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Sep 2025, at 10:33, Johannes Weiner wrote: > On Tue, Sep 02, 2025 at 02:58:07AM +0000, Wei Yang wrote: >> We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large bu= ddy. >> While if the order is less than start_pfn aligned order, we would get = the >> same pfn and do the same check again. >> >> Iterate from start_pfn aligned order to reduce duplicated work. >> >> Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@= gmail.com >> Signed-off-by: Wei Yang >> Cc: Johannes Weiner >> Cc: Zi Yan >> Cc: Vlastimil Babka >> Cc: David Hildenbrand >> Signed-off-by: Andrew Morton >> Reviewed-by: Zi Yan >> >> --- >> v2: add comment on assignment of order >> --- >> mm/page_alloc.c | 8 +++++++- >> 1 file changed, 7 insertions(+), 1 deletion(-) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 07d79ae557f8..5d9ceca869e5 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zo= ne, struct page *page, >> /* Look for a buddy that straddles start_pfn */ >> static unsigned long find_large_buddy(unsigned long start_pfn) >> { >> - int order =3D 0; >> + /* >> + * If start_pfn is not an order-0 PageBuddy, next PageBuddy containi= ng >> + * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checki= ng >> + * the order with __ffs(start_pfn). If start_pfn is order-0 PageBudd= y, >> + * the starting order does not matter. >> + */ >> + int order =3D start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER; > > This should be __ffs(start_pfn) - 1, no? > > If you have the lowest bit set in the pfn, you should check the > order-1 buddy to the left first. But ffs(1) is already 1, which means > the loop will check order-2 next. __ffs() seems different from usespace ffs() and is 0-index, so __ffs() is= what you mean ffs() - 1. Best Regards, Yan, Zi