Re: [PATCH v1 26] mm allow reuse of the lower 16 bit of the page type with an actual type

[wang wei]

---
> As long as the owner sets a page type first, we can allow reuse of the
> lower 18 bit: sufficient to store an offset into a 64 KiB page, which
> is the maximum base page size in *common* configurations (ignoring the
> 256 KiB variant). Restrict it to the head page.
> 
> We'll use that for zsmalloc next, to set a proper type while still
> reusing that field to store information (offset into a base page) that
> cannot go elsewhere for now.
> 
> Fear of running out of bits for storing the actual type? Actually, we
> don't need one bit per type, we could store a single value instead.
> Further, we could likely limit PAGE_TYPE_BASE to a single (highest) bit.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  include/linux/mm_types.h   |  5 +++++
>  include/linux/page-flags.h | 20 ++++++++++++--------
>  2 files changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6b2aeba792c4..598cfedbbfa0 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -157,6 +157,11 @@ struct page {
>  		 *
>  		 * See page-flags.h for a list of page types which are currently
>  		 * stored here.
> +		 *
> +		 * Owners of typed folios may reuse the lower 16 bit of the
> +		 * head page page_type field after setting the page type,
> +		 * but must reset these 16 bit to -1 before clearing the
> +		 * page type.
>  		 */
>  		unsigned int page_type;
>  
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 104078afe0b1..b43e380ffa0b 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -945,14 +945,18 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
>   */
>  
>  #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
> +/*
> + * Reserve		0x0000ffff to catch underflows of _mapcount and
> + * allow owners that set a type to reuse the lower 16 bit for their own
> + * purposes.
> + */
> +#define PAGE_MAPCOUNT_RESERVE	-65536
> +#define PG_buddy	0x00010000
> +#define PG_offline	0x00020000
> +#define PG_table	0x00040000
> +#define PG_guard	0x00080000
> +#define PG_hugetlb	0x00100800

Every PG_XX occupies one bit in my understanding.   But why PG_hugetlb occupies two bits?

> +#define PG_slab		0x00200000
>  
>  #define PageType(page, flag)						\
>  	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)

-- 
2.25.1