linux-kernel - Re: [RFC] Tight check of pfn

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Message-ID: <20100713072009.GA19839@n2100.arm.linux.org.uk>
Date:	Tue, 13 Jul 2010 08:20:09 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shaohua Li <shaohua.li@...el.com>,
	Yakui Zhao <yakui.zhao@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	arm-kernel@...ts.infradead.org, kgene.kim@...sung.com,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [RFC] Tight check of pfn_valid on sparsemem

On Tue, Jul 13, 2010 at 03:04:00PM +0900, Minchan Kim wrote:
> > __get_user() works with TLB and page table, the vaddr is really mapped or not.
> > If you got SEGV, __get_user() returns -EFAULT. It works per page granule.

Not in kernel space.  It works on 1MB sections there.

Testing whether a page is mapped by __get_user is a hugely expensive
way to test whether a PFN is valid.  It'd be cheaper to use our
flatmem implementation of pfn_valid() instead.
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