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.
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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