linux-kernel - Re: [PATCH] Tight check of pfn_valid on sparsemem

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Message-ID: <alpine.DEB.2.00.1007261136160.5438@router.home>
Date:	Mon, 26 Jul 2010 11:40:44 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Minchan Kim <minchan.kim@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Kukjin Kim <kgene.kim@...sung.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	linux-mm <linux-mm@...ck.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

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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