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SubjectRe: [PATCH 1/1] handle initialising compound pages at orders greater than MAX_ORDER
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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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