Re: [PATCH v4] mm: convert page type macros to enum

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 17 Jun 2024 13:34:21 -0700 Stephen Brennan <stephen.s.brennan@oracle.com> wrote:

> >>> Fixes: 46df8e73a4a3 ("mm: free up PG_slab")
> >>
> >> Should we backport this into 6.9.x?
> >
> > Hi Andrew,
> >
> > Looks like commit 46df8e73a4a3 ("mm: free up PG_slab") is introduced in
> > the v6.10-rc's, and not backported to 6.9. So PG_slab is still part of
> 
> Hi Andrew,
> 
> I saw that you've merged this into mm-unstable, thank you!
> 
> Since 46df8e73a4a3 ("mm: free up PG_slab") is part of the current 6.10
> RC, it would be great if this patch could be part of the 6.10 release so
> we don't release a kernel missing the PG_slab info.
> 
> Can you confirm if mm-unstable will get merged in this release cycle? Or
> else, would it be possible to include it in a branch that will?

Turns out the patch as sent was based on David's "mm: allow reuse of
the lower 16 bit of the page type with an actual type", which changed
the page flags a lot,  I redid this patch thusly:

--- a/include/linux/page-flags.h~mm-convert-page-type-macros-to-enum
+++ a/include/linux/page-flags.h
@@ -944,15 +944,22 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpois
  * mistaken for a page type value.
  */
 
-#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 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	= 0x00000080,
+	PG_offline	= 0x00000100,
+	PG_table	= 0x00000200,
+	PG_guard	= 0x00000400,
+	PG_hugetlb	= 0x00000800,
+	PG_slab		= 0x00001000,
+
+	PAGE_TYPE_BASE	= 0xf0000000,
+	PAGE_MAPCOUNT_RESERVE	= -128,
+};
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
_

(please check carefully)

and David's later "mm: allow reuse of the lower 16 bit of the page type
with an actual type" becomes 

 include/linux/mm_types.h   |    5 +++++
 include/linux/page-flags.h |   16 ++++++++--------
 2 files changed, 13 insertions(+), 8 deletions(-)

--- a/include/linux/mm_types.h~mm-allow-reuse-of-the-lower-16-bit-of-the-page-type-with-an-actual-type
+++ a/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;
 
--- a/include/linux/page-flags.h~mm-allow-reuse-of-the-lower-16-bit-of-the-page-type-with-an-actual-type
+++ a/include/linux/page-flags.h
@@ -951,15 +951,15 @@ enum pagetype {
 	 * allow owners that set a type to reuse the lower 16 bit for their own
 	 * purposes.
 	 */
-	PG_buddy	= 0x00000080,
-	PG_offline	= 0x00000100,
-	PG_table	= 0x00000200,
-	PG_guard	= 0x00000400,
-	PG_hugetlb	= 0x00000800,
-	PG_slab		= 0x00001000,
+	PG_buddy	= 0x40000000,
+	PG_offline	= 0x20000000,
+	PG_table	= 0x10000000,
+	PG_guard	= 0x08000000,
+	PG_hugetlb	= 0x04008000,
+	PG_slab		= 0x02000000,
 
-	PAGE_TYPE_BASE	= 0xf0000000,
-	PAGE_MAPCOUNT_RESERVE	= -128,
+	PAGE_TYPE_BASE	= 0x80000000,
+	PAGE_MAPCOUNT_RESERVE	=  (~0x0000ffff),
 };
 
 #define PageType(page, flag)						\
_

and that patch's fixup becomes

--- a/include/linux/page-flags.h~mm-allow-reuse-of-the-lower-16-bit-of-the-page-type-with-an-actual-type-fix
+++ a/include/linux/page-flags.h
@@ -955,7 +955,7 @@ enum pagetype {
 	PG_offline	= 0x20000000,
 	PG_table	= 0x10000000,
 	PG_guard	= 0x08000000,
-	PG_hugetlb	= 0x04008000,
+	PG_hugetlb	= 0x04000000,
 	PG_slab		= 0x02000000,
 
 	PAGE_TYPE_BASE	= 0x80000000,
_

and "mm/zsmalloc: use a proper page type" becomes, in part,

--- a/include/linux/page-flags.h~mm-zsmalloc-use-a-proper-page-type
+++ a/include/linux/page-flags.h
@@ -957,6 +957,7 @@ enum pagetype {
 	PG_guard	= 0x08000000,
 	PG_hugetlb	= 0x04000000,
 	PG_slab		= 0x02000000,
+	PG_zsmalloc	= 0x01000000,
 
 	PAGE_TYPE_BASE	= 0x80000000,
 	PAGE_MAPCOUNT_RESERVE	=  (~0x0000ffff),



and the end result is identical to yesterday's mm-everything so that's
all good.

However I wouldn't want to send the altered version of "mm: convert
page type macros to enum" into 6.10-rcX because it gets so altered by
David's mm-unstable changes for the next merge window.  The new version
of the hotfixes patch won't have had any valid testing on its own.

So I'll temporarily drop David's "mm: page_type, zsmalloc and
page_mapcount_reset()" series from mm-unstable.  To permit the new "mm:
convert page type macros to enum" to get some linux-next exposure. 
David, please remind me to restore that series in a week or so?