About SECTION_SIZE_BITS for Sparsemem

[Kukjin Kim]

Minchan Kim wrote:
> 
> Hello.
> 
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.
> 
Yes, I did it, and no kernel panic happens :-)

Same test...
[root at Samsung ~]# cat /proc/sys/vm/min_free_kbytes
2736
[root at Samsung ~]# echo "2730" > /proc/sys/vm/min_free_kbytes
[root at Samsung ~]#
[root at Samsung ~]# cat /proc/sys/vm/min_free_kbytes
2730


> @@ -2824,8 +2825,13 @@ static void setup_zone_migrate_reserve(struct zone
> *zone)
>         for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
>                 if (!pfn_valid(pfn))
>                         continue;
> +
>                 page = pfn_to_page(pfn);
> 
> +                /* Watch for unexpected holes punched in the memmap */
> +                if (!memmap_valid_within(pfn, page, zone))
> +                        continue;
> +
>                 /* Watch out for overlapping nodes */
>                 if (page_to_nid(page) != zone_to_nid(zone))
>                         continue;
> 
> 
> 

...Could you please explain about this issue?

Thanks.

Best regards,
Kgene.
--
Kukjin Kim <kgene.kim at samsung.com>, Senior Engineer,
SW Solution Development Team, Samsung Electronics Co., Ltd.