Re: [RFC] Tight check of pfn_valid on sparsemem

[Minchan Kim <minchan.kim@gmail.com>]

On Tue, Jul 13, 2010 at 12:19 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Tue, 13 Jul 2010 00:53:48 +0900
> Minchan Kim <minchan.kim@gmail.com> 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@gmail.com>
>> Reported-by: Kukjin Kim <kgene.kim@samsung.com>
>>
>> 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.

>  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?




-- 
Kind regards,
Minchan Kim

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