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

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

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

?

Or, as "_mm_ids field only works under CONFIG_MM_ID", make it
not-even-present when !CONFIG_MM_ID?

--- a/include/linux/mm_types.h~a
+++ a/include/linux/mm_types.h
@@ -438,7 +438,9 @@ struct folio {
 					mm_id_mapcount_t _mm_id_mapcount[2];
 					union {
 						mm_id_t _mm_id[2];
+#ifdef CONFIG_MM_ID
 						unsigned long _mm_ids;
+#endif
 					};
 	/* private: the union with struct page is transitional */
 				};
_

or

--- a/include/linux/mm_types.h~a
+++ a/include/linux/mm_types.h
@@ -436,10 +436,12 @@ struct folio {
 					atomic_t _pincount;
 #endif /* CONFIG_64BIT */
 					mm_id_mapcount_t _mm_id_mapcount[2];
+#ifdef CONFIG_MM_ID
 					union {
 						mm_id_t _mm_id[2];
 						unsigned long _mm_ids;
 					};
+#endif
 	/* private: the union with struct page is transitional */
 				};
 				unsigned long _usable_1[4];
_



I dunno, it sounds like something hasn't been fully thought through
here.  It's hard to say because the changelog is unclear.  Perhaps
start out by fully describing what problem the patch is addressing?