linux-kernel - Re: Possible partial miss in pages needed for zone's memory map?

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Message-Id: <20080519231937.5fee7cf7.akpm@linux-foundation.org>
Date:	Mon, 19 May 2008 23:19:37 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...urebad.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org
Subject: Re: Possible partial miss in pages needed for zone's memory map?

On Sat, 17 May 2008 14:19:15 +0200 Johannes Weiner <hannes@...urebad.de> wrote:

> Hi,
> 
> I stumbled over the following in the zone initialization code.  Please
> let me know if I overlooked something here.
> 

hm, no takers.  Let's add linux-mm.

> 
> From: Johannes Weiner <hannes@...urebad.de>
> Subject: [PATCH] Don't drop a partial page in a zone's memory map size
> 
> In a zone's present pages number, account for all pages occupied by the
> memory map, including a partial.
> 
> Signed-off-by: Johannes Weiner <hannes@...urebad.de>
> ---
> 
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3378,7 +3378,8 @@ static void __paginginit free_area_init_
>  		 * is used by this zone for memmap. This affects the watermark
>  		 * and per-cpu initialisations
>  		 */
> -		memmap_pages = (size * sizeof(struct page)) >> PAGE_SHIFT;
> +		memmap_pages =
> +			PAGE_ALIGN(size * sizeof(struct page)) >> PAGE_SHIFT;
>  		if (realsize >= memmap_pages) {
>  			realsize -= memmap_pages;
>  			printk(KERN_DEBUG

I looked in there for 30 seconds and collapsed in confusion over which
variables are in which units.

Hint: never ever name a variable or a /proc file or your cat 
or anything else anything dimensionless like "size".  It can always be
replaced with something which communicates its units.  zones_nrbytes,
zholes_nrpages, etc.

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