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

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Date:	Wed, 14 Jul 2010 16:35:22 +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

On Wed, Jul 14, 2010 at 4:10 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Wed, 14 Jul 2010 15:44:41 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
>
>> 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).
> yes.
>
>> 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.
>
> yes. but in that case, there are valid page decriptor for 64pages of holes.
> pfn_valid() should return true but PG_reserved is set.
> (This is usual behavior.)
>
> My intention is that
>
>  - When all 128 page descriptors are unused, free_memmap() will free it.
>   In that case, clear page->private of a page for freed page descriptors.
>
>  - When some of page descriptors are used, free_memmap() can't free it
>   and page->private points to &mem_section. We may have memmap for memory
>   hole but pfn_valid() is a function to check there is memmap or not.
>   The bahavior of pfn_valid() is valid.
>   Anyway, you can't free only half of page.

Okay. I missed PageReserved.
Your idea seems to be good. :)

I looked at pagetypeinfo_showblockcount_print.
It doesn't check PageReserved. Instead of it, it does ugly memmap_valid_within.
Can't we remove it and change it with PageReserved?


-- 
Kind regards,
Minchan Kim
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