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

[Andrew Morton <akpm@linux-foundation.org>]

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.