Re: [PATCH v7 1/2] mm/memory_hotplug: make shrink_zone_span() more robust

[Wei Yang]

On Tue, Aug 18, 2026 at 04:57:01AM -0400, Yuan Liu wrote:
>From: "David Hildenbrand (Arm)" <david@xxxxxxxxxx>
>
>Let's make shrink_zone_span() more robust by checking in
>find_smallest_section_pfn() / find_biggest_section_pfn() that the
>start and end PFNs of the subsection are within the zone.
>
>While at it, clean up the function by factoring the core check out
>into subsection_overlaps_zone().
>
>There likely is no need to check the nid first. We require
>SPARSEMEM_VMEMMAP_ENABLE, where pfn_to_page() is cheap, and
>pfn_to_nid() on CONFIG_NUMA would call pfn_to_page() either way.
>So let's just drop that for now.
>
>Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>Tested-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
>Signed-off-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
>---
> mm/memory_hotplug.c | 59 ++++++++++++++++++---------------------------
> 1 file changed, 24 insertions(+), 35 deletions(-)
>
>diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>index 7ac19fab2263..cd82e79f0782 100644
>--- a/mm/memory_hotplug.c
>+++ b/mm/memory_hotplug.c
>@@ -422,49 +422,39 @@ int __add_pages(int nid, unsigned long pfn, unsigned long nr_pages,
> 	return err;
> }
> 
>-/* find the smallest valid pfn in the range [start_pfn, end_pfn) */
>-static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
>-				     unsigned long start_pfn,
>-				     unsigned long end_pfn)
>+static bool subsection_overlaps_zone(unsigned long pfn, struct zone *zone)
> {
>-	for (; start_pfn < end_pfn; start_pfn += PAGES_PER_SUBSECTION) {
>-		if (unlikely(!pfn_to_online_page(start_pfn)))
>-			continue;
>+	const unsigned long start_pfn = ALIGN_DOWN(pfn, PAGES_PER_SUBSECTION);
>+	const unsigned long end_pfn = start_pfn + PAGES_PER_SUBSECTION - 1;
> 
>-		if (unlikely(pfn_to_nid(start_pfn) != nid))
>-			continue;

Would we have a memory layout with node interleave?

-- 
Wei Yang
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