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Date: Thu, 6 Jun 2024 10:34:33 -0700
From: Omar Sandoval <osandov@osandov.com>
To: Stephen Brennan <stephen.s.brennan@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, David Hildenbrand <david@redhat.com>,
	Hao Ge <gehao@kylinos.cn>, Vlastimil Babka <vbabka@suse.cz>,
	"Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	linux-debuggers@vger.kernel.org
Subject: Re: [PATCH] mm: convert page type macros to enum
Message-ID: <ZmHzKRkgKOGW8Xmk@telecaster.dhcp.thefacebook.com>
References: <20240606172530.829790-1-stephen.s.brennan@oracle.com>
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In-Reply-To: <20240606172530.829790-1-stephen.s.brennan@oracle.com>

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
>