linux-kernel - Re: [PATCH v2 2/6] mm: allow reuse of the lower 16 bit of the page type with an actual type

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Date: Wed, 29 May 2024 18:00:15 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Mike Rapoport <rppt@...nel.org>, Minchan Kim <minchan@...nel.org>,
 Sergey Senozhatsky <senozhatsky@...omium.org>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, wang wei <a929244872@....com>
Subject: Re: [PATCH v2 2/6] mm: allow reuse of the lower 16 bit of the page
 type with an actual type

>   
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index d1bdbaaccc964..f060db808102c 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -945,15 +945,19 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
>    * mistaken for a page type value.
>    */
>   
> -#define PAGE_TYPE_BASE	0xf0000000
> -/* Reserve		0x0000007f to catch underflows of _mapcount */
> -#define PAGE_MAPCOUNT_RESERVE	-128
> -#define PG_buddy	0x00000080
> -#define PG_offline	0x00000100
> -#define PG_table	0x00000200
> -#define PG_guard	0x00000400
> -#define PG_hugetlb	0x00000800
> -#define PG_slab		0x00001000
> +#define PAGE_TYPE_BASE	0x80000000
> +/*
> + * Reserve 0xffff0000 - 0xfffffffe to catch _mapcount underflows and
> + * allow owners that set a type to reuse the lower 16 bit for their own
> + * purposes.
> + */
> +#define PG_buddy	0x40000000
> +#define PG_offline	0x20000000
> +#define PG_table	0x10000000
> +#define PG_guard	0x08000000
> +#define PG_hugetlb	0x04008000

As Wang Wei points out, the 0 and 8 look too similar on my screen ;)

This should be

#define PG_hugetlb	0x04000000

-- 
Cheers,

David / dhildenb