Re: [PATCH 19/22] Update NR_FREE_PAGES only as necessary

Andrew Morton
2009-04-23T23:13:26Z

On Wed, 22 Apr 2009 14:53:24 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> When pages are being freed to the buddy allocator, the zone
> NR_FREE_PAGES counter must be updated. In the case of bulk per-cpu page
> freeing, it's updated once per page. This retouches cache lines more
> than necessary. Update the counters one per per-cpu bulk free.
>
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -460,7 +460,6 @@ static inline void __free_one_page(struct page *page,
> int migratetype)
> {
> unsigned long page_idx;
> - int order_size = 1 << order;
>
> if (unlikely(PageCompound(page)))
> if (unlikely(destroy_compound_page(page, order)))
> @@ -470,10 +469,9 @@ static inline void __free_one_page(struct page *page,
>
> page_idx = page_to_pfn(page) & ((1 << MAX_ORDER) - 1);
>
> - VM_BUG_ON(page_idx & (order_size - 1));
> + VM_BUG_ON(page_idx & ((1 << order) - 1));
> VM_BUG_ON(bad_range(zone, page));
>



Is this all a slow and obscure way of doing

VM_BUG_ON(order > MAX_ORDER);

?

If not, what _is_ it asserting?