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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/1] handle initialising compound pages at orders greater than MAX_ORDER
Date: Fri, 3 Oct 2008 16:43:28 +1000
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Cc: Andy Whitcroft <apw@shadowen.org>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, kniht@linux.vnet.ibm.com, mel@csn.ul.ie
References: <1222964396-25031-1-git-send-email-apw@shadowen.org> <20081002143004.5fec3952.akpm@linux-foundation.org>
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On Friday 03 October 2008 07:30, Andrew Morton wrote:
> 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?

I think the problem is that pfn_to_page isn't always trivial. I would
prefer to have seen a new function for hugetlb to use, and keep the
branch-less version for the page allocator itself.
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