Re: [RESEND PATCH v2 1/1] mm/rmap: add CONFIG_MM_ID guard for folio_test_large_maybe_mapped_shared()

["Lance Yang" <lance.yang@linux.dev>]

April 21, 2025 at 7:29 AM, "Andrew Morton" <akpm@linux-foundation.org> wrote:



> 
> On Fri, 18 Apr 2025 23:22:28 +0800 Lance Yang <ioworker0@gmail.com> wrote:
> 
> > 
> > From: Lance Yang <lance.yang@linux.dev>
> > 
> >  
> > 
> >  To prevent folio_test_large_maybe_mapped_shared() from being used without
> > 
> >  CONFIG_MM_ID, we add a compile-time check rather than wrapping it in
> > 
> >  '#ifdef', avoiding even more #ifdef in callers that already use
> > 
> >  IS_ENABLED(CONFIG_MM_ID).
> > 
> >  
> > 
> >  Also, we used plenty of IS_ENABLED() on purpose to keep the code free of
> > 
> >  '#ifdef' mess.
> > 
> 
> I dunno, this just seems really whacky.

I'd hope David could leave some comments on that.

> 
> > 
> > --- 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. */
> > 
> 
> A correcter comment would be "This function should never be compiled
> 
> without CONFIG_MM_ID enabled".

Yes, that is more exact ;)

> 
> Which lets the cat out of the bag. Why the heck is it being compiled
> 
> with CONFIG_MM_ID=n?? We have tools to prevent that.
> 
> Can we just slap "#ifdef CONFIG_MM_ID" around the whole function? It
> 
> should have no callers, right? If the linker ends up complaining then
> 
> something went wrong.

The reason we can't simply add #ifdef CONFIG_MM_ID around folio_test_large_maybe_mapped_shared()
is because its caller folio_maybe_mapped_shared() relies on IS_ENABLED(CONFIG_MM_ID).

If we do, with CONFIG_TRANSPARENT_HUGEPAGE=N, we'll hit compilation errors like:

./include/linux/mm.h: In function ‘folio_maybe_mapped_shared’:
./include/linux/mm.h:2337:16: error: implicit declaration of function ‘folio_test_large_maybe_mapped_shared’; did you mean ‘folio_maybe_mapped_shared’? [-Werror=implicit-function-declaration]
 2337 |         return folio_test_large_maybe_mapped_shared(folio);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                folio_maybe_mapped_shared
cc1: some warnings being treated as errors


Thanks,
Lance
>