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

[David Hildenbrand <david@redhat.com>]

On 18.04.25 09:26, Lance Yang wrote:
> Hi Andrew,
> 
> Thanks for taking the time to review!
> 
> On Fri, Apr 18, 2025 at 6:02 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>>
>> On Thu, 17 Apr 2025 20:49:08 +0800 Lance Yang <ioworker0@gmail.com> wrote:
>>
>>> Add a compile-time check to make sure folio_test_large_maybe_mapped_shared()
>>> is only used with CONFIG_MM_ID enabled, as it directly accesses the _mm_ids
>>> field that only works under CONFIG_MM_ID.
>>>
>>> ...
>>>
>>> --- 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
>>
>> I don't get it.  Sounds like we're adding a compile-time check to check
>> for a compilation error which would have happened anyway.
>>
>> If folio_test_large_maybe_mapped_shared() is only used with
>> CONFIG_MM_ID enabled, then do
>>
>> #ifdef CONFIG_MM_ID
>> static inline bool folio_test_large_maybe_mapped_shared(...)
>> {
>> }
>> #endif
>>
>> ?
> 
> Hmm... we considered using '#ifdef CONFIG_MM_ID' for
> folio_test_large_maybe_mapped_shared(),
> but since this function should never be called without CONFIG_MM_ID
> enabled, compile-time errors might be the way to go -- and a compile-time
> check here does the trick ;)

Yeah, I deliberately used plenty of IS_ENABLED to avoid a #ifdef mess 
all over the place.

Maybe clarify in the patch description that we want to prevent the 
function from getting used without CONFIG_MM_ID, and we don't want to 
use #ifdef because then we'd have to add even more #ifdef in callers 
that use IS_ENABLED(CONFIG_MM_ID).

-- 
Cheers,

David / dhildenb