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

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Message-ID: <2e501e48-8604-4813-b76a-d467cad67f53@redhat.com>
Date: Sun, 20 Apr 2025 09:12:10 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: mingzhe.yang@...com, willy@...radead.org, ziy@...dia.com,
 mhocko@...e.com, vbabka@...e.cz, surenb@...gle.com, linux-mm@...ck.org,
 jackmanb@...gle.com, hannes@...xchg.org, linux-kernel@...r.kernel.org,
 Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH 1/1] mm/rmap: optimize MM-ID mapcount handling with union


>   	/* 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