[PATCH] mm: convert page type macros to enum

[PATCH] mm: convert page type macros to enum

[Stephen Brennan]

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@oracle.com>
---
 include/linux/page-flags.h | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 104078afe0b16..64fc191a75e8d 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -947,12 +947,15 @@ 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
+
+enum page_type {
+	PG_buddy	= 0x00000080,
+	PG_offline	= 0x00000100,
+	PG_table	= 0x00000200,
+	PG_guard	= 0x00000400,
+	PG_hugetlb	= 0x00000800,
+	PG_slab		= 0x00001000,
+};
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
-- 
2.43.0

Re: [PATCH] mm: convert page type macros to enum

[Omar Sandoval]

On Thu, Jun 06, 2024 at 10:25:29AM -0700, 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@oracle.com>
> ---
>  include/linux/page-flags.h | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 104078afe0b16..64fc191a75e8d 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -947,12 +947,15 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
>  #define PAGE_TYPE_BASE	0xf0000000
>  /* Reserve		0x0000007f to catch underflows of _mapcount */
>  #define PAGE_MAPCOUNT_RESERVE	-128

Adding linux-debuggers to Cc.

Can we also get PAGE_TYPE_BASE and PAGE_MAPCOUNT_RESERVE as enums? That
would make it possible for debuggers to implement PageType() and
page_has_type().

> -#define PG_buddy	0x00000080
> -#define PG_offline	0x00000100
> -#define PG_table	0x00000200
> -#define PG_guard	0x00000400
> -#define PG_hugetlb	0x00000800
> -#define PG_slab		0x00001000
> +
> +enum page_type {
> +	PG_buddy	= 0x00000080,
> +	PG_offline	= 0x00000100,
> +	PG_table	= 0x00000200,
> +	PG_guard	= 0x00000400,
> +	PG_hugetlb	= 0x00000800,
> +	PG_slab		= 0x00001000,
> +};
>  
>  #define PageType(page, flag)						\
>  	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> -- 
> 2.43.0
>