Date: Thu, 2 Oct 2008 14:30:04 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Andy Whitcroft <apw@shadowen.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kniht@linux.vnet.ibm.com,
       mel@csn.ul.ie, apw@shadowen.org
Subject: Re: [PATCH 1/1] handle initialising compound pages at orders
 greater than MAX_ORDER
Message-Id: <20081002143004.5fec3952.akpm@linux-foundation.org>
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References: <1222964396-25031-1-git-send-email-apw@shadowen.org>
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On Thu,  2 Oct 2008 17:19:56 +0100
Andy Whitcroft <apw@shadowen.org> wrote:

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -268,13 +268,14 @@ void prep_compound_page(struct page *page, unsigned long order)
>  {
>  	int i;
>  	int nr_pages = 1 << order;
> +	struct page *p = page + 1;
>  
>  	set_compound_page_dtor(page, free_compound_page);
>  	set_compound_order(page, order);
>  	__SetPageHead(page);
> -	for (i = 1; i < nr_pages; i++) {
> -		struct page *p = page + i;
> -
> +	for (i = 1; i < nr_pages; i++, p++) {
> +		if (unlikely((i & (MAX_ORDER_NR_PAGES - 1)) == 0))
> +			p = pfn_to_page(page_to_pfn(page) + i);
>  		__SetPageTail(p);
>  		p->first_page = page;
>  	}

gad.  Wouldn't it be clearer to do

	for (i = 1; i < nr_pages; i++) {
		struct page *p = pfn_to_page(i);
		__SetPageTail(p);
		p->first_page = page;
	}

Oh well, I guess we can go with the obfuscated, uncommented version for
now :(

This patch applies to 2.6.26 (and possibly earlier) but I don't think
those kernels can trigger the bug?

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