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

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Message-Id: <20100714161045.ef028769.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 14 Jul 2010 16:10:45 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.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, 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.

If my code doesn't seem to work as above, it's bug.


Thanks,
-Kame

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