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

[Mel Gorman <mgorman@suse.de>]

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.

-- 
Mel Gorman
SUSE Labs