linux-kernel - Re: [rfc 01/10] Pageflags: Use an enum for the flags

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Date:	Mon, 3 Mar 2008 10:09:23 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <clameter@....com>
Cc:	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, apw@...dowen.org,
	linux-mm@...ck.org
Subject: Re: [rfc 01/10] Pageflags: Use an enum for the flags

On (29/02/08 20:05), Christoph Lameter didst pronounce:
> Use an enum to ease the maintenance of page flags. This is going to change the
> numbering from 0 to 18.
> 
> Signed-off-by: Christoph Lameter <clameter@....com>
> 
> ---
>  include/linux/page-flags.h |   54 ++++++++++++++++++++-------------------------
>  1 file changed, 24 insertions(+), 30 deletions(-)
> 
> Index: linux-2.6/include/linux/page-flags.h
> ===================================================================
> --- linux-2.6.orig/include/linux/page-flags.h	2008-02-29 14:54:22.000000000 -0800
> +++ linux-2.6/include/linux/page-flags.h	2008-02-29 15:00:00.000000000 -0800
> @@ -67,35 +67,28 @@
>   * FLAGS_RESERVED which defines the width of the fields section
>   * (see linux/mmzone.h).  New flags must _not_ overlap with this area.
>   */
> -#define PG_locked	 	 0	/* Page is locked. Don't touch. */
> -#define PG_error		 1
> -#define PG_referenced		 2
> -#define PG_uptodate		 3
> -
> -#define PG_dirty	 	 4
> -#define PG_lru			 5
> -#define PG_active		 6
> -#define PG_slab			 7	/* slab debug (Suparna wants this) */
> -
> -#define PG_owner_priv_1		 8	/* Owner use. If pagecache, fs may use*/
> -#define PG_arch_1		 9
> -#define PG_reserved		10
> -#define PG_private		11	/* If pagecache, has fs-private data */
> -
> -#define PG_writeback		12	/* Page is under writeback */
> -#define PG_compound		14	/* Part of a compound page */
> -#define PG_swapcache		15	/* Swap page: swp_entry_t in private */
> -
> -#define PG_mappedtodisk		16	/* Has blocks allocated on-disk */
> -#define PG_reclaim		17	/* To be reclaimed asap */
> -#define PG_buddy		19	/* Page is free, on buddy lists */
> -
> -/* PG_readahead is only used for file reads; PG_reclaim is only for writes */
> -#define PG_readahead		PG_reclaim /* Reminder to do async read-ahead */
> -
> -/* PG_owner_priv_1 users should have descriptive aliases */
> -#define PG_checked		PG_owner_priv_1 /* Used by some filesystems */
> -#define PG_pinned		PG_owner_priv_1	/* Xen pinned pagetable */

These two verbose descriptions appear to go missing in the enum below.
It probably is intentional because they are aliases but you can preseve them
being aliases with

PG_checked = PG_owner_priv_1,
PG_pinned = PG_owner_priv_1,

That would keep the comments about flags in one place. I note that you
move the comment to the macro definitions in a later patch but maybe having
comments about some flags in multiple places is not very desirable?

> +enum pageflags {
> +	PG_locked,		/* Page is locked. Don't touch. */
> +	PG_error,
> +	PG_referenced,
> +	PG_uptodate,
> +	PG_dirty,
> +	PG_lru,
> +	PG_active,
> +	PG_slab,
> +	PG_owner_priv_1,	/* Owner use. If pagecache, fs may use*/
> +	PG_arch_1,
> +	PG_reserved,
> +	PG_private,		/* If pagecache, has fs-private data */
> +	PG_writeback,		/* Page is under writeback */
> +	PG_compound,		/* A compound page */
> +	PG_swapcache,		/* Swap page: swp_entry_t in private */
> +	PG_mappedtodisk,	/* Has blocks allocated on-disk */
> +	PG_reclaim,		/* To be reclaimed asap */
> +	/* PG_readahead is only used for file reads; PG_reclaim is only for writes */
> +	PG_readahead = PG_reclaim, /* Reminder to do async read-ahead */
> +	PG_buddy,		/* Page is free, on buddy lists */
> +	NR_PAGEFLAGS,		/* For verification purposes */
>  
>  #if (BITS_PER_LONG > 32)
>  /*
> @@ -105,8 +98,9 @@
>   * 64 bit  |           FIELDS             | ??????         FLAGS         |
>   *         63                            32                              0
>   */
> -#define PG_uncached		31	/* Page has been mapped as uncached */
> +	PG_uncached = 31,		/* Page has been mapped as uncached */
>  #endif
> +};
>  
>  /*
>   * Manipulation of page state flags
> 
> -- 
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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