OK, so let me try again. What is the difference for a pfn walker to
start at an offline pfn start from any other offlined section withing a
zone boundary? I believe there is none because the pfn walker needs to
skip over offline pfns anyway whether they start at a zone boundary or
not.
I checked most of the users of zone_start_pnf:
* split_huge_pages_set:
- It uses pfn_valid().
I guess this is fine as it will check if the section still has a
map.
* __reset_isolation_suitable():
- Safe as it uses pfn_to_online_page().
* isolate_freepages_range():
* isolate_migratepages_range():
* isolate_migratepages():
- They use pageblock_pfn_to_page().
If !zone->contiguos, it will use
__pageblock_pfn_to_page()->pfn_to_online_page()
If zone->contiguos is true, it will use
pageblock_pfn_to_page()->pfn_to_page(),
which is bad because it will not skip over offlined pfns.
* count_highmem_pages:
* count_data_pages:
* copy_data_pages:
- page_is_saveable()->pfn_valid().
I guess this is fine as it will check if the section still has a
map.
So, leaving out isolate_* functions, it seems that we should be safe.
isolate_* functions would depend on !zone->contiguos to call
__pageblock_pfn_to_page()->pfn_to_online_page().
So whenever we remove a section in a zone, we should clear
zone->contiguos.
But this really calls for some deep check that we will not shoot in the
foot.
What I can do for now is to drop this patch from the patchset and
re-send
v3 without it.
