linux-kernel - Re: [PATCH 19/22] Update NR_FREE

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Date:	Thu, 23 Apr 2009 16:06:10 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	mel@....ul.ie, linux-mm@...ck.org, kosaki.motohiro@...fujitsu.com,
	cl@...ux-foundation.org, npiggin@...e.de,
	linux-kernel@...r.kernel.org, ming.m.lin@...el.com,
	yanmin_zhang@...ux.intel.com, peterz@...radead.org,
	penberg@...helsinki.fi
Subject: Re: [PATCH 19/22] Update NR_FREE_PAGES only as necessary

On Wed, 22 Apr 2009 14:53:24 +0100
Mel Gorman <mel@....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));
>  

<head spins>

Is this all a slow and obscure way of doing

	VM_BUG_ON(order > MAX_ORDER);

?

If not, what _is_ it asserting?
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