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

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

[Lance Yang]

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.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Acked-by: David Hildenbrand <david@redhat.com>
---
v1 -> v2:
 * Update the changelog, suggested by Andrew and David
 * https://lore.kernel.org/linux-mm/20250417124908.58543-1-ioworker0@gmail.com

 include/linux/page-flags.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d3909cb1e576..6bd9b9043976 100644
--- 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
-- 
2.49.0

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

[Andrew Morton]

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.

> --- 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".

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.

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

[Lance Yang]

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
>

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

[Lance Yang]

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



> 
> 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).

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
> > 
> > 
> >
>

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

[David Hildenbrand]

On 21.04.25 07:16, Lance Yang wrote:
> April 21, 2025 at 1:13 PM, "Lance Yang" <lance.yang@linux.dev> wrote:
> 
> 
> 
>>
>> 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).
> 
> 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.

We could #ifdef in folio_maybe_mapped_shared(), which I find rather 
suboptimal ...

or simply inline it into the 4 callers.

That might be the best approach, given that only selected user should be 
using the low-level primitive and everybody else should be using 
folio_maybe_mapped_shared().

-- 
Cheers,

David / dhildenb

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

[Lance Yang]

April 21, 2025 at 3:17 PM, "David Hildenbrand" <david@redhat.com> wrote:



> 
> On 21.04.25 07:16, Lance Yang wrote:
> 
> > 
> > April 21, 2025 at 1:13 PM, "Lance Yang" <lance.yang@linux.dev> wrote:
> > 
> >  > >>
> > 
> > > 
> > > 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).
> > > 
> > 
> >  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.
> > 
> 
> We could #ifdef in folio_maybe_mapped_shared(), which I find rather suboptimal ...
> 
> or simply inline it into the 4 callers.
> 
> That might be the best approach, given that only selected user should be using the low-level primitive and everybody else should be using folio_maybe_mapped_shared().

Yep, I tend to prefer the second approach as well, given there are only
four callers. Andrew, wdyt?

Thanks,
Lance

> 
> -- Cheers,
> 
> David / dhildenb
>

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

[Andrew Morton]

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.

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

[Lance Yang]

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

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

[David Hildenbrand]

On 22.04.25 06:35, Lance Yang wrote:
> 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

Fine with me. At this point, I do prefer inlining the function, though.

-- 
Cheers,

David / dhildenb

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

[Andrew Morton]

On Tue, 22 Apr 2025 09:00:54 +0200 David Hildenbrand <david@redhat.com> wrote:

> > --- 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
> 
> Fine with me. At this point, I do prefer inlining the function, though.

The above has the advantage that if the compiler unintendedly emits a
call to folio_test_large_maybe_mapped_shared(), the link will fail.

I do think a little comment which explains this trick is needed.