From bdb5439c3f5d1dbf50a149c6ecb1cbdd1d3b182d Mon Sep 17 00:00:00 2001 From: Keir Fraser Date: Wed, 1 Sep 2010 10:17:49 +0100 Subject: [PATCH] x86_64: Ensure frame-table compression leaves MAX_ORDER aligned contiguous ranges of page_info structs. This allows page-pointer arithmetic in places like our buddy allocator. This restriction was already implicitly guaranteed, but it is good to make it explicit in the pdx-related initialisation. Signed-off-by: Keir Fraser --- xen/arch/x86/x86_64/mm.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c index 271cea5ef3..126b2c9421 100644 --- a/xen/arch/x86/x86_64/mm.c +++ b/xen/arch/x86/x86_64/mm.c @@ -163,9 +163,15 @@ void *do_page_walk(struct vcpu *v, unsigned long addr) void __init pfn_pdx_hole_setup(unsigned long mask) { - unsigned int i, j, bottom_shift, hole_shift; + unsigned int i, j, bottom_shift = 0, hole_shift = 0; - for ( hole_shift = bottom_shift = j = 0; ; ) + /* + * We skip the first MAX_ORDER bits, as we never want to compress them. + * This guarantees that page-pointer arithmetic remains valid within + * contiguous aligned ranges of 2^MAX_ORDER pages. Among others, our + * buddy allocator relies on this assumption. + */ + for ( j = MAX_ORDER-1; ; ) { i = find_next_zero_bit(&mask, BITS_PER_LONG, j); j = find_next_bit(&mask, BITS_PER_LONG, i); -- 2.30.2