On 25.11.20 14:33, Mel Gorman wrote: > On Wed, Nov 25, 2020 at 12:04:15PM +0100, David Hildenbrand wrote: >> On 25.11.20 11:39, Mel Gorman wrote: >>> On Wed, Nov 25, 2020 at 07:45:30AM +0100, David Hildenbrand wrote: >>>>> Something must have changed more recently than v5.1 that caused the >>>>> zoneid of reserved pages to be wrong, a possible candidate for the >>>>> real would be this change below: >>>>> >>>>> + __init_single_page(pfn_to_page(pfn), pfn, 0, 0); >>>>> >>>> >>>> Before that change, the memmap of memory holes were only zeroed out. So the zones/nid was 0, however, pages were not reserved and had a refcount of zero - resulting in other issues. >>>> >>>> Most pfn walkers shouldn???t mess with reserved pages and simply skip them. That would be the right fix here. >>>> >>> >>> Ordinarily yes, pfn walkers should not care about reserved pages but it's >>> still surprising that the node/zone linkages would be wrong for memory >>> holes. If they are in the middle of a zone, it means that a hole with >>> valid struct pages could be mistaken for overlapping nodes (if the hole >>> was in node 1 for example) or overlapping zones which is just broken. >> >> I agree within zones - but AFAIU, the issue is reserved memory between >> zones, right? >> > > It can also occur in the middle of the zone. > >> Assume your end of memory falls within a section - what would be the >> right node/zone for such a memory hole at the end of the section? > > Assuming a hole is not MAX_ORDER-aligned but there is real memory within > the page block, then the node/zone for the struct pages backing the hole > should match the real memorys node and zone. > > As it stands, with the uninitialised node/zone, certain checks like > page_is_buddy(): page_zone_id(page) != page_zone_id(buddy) may only > work by co-incidence. page_is_buddy() happens to work anyway because > PageBuddy(buddy) would never be true for a PageReserved page. > >> With >> memory hotplug after such a hole, we can easily have multiple >> nodes/zones spanning such a hole, unknown before hotplug. >> > > When hotplugged, the same logic would apply. Where the hole is not aligned, > the struct page linkages should match the "real" memory". > >>> It would partially paper over the issue that setting the pageblock type >>> based on a reserved page. I agree that compaction should not be returning >>> pfns that are outside of the zone range because that is buggy in itself >>> but valid struct pages should have valid information. I don't think we >>> want to paper over that with unnecessary PageReserved checks. >> >> Agreed as long as we can handle that issue using range checks. >> > > I think it'll be ok as long as the struct pages within a 1<<(MAX_ORDER-1) > range have proper linkages. Agreed. -- Thanks, David / dhildenb
