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? 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? With memory hotplug after such a hole, we can easily have multiple nodes/zones spanning such a hole, unknown before hotplug. IMHO, marking memory holes properly (as discussed) would be the cleanest approach. For now, we use node/zone 0 + PageReserved - because memory hotunplug (zone shrinking etc.) doesn't really care about ZONE_DMA. > >>> >>> Whenever pfn_valid is true, it's better that the zoneid/nid is correct >>> all times, otherwise if the second stage fails we end up in a bug with >>> weird side effects. >> >> Memory holes with a valid memmap might not have a zone/nid. For now, skipping reserved pages should be good enough, no? >> > > 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. -- Thanks, David / dhildenb
