taking the time to review!
>
> On Fri, Apr 18, 2025 at 6:02 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, 17 Apr 2025 20:49:08 +0800 Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >
> > > Add a compile-time check to make sure folio_test_large_maybe_mapped_shared()
> > > is only used with CONFIG_MM_ID enabled, as it directly accesses the _mm_ids
> > > field that only works under CONFIG_MM_ID.
> > >
> > > ...
> > >
> > > --- a/include/linux/page-flags.h
> > > +++ b/include/linux/page-flags.h
> > > @@ -1232,6 +1232,8 @@ static inline int folio_has_private(const struct folio *folio)
> > >
> > >   static inline bool folio_test_large_maybe_mapped_shared(const struct folio *folio)
> > >   {
> > > +     /* This function should never be called without CONFIG_MM_ID enabled. */
> > > +     BUILD_BUG_ON(!IS_ENABLED(CONFIG_MM_ID));
> > >        return test_bit(FOLIO_MM_IDS_SHARED_BITNUM, &folio->_mm_ids);
> > >   }
> > >   #undef PF_ANY
> >
> > I don't get it.  Sounds like we're adding a compile-time check to check
> > for a compilation error which would have happened anyway.
> >
> > If folio_test_large_maybe_mapped_shared() is only used with
> > CONFIG_MM_ID enabled, then do
> >
> > #ifdef CONFIG_MM_ID
> > static inline bool folio_test_large_maybe_mapped_shared(...)
> > {
> > }
> > #endif
> >
> > ?
>
> Hmm... we considered using '#ifdef CONFIG_MM_ID' for
> folio_test_large_maybe_mapped_shared(),
> but since this function should never be called without CONFIG_MM_ID
> enabled, compile-time errors might be the way to go -- and a compile-time
> check here does the trick ;)

Yeah, I deliberately used plenty of IS_ENABLED to avoid a #ifdef mess 
all over the place.

Maybe clarify in the patch description that we want to prevent the 
function from getting used without CONFIG_MM_ID, and we don't want to 
use #ifdef because then we'd have to add even more #ifdef in callers 
that use IS_ENABLED(CONFIG_MM_ID).

--
Cheers,

David / dhildenb