Re: [PATCH 1/1] mm/rmap: optimize MM-ID mapcount handling with union

[David Hildenbrand <david@redhat.com>]

>   	/* Note: mapcounts start at -1. */
>   	atomic_set(&folio->_large_mapcount, mapcount - 1);
> diff --git a/mm/internal.h b/mm/internal.h
> index 838f840ded83..1505174178f4 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -772,8 +772,13 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
>   		atomic_set(&folio->_nr_pages_mapped, 0);
>   	if (IS_ENABLED(CONFIG_MM_ID)) {
>   		folio->_mm_ids = 0;
> -		folio->_mm_id_mapcount[0] = -1;
> -		folio->_mm_id_mapcount[1] = -1;
> +		/*
> +		 * One-shot initialization of both mapcount slots to -1.
> +		 * Using 'unsigned long' ensures cross-arch compatibility:
> +		 * - 32-bit: Fills both short slots (0xFFFF each)
> +		 * - 64-bit: Fills both int slots (0xFFFFFFFF each)
> +		 */
> +		folio->_mm_id_mapcounts = -1UL;

Are we sure the compiler cannot optimize that itself?

On x86-64 I get with gcc 14.2.1:


;               folio->_mm_id_mapcount[0] = -1;
     3f2f: 48 c7 42 60 ff ff ff ff       movq    $-0x1, 0x60(%rdx)

Which should be a quadword (64bit) setting, so exactly what you want to achieve.

-- 
Cheers,

David / dhildenb