About SECTION_SIZE_BITS for Sparsemem

[Russell King - ARM Linux]

On Mon, Jul 12, 2010 at 06:52:28PM +0900, Minchan Kim wrote:
> Hello.
> 
> On Mon, Jul 12, 2010 at 5:32 PM, Kukjin Kim <kgene.kim at samsung.com> wrote:
> > Russell,
> >
> > Hi,
> >
> > Kukjin Kim wrote:
> >> Russell wrote:
> >> > So, memory starts at 0x20000000 and finishes at 0x25000000.  That's
> > fine.
> >> > That doesn't mean the section size is 16MB.
> >> >
> >> > As I've already said, the section size has _nothing_ what so ever to do
> >> > with the size of memory, or the granularity of the size of memory.  By
> >> > way of illustration, it is perfectly legal to have a section size of
> >> > 256MB but only have 1MB in a section and this is perfectly legal.  So
> >> > sections do not have to be completely filled.
> >> >
> >> Actually, as you know, the hole's area of mem_map is freed from bootmem if
> > a
> >> section has a hole when initializing sparse memory.
> >>
> >> I identified that a section doesn't need to be a contiguous area of
> > physical
> >> memory when reading your comment with the fact that the mem_map of a
> > section
> >> can be smaller than the size of a section.
> >>
> >> I found, however, the kernel panics when modifying min_free_kbytes file in
> >> the proc filesystem if a section has a hole.
> >>
> >> While processing the change of min_free_kbytes in the kernel, page
> >> descriptors in a hole of an online section is accessed.
> >
> > As I said, following error happens.
> > It would be helpful to me if any opinions or comments.
> >
> 
> Could you test below patch?
> Also, you should select ARCH_HAS_HOLES_MEMORYMODEL in your config.

But hang on.  Where are the hole(s)?

The DRAM setup is:
0x20000000-0x25000000, 0x40000000-0x50000000, 0x50000000-0x58000000

which with SECTION_SIZE_BITS set to 28 gives three sections of memory,
and each sparsemem section does not have a hole.

No zone should cross a sparsemem section boundary.

Moreover, our pfn_valid() now returns false for any and all invalid PFNs.