Re: [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order

[Johannes Weiner <hannes@cmpxchg.org>]

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 buddy.
> 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 <richard.weiyang@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: David Hildenbrand <david@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> 
> ---
> 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 *zone, struct page *page,
>  /* Look for a buddy that straddles start_pfn */
>  static unsigned long find_large_buddy(unsigned long start_pfn)
>  {
> -	int order = 0;
> +	/*
> +	 * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing
> +	 * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking
> +	 * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy,
> +	 * the starting order does not matter.
> +	 */
> +	int order = 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.