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

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Message-ID: <20090424130648.GI14283@csn.ul.ie>
Date:	Fri, 24 Apr 2009 14:06:48 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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 Thu, Apr 23, 2009 at 04:06:10PM -0700, Andrew Morton wrote:
> 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);
> 
> ?

Nope.

> 
> If not, what _is_ it asserting?
> 

That the page is properly placed. The start of the page being freed has to
be size-of-page-aligned. If it isn't the calculation that works out where
the buddy is will break.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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