linux-kernel - Re: [RESEND PATCH v2 1/1] mm/rmap: add CONFIG_MM_ID guard for folio_test_large_maybe_mapped

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Message-ID: <1c81b176893c349dfb12feaebd17e0694fedec0b@linux.dev>
Date: Mon, 21 Apr 2025 05:16:50 +0000
From: "Lance Yang" <lance.yang@...ux.dev>
To: "Andrew Morton" <akpm@...ux-foundation.org>, "David Hildenbrand"
 <david@...hat.com>
Cc: mingzhe.yang@...com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 "Lance Yang" <ioworker0@...il.com>
Subject: Re: [RESEND PATCH v2 1/1] mm/rmap: add CONFIG_MM_ID guard for
 folio_test_large_maybe_mapped_shared()

April 21, 2025 at 1:13 PM, "Lance Yang" <lance.yang@...ux.dev> wrote:



> 
> April 21, 2025 at 7:29 AM, "Andrew Morton" <akpm@...ux-foundation.org> wrote:
> 
> > 
> > On Fri, 18 Apr 2025 23:22:28 +0800 Lance Yang <ioworker0@...il.com> wrote:
> > 
> >  
> > 
> >  
> > 
> >  From: Lance Yang <lance.yang@...ux.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).

static inline bool folio_maybe_mapped_shared(struct folio *folio)
{
        [...]
        if (!IS_ENABLED(CONFIG_MM_ID))
                return true;
        [...]
        return folio_test_large_maybe_mapped_shared(folio);
}

folio_maybe_mapped_shared() is always available - doesn't depend on
CONFIG_MM_ID or CONFIG_TRANSPARENT_HUGEPAGE.

Thanks,
Lance


> 
> 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
> > 
> > 
> >
>