Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages

[David Hildenbrand]

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