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

[Tianyou Li <tianyou.li@intel.com>]

From: Yuan Liu <yuan1.liu@intel.com>

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: the fast path relies on the zone
contiguous state may skipped.

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: Michal Hocko <mhocko@suse.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nanhai Zou <nanhai.zou@intel.com>
Signed-off-by: Yuan Liu <yuan1.liu@intel.com>
Signed-off-by: Tianyou Li <tianyou.li@intel.com>
---
Separated from: https://lore.kernel.org/linux-mm/20260120143346.1427837-1-tianyou.li@intel.com/

 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);
 }
 
 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