Re: [PATCH v8 1/3] mm/memory hotplug: Fix zone->contiguous always false when hotplug

[Li, Tianyou]

Appreciated for your review, Mike.

On 1/22/2026 7:16 PM, Mike Rapoport wrote:
> Hi,
>
> On Tue, Jan 20, 2026 at 10:33:44PM +0800, Tianyou Li wrote:
> > From: Yuan Liu <yuan1.liu@xxxxxxxxx>
> >
> > set_zone_contiguous() uses __pageblock_pfn_to_page() to detect
> > pageblocks that either do not exist (hole) or that do not belong
> > to the same zone.
> >
> > __pageblock_pfn_to_page(), however, relies on pfn_to_online_page(),
> > effectively always returning NULL for memory ranges that were not
> > onlined yet. So when called on a range-to-be-onlined, it indicates
> > a memory hole to set_zone_contiguous().
> >
> > Consequently, the set_zone_contiguous() call in move_pfn_range_to_zone(),
> > which happens early during memory onlining, will never detect a
> > zone as being contiguous. Bad.
> >
> > To fix the issue, move the set_zone_contiguous() call to a later
> > stage in memory onlining, where pfn_to_online_page() will succeed:
> > after we mark the memory sections to be online.
> >
> > Fixes: 2d070eab2e82 ("mm: consider zone which is not fully populated to have holes")
> cc stable@ perhaps?

Yes, will do. Thanks.

> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Reviewed-by: Nanhai Zou <nanhai.zou@xxxxxxxxx>
> > Signed-off-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
> > Signed-off-by: Tianyou Li <tianyou.li@xxxxxxxxx>
> > ---
> >   mm/memory_hotplug.c | 9 +++++++--
> >   1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index a63ec679d861..c8f492b5daf0 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -782,8 +782,6 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
> >   	memmap_init_range(nr_pages, nid, zone_idx(zone), start_pfn, 0,
> >   			 MEMINIT_HOTPLUG, altmap, migratetype,
> >   			 isolate_pageblock);
> > -
> > -	set_zone_contiguous(zone);
> move_pfn_range_to_zone() is also called from memremap::pagemap_range().
> Shouldn't we add set_zone_contiguous() there as well?

I did not find the place where the online_pages was invoked along path 
of the memremap:pagemap_range() function. Would there be other functions 
to online the pages remapped? Much appreciated for the guidance.

I leave the zone contiguous state remains the same as the optimization 
not took place in case any unexpected behavior.

> >   }
> >   
> >   struct auto_movable_stats {
> > @@ -1205,6 +1203,13 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
> >   	}
> >   
> >   	online_pages_range(pfn, nr_pages);
> > +
> > +	/*
> > +	 * Now that the ranges are indicated as online, check whether the whole
> > +	 * zone is contiguous.
> > +	 */
> > +	set_zone_contiguous(zone);
> > +
> >   	adjust_present_page_count(pfn_to_page(pfn), group, nr_pages);
> >   
> >   	if (node_arg.nid >= 0)
> > --
> > 2.47.1