Date: Mon, 26 Jul 2010 11:40:44 -0500 (CDT)
From: Christoph Lameter <cl@linux-foundation.org>
To: Minchan Kim <minchan.kim@gmail.com>
cc: Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Kukjin Kim <kgene.kim@samsung.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-mm <linux-mm@kvack.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Mel Gorman <mel@csn.ul.ie>, Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4
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On Tue, 27 Jul 2010, Minchan Kim wrote:

> This patch registers address of mem_section to memmap itself's page struct's
> pg->private field. This means the page is used for memmap of the section.
> Otherwise, the page is used for other purpose and memmap has a hole.

What if page->private just happens to be the value of the page struct?
Even if that is not possible today, someday someone may add new
functionality to the kernel where page->pivage == page is used for some
reason.

Checking for PG_reserved wont work?

> +void mark_valid_memmap(unsigned long start, unsigned long end);
> +
> +#ifdef CONFIG_ARCH_HAS_HOLES_MEMORYMODEL
> +static inline int memmap_valid(unsigned long pfn)
> +{
> +	struct page *page = pfn_to_page(pfn);
> +	struct page *__pg = virt_to_page(page);
> +	return page_private(__pg) == (unsigned long)__pg;

Hmmm.. hmmm....
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