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

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Message-ID: <AANLkTin8tdw4VmfCPHE0TR3f-l7ao8ngQJcepTDPpMAC@mail.gmail.com>
Date:	Wed, 14 Jul 2010 15:44:41 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>, linux@....linux.org.uk,
	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

Hi, Kame.

On Wed, Jul 14, 2010 at 9:23 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Wed, 14 Jul 2010 01:44:23 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
>
>> > If you _really_ can't make the section size smaller, and the vast
>> > majority of the sections are fully populated, you could hack something
>> > in.  We could, for instance, have a global list that's mostly readonly
>> > which tells you which sections need to be have their sizes closely
>> > inspected.  That would work OK if, for instance, you only needed to
>> > check a couple of memory sections in the system.  It'll start to suck if
>> > you made the lists very long.
>>
>> Thanks for advise. As I say, I hope Russell accept 16M section.
>>
>
> It seems what I needed was good sleep....
> How about this if 16M section is not acceptable ?
>
> == NOT TESTED AT ALL, EVEN NOT COMPILED ==
>
> register 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.

It's a very good idea. :)
But can this handle case that a page on memmap pages have struct page
descriptor of hole?
I mean one page can include 128 page descriptor(4096 / 32).
In there, 64 page descriptor is valid but remain 64 page descriptor is on hole.
In this case, free_memmap doesn't free the page.

I think most of system will have aligned memory of 512K(4K * 128).
But I am not sure.
-- 
Kind regards,
Minchan Kim
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