linux-kernel - Re: [PATCH 2/8] mm: alloc_contig_freed

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Date:	Wed, 21 Sep 2011 07:07:36 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Russell King <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ankita Garg <ankita@...ibm.com>,
	Daniel Walker <dwalker@...eaurora.org>,
	Mel Gorman <mel@....ul.ie>, Arnd Bergmann <arnd@...db.de>,
	Jesse Barker <jesse.barker@...aro.org>,
	Jonathan Corbet <corbet@....net>,
	Shariq Hasnain <shariq.hasnain@...aro.org>,
	Chunsang Jeong <chunsang.jeong@...aro.org>
Subject: Re: [PATCH 2/8] mm: alloc_contig_freed_pages() added

On Wed, 2011-09-21 at 15:17 +0200, Michal Nazarewicz wrote:
> > This 'struct page *'++ stuff is OK, but only for small, aligned areas.
> > For at least some of the sparsemem modes (non-VMEMMAP), you could walk
> > off of the end of the section_mem_map[] when you cross a MAX_ORDER
> > boundary.  I'd feel a little bit more comfortable if pfn_to_page() was
> > being done each time, or only occasionally when you cross a section
> > boundary.
> 
> I'm fine with that.  I've used pointer arithmetic for performance reasons
> but if that may potentially lead to bugs then obviously pfn_to_page()  
> should be used

pfn_to_page() on x86 these days is usually:

	#define __pfn_to_page(pfn)      (vmemmap + (pfn))

Even for the non-vmemmap sparsemem it stays pretty quick because the
section array is in cache as you run through the loop.

There are ways to _minimize_ the number of pfn_to_page() calls by
checking when you cross a section boundary, or even at a
MAX_ORDER_NR_PAGES boundary.  But, I don't think it's worth the trouble.

-- Dave

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