---
> 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
