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

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Date: Tue, 18 Jun 2024 14:14:24 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Stephen Brennan <stephen.s.brennan@...cle.com>
Cc: linux-mm@...ck.org, Vlastimil Babka <vbabka@...e.cz>,
 "Vishal Moola (Oracle)" <vishal.moola@...il.com>, Omar Sandoval
 <osandov@...ndov.com>, David Hildenbrand <david@...hat.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, Hao Ge <gehao@...inos.cn>,
 linux-debuggers@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm: convert page type macros to enum

On Mon, 17 Jun 2024 15:14:58 -0700 Stephen Brennan <stephen.s.brennan@...cle.com> wrote:

> > -#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.
> > +	 */
> 
> This comment is a bit out of place now, because it refers to David's
> change which has not yet been applied. Maybe it should continue to read
> 
> "Reserve	0x0000007f to catch underflows of _mapcount"
> 
> until "mm: allow reuse of the lower 16 bit of the page type with an
> actual type" changes it?

Updated, thanks.

The post-Stephen code is now

/*
 * For pages that are never mapped to userspace,
 * page_type may be used.  Because it is initialised to -1, we invert the
 * sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
 * __ClearPageFoo *sets* the bit used for PageFoo.  We reserve a few high and
 * low bits so that an underflow or overflow of _mapcount won't be
 * mistaken for a page type value.
 */

enum pagetype {
	PG_buddy	= 0x00000080,
	PG_offline	= 0x00000100,
	PG_table	= 0x00000200,
	PG_guard	= 0x00000400,
	PG_hugetlb	= 0x00000800,
	PG_slab		= 0x00001000,

	PAGE_TYPE_BASE	= 0xf0000000,
	/* Reserve 0x0000007f to catch underflows of _mapcount */
	PAGE_MAPCOUNT_RESERVE	= -128,
};


And the post-David code is now:

/*
 * For pages that are never mapped to userspace,
 * page_type may be used.  Because it is initialised to -1, we invert the
 * sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
 * __ClearPageFoo *sets* the bit used for PageFoo.  We reserve a few high and
 * low bits so that an underflow or overflow of _mapcount won't be
 * mistaken for a page type value.
 */

enum pagetype {
	PG_buddy	= 0x40000000,
	PG_offline	= 0x20000000,
	PG_table	= 0x10000000,
	PG_guard	= 0x08000000,
	PG_hugetlb	= 0x04008000,
	PG_slab		= 0x02000000,

	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.
	 */
	PAGE_MAPCOUNT_RESERVE	= ~0x0000ffff,
};