[PATCH v4] 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 pagetype.

Fixes: 46df8e73a4a3 ("mm: free up PG_slab")

Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
v3 -> v4: rename to enum pagetype, avoiding conflict in f2fs.h and matching
          the name of enum pageflags
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..32722c6e8397b 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 pagetype {
+	/*
+	 * 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)
-- 
2.43.0