linux-kernel - Re: Regression - locking (all from 2.6.28)

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Message-Id: <1236276274.18136.32.camel@pc1117.cambridge.arm.com>
Date:	Thu, 05 Mar 2009 18:04:34 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	jan sonnek <ha2nny@...il.com>, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Whitcroft <apw@...dowen.org>
Subject: Re: Regression - locking (all from 2.6.28)

On Wed, 2009-03-04 at 16:54 -0800, Dave Hansen wrote:
> On Tue, 2009-03-03 at 15:01 +0000, Catalin Marinas wrote:
> > > +     /* mem_map scanning */
> > > +     for_each_online_node(i) {
> > > +             struct page *page, *end;
> > > +
> > > +             page = NODE_MEM_MAP(i);
> > > +             end  = page + NODE_DATA(i)->node_spanned_pages;
> > > +
> > > +             scan_block(page, end, NULL);
> > > +     }
[...]
> The above is *not* a valid code sequence.
> 
> It is valid with discontig, but isn't valid for sparsemem.  You simply
> can't expect to do math on 'struct page' pointers for any granularity
> larger than MAX_ORDER_NR_PAGES.
> 
> Also, we don't even define NODE_MEM_MAP() for all configurations so that
> code snippet won't even compile.  We would be smart to kill that macro.
> 
> One completely unoptimized thing you can do which will scan a 'struct
> page' at a time is this:
> 
> 	for_each_online_node(i) {
> 		unsigned long pfn;
> 		for (pfn = node_start_pfn(i); pfn < node_end_pfn(i); pfn++) {
> 			struct page *page;
> 			if (!pfn_valid(pfn))
> 				continue;
> 			page = pfn_to_page(pfn);
> 			scan_block(page, page+1, NULL);
> 		}
> 	}
> 		
> The way to optimize it would be to call scan_block() only once for each
> MAX_ORDER_NR_PAGES that you encounter.  The other option would be to use
> the active_regions functions to walk the memory.  
> 
> Is there a requirement to reduce the number of calls to scan_block()
> here?

I think the improvement wouldn't be that big since scan_block() is
pretty time consuming as it checks every value that looks like a pointer
against a prio_tree.

BTW, is there a way to know whether the page is in use or on the free
list? Is page_count(page) feasible?

Thanks.

-- 
Catalin

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