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

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Message-Id: <20100713154025.7c60c76b.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 13 Jul 2010 15:40:25 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	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 Tue, 13 Jul 2010 15:04:00 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> >> >  2. This can't be help for a case where a section has multiple small holes.
> >>
> >> I agree. But this(not punched hole but not filled section problem)
> >> isn't such case. But it would be better to handle it altogether. :)
> >>
> >> >
> >> > Then, my proposal for HOLES_IN_MEMMAP sparsemem is below.
> >> > ==
> >> > Some architectures unmap memmap[] for memory holes even with SPARSEMEM.
> >> > To handle that, pfn_valid() should check there are really memmap or not.
> >> > For that purpose, __get_user() can be used.
> >>
> >> Look at free_unused_memmap. We don't unmap pte of hole memmap.
> >> Is __get_use effective, still?
> >>
> > __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.
> 
> I mean following as.
> For example, there is a struct page in on 0x20000000.
> 
> int pfn_valid_mapped(unsigned long pfn)
> {
>        struct page *page = pfn_to_page(pfn); /* hole page is 0x2000000 */
>        char *lastbyte = (char *)(page+1)-1;  /* lastbyte is 0x2000001f */
>        char byte;
> 
>        /* We pass this test since free_unused_memmap doesn't unmap pte */
>        if(__get_user(byte, page) != 0)				
>                return 0;

why ? When the page size is 4096 byte.

      0x1ffff000 - 0x1ffffffff
      0x20000000 - 0x200000fff are on the same page. And memory is mapped per page.

What we access by above __get_user() is a byte at [0x20000000, 0x20000001)
and it's unmapped if 0x20000000 is unmapped.

Thanks,
-Kame

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