Re: [RFC] Tight check of pfn_valid on sparsemem

[Minchan Kim]

On Tue, Jul 13, 2010 at 3:40 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Tue, 13 Jul 2010 15:04:00 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> 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.

sizeof(struct page) is 32 byte.
So lastbyte is address of struct page + 32 byte - 1.

> What we access by above __get_user() is a byte at [0x20000000, 0x20000001)

Right.

> and it's unmapped if 0x20000000 is unmapped.

free_unused_memmap doesn't unmap pte although it returns the page to
free list of buddy.

>
> Thanks,
> -Kame
>
>



-- 
Kind regards,
Minchan Kim
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