Kernel panic due to page migration accessing memory holes

[mbohan@codeaurora.org (Michael Bohan)]

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.  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.

Thanks,
Michael