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Date: Fri, 7 Jun 2024 09:19:09 +0200
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Subject: Re: [PATCH v3] mm: convert page type macros to enum
To: Stephen Brennan <stephen.s.brennan@oracle.com>,
 Andrew Morton <akpm@linux-foundation.org>
Cc: "Vishal Moola (Oracle)" <vishal.moola@gmail.com>, linux-mm@kvack.org,
 Omar Sandoval <osandov@osandov.com>, linux-debuggers@vger.kernel.org,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org,
 Hao Ge <gehao@kylinos.cn>
References: <20240607001116.1061485-1-stephen.s.brennan@oracle.com>
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From: Vlastimil Babka <vbabka@suse.cz>
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In-Reply-To: <20240607001116.1061485-1-stephen.s.brennan@oracle.com>
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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@oracle.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> 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)