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

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Message-Id: <20100713163417.17895202.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 13 Jul 2010 16:34:17 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Minchan Kim <minchan.kim@...il.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, 13 Jul 2010 08:20:09 +0100
Russell King - ARM Linux <linux@....linux.org.uk> wrote:

> 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.

Note: pfn_valid() is for checking "there is memmap". 

> It'd be cheaper to use our flatmem implementation of pfn_valid() instead.
> 
Hmm. IIUC, pfn_valid() succeeds in almost all case if there is a section.
But yes, I'm not familar with ARM. 

I love another idea as I've already shown as preparing _a_ page filled with
0x00004000 and map it into the all holes. PG_reserved will help almost all case
even if it's ugly.

Anyway, sparsemem is designed to be aligned to SECTION_SIZE of memmap.
Please avoid adding new Spaghetti code without proper configs.
Thanks,
-Kame

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