Re: [RFC] Tight check of pfn_valid on sparsemem

[Minchan Kim]

On Tue, Jul 13, 2010 at 1:23 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Tue, 13 Jul 2010 13:11:14 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>
>> On Tue, Jul 13, 2010 at 12:19 PM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
>> > On Tue, 13 Jul 2010 00:53:48 +0900
>> > Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>> >
>> >> Kukjin, Could you test below patch?
>> >> I don't have any sparsemem system. Sorry.
>> >>
>> >> -- CUT DOWN HERE --
>> >>
>> >> Kukjin reported oops happen while he change min_free_kbytes
>> >> http://www.spinics.net/lists/arm-kernel/msg92894.html
>> >> It happen by memory map on sparsemem.
>> >>
>> >> The system has a memory map following as.
>> >>      section 0             section 1              section 2
>> >> 0x20000000-0x25000000, 0x40000000-0x50000000, 0x50000000-0x58000000
>> >> SECTION_SIZE_BITS 28(256M)
>> >>
>> >> It means section 0 is an incompletely filled section.
>> >> Nontheless, current pfn_valid of sparsemem checks pfn loosely.
>> >>
>> >> It checks only mem_section's validation.
>> >> So in above case, pfn on 0x25000000 can pass pfn_valid's validation check.
>> >> It's not what we want.
>> >>
>> >> The Following patch adds check valid pfn range check on pfn_valid of sparsemem.
>> >>
>> >> Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
>> >> Reported-by: Kukjin Kim <kgene.kim@xxxxxxxxxxx>
>> >>
>> >> P.S)
>> >> It is just RFC. If we agree with this, I will make the patch on mmotm.
>> >>
>> >> --
>> >>
>> >> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> >> index b4d109e..6c2147a 100644
>> >> --- a/include/linux/mmzone.h
>> >> +++ b/include/linux/mmzone.h
>> >> @@ -979,6 +979,8 @@ struct mem_section {
>> >>         struct page_cgroup *page_cgroup;
>> >>         unsigned long pad;
>> >>  #endif
>> >> +       unsigned long start_pfn;
>> >> +       unsigned long end_pfn;
>> >>  };
>> >>
>> >
>> > I have 2 concerns.
>> >  1. This makes mem_section twice. Wasting too much memory and not good for cache.
>> >    But yes, you can put this under some CONFIG which has small number of mem_section[].
>> >
>>
>> I think memory usage isn't a big deal. but for cache, we can move
>> fields into just after section_mem_map.
>>
> I don't think so. This addtional field can eat up the amount of memory you saved
> by unmap.

Agree.

>
>> >  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;
	 /*
	  * (0x20000000 & PAGE_MASK) == (0x2000001f & PAGE_MASK)
	  * So, return 1, it is wrong result.
	  */
       if ((((unsigned long)page) & PAGE_MASK) ==
           (((unsigned long)lastbyte) & PAGE_MASK))
               return 1;
       return (__get_user(byte,lastbyte) == 0);
}

Am I missing something?


-- 
Kind regards,
Minchan Kim
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