On Thu, 18 Feb 2010 21:48:37 -0800
Michael Bohan <mbohan@xxxxxxxxxxxxxx> wrote:
> On 2/18/2010 6:00 PM, KAMEZAWA Hiroyuki wrote:
> > memmap for memory holes should be marked as PG_reserved and never be freed
> > by free_bootmem(). Then, memmap for memory holes will not be in buddy allocator.
> >
> > Again, pfn_valid() just show "there is memmap", not for "there is a valid page"
> >
>
> ARM seems to have been freeing the memmap holes for a long time.
Ouch.
> I'm pretty sure there would be a lot of pushback if we tried to change
> that. For example, in my memory map running FLATMEM, I would be
> consuming an extra ~7 MB of memory if these structures were not freed.
>
> As a compromise, perhaps we could free everything except the first
> 'pageblock_nr_pages' in a hole? This would guarantee that
> move_freepages() doesn't deference any memory that doesn't belong to the
> memmap -- but still only waste a relatively small amount of memory. For
> a 4 MB page block, it should only consume an extra 32 KB per hole in the
> memory map.
>
No. You have to implement pfn_valid() to return correct value as
"pfn_valid() returnes true if there is memmap." even if you do that.
Otherwise, many things will go bad.
You have 2 or 3 ways.
1. re-implement pfn_valid() which returns correct value.
maybe not difficult. but please take care of defining CONFIG_HOLES_IN_....
etc.
2. use DISCONTIGMEM and treat each bank and NUMA node.
There will be no waste for memmap. But other complication of CONFIG_NUMA.
3. use SPARSEMEM.
You have even 2 choisce here.
a - Set your MAX_ORDER and SECTION_SIZE to be proper value.
b - waste some amount of memory for memmap on the edge of section.
(and don't free memmap for the edge.)
Thanks,
-Kame
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