About SECTION_SIZE_BITS for Sparsemem

[Minchan Kim]

Hi, Mel and Kame.

On Mon, Jul 12, 2010 at 7:13 PM, Kukjin Kim <kgene.kim at samsung.com> wrote:
> 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?

The setup_zone_migrate_reserve doesn't check memmap hole. I think
compaction would have the  same problem, too. I don't know there is a
problem in elsewhere.
Anyway, I think memmap_valid_within calling whenever walking whole pfn
range isn't a good solution. We already have pfn_valid. Could we check
this in there?
For example, mem_section have a valid pfn range and then valid section
can test it in pfn_valid.

What do you think about it?

P.S)
I know Mel is very busy to test to avoid writeback in direct reclaim.
I can wait on you. :)
Kukjin. Is this a urgent issue? If it isn't, please wait on our mm guys. :)

-- 
Kind regards,
Minchan Kim