On 6/7/24 2:11 AM, Stephen Brennan wrote:
> Changing PG_slab from a page flag to a page type in commit 46df8e73a4a3
> ("mm: free up PG_slab") in has the unintended consequence of removing
> the PG_slab constant from kernel debuginfo. The commit does add the
> value to the vmcoreinfo note, which allows debuggers to find the value
> without hardcoding it. However it's most flexible to continue
> representing the constant with an enum. To that end, convert the page
> type fields into an enum. Debuggers will now be able to detect that
> PG_slab's type has changed from enum pageflags to enum page_type.
>
> Fixes: 46df8e73a4a3 ("mm: free up PG_slab")
>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> v2 -> v3: rebase on mm-unstable
> v1 -> v2: include PAGE_TYPE_BASE and PAGE_MAPCOUNT_RESERVE
>
> include/linux/page-flags.h | 31 +++++++++++++++++--------------
> 1 file changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index f04fea86324d9..7c0a8fd2c8c17 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -945,20 +945,23 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
> * mistaken for a page type value.
> */
>
> -#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 0x04000000
> -#define PG_slab 0x02000000
> -#define PG_zsmalloc 0x01000000
> -#define PAGE_MAPCOUNT_RESERVE (~0x0000ffff)
> +enum page_type {
> + /*
> + * Reserve 0xffff0000 - 0xfffffffe to catch _mapcount underflows and
> + * allow owners that set a type to reuse the lower 16 bit for their own
> + * purposes.
> + */
> + PG_buddy = 0x40000000,
> + PG_offline = 0x20000000,
> + PG_table = 0x10000000,
> + PG_guard = 0x08000000,
> + PG_hugetlb = 0x04000000,
> + PG_slab = 0x02000000,
> + PG_zsmalloc = 0x01000000,
> +
> + PAGE_TYPE_BASE = 0x80000000,
> + PAGE_MAPCOUNT_RESERVE = ~0x0000ffff,
> +};
>
> #define PageType(page, flag) \
> ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
