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 22, 2025 at 3:22 AM, "Andrew Morton" <akpm@linux-foundation.org> wrote:



> 
> On Mon, 21 Apr 2025 05:13:03 +0000 "Lance Yang" <lance.yang@linux.dev> wrote:
> 
> > 
> > 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
> > 
> 
> That's OK - provide a declaration of folio_maybe_mapped_shared() but no
> 
> definition. So the compiled-out code can be compiled and the linker
> 
> will confirm that it's never actually called.
>

Got it, that works as well ;)

So if David is cool with it, I'll send out the new version like this:

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d3909cb1e576..a762e4b4eab4 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1230,10 +1230,15 @@ static inline int folio_has_private(const struct folio *folio)
 	return !!(folio->flags & PAGE_FLAGS_PRIVATE);
 }

+#ifdef CONFIG_MM_ID
 static inline bool folio_test_large_maybe_mapped_shared(const struct folio *folio)
 {
 	return test_bit(FOLIO_MM_IDS_SHARED_BITNUM, &folio->_mm_ids);
 }
+#else
+bool folio_test_large_maybe_mapped_shared(const struct folio *folio);
+#endif
+

Thanks,
Lance