Re: [PATCH v6 2/2] mm/memory_hotplug: improve shrink_zone_span() subsection boundary checks

[Wei Yang]

On Thu, Jul 23, 2026 at 04:49:46AM -0400, Yuan Liu wrote:
>When shrinking a zone span after removing a PFN range,
>find_smallest_section_pfn() and find_biggest_section_pfn()
>only checked one edge PFN in each subsection for nid/zone matching.
>
>If a memory or hole boundary falls in the middle of a subsection,
>that edge PFN may belong to a different nid/zone, causing the helpers
>to miss a valid PFN within that subsection.
>
>Fix this by checking both subsection edge PFNs for nid/zone matching.
>Keep a single pfn_to_online_page() check per subsection, since online
>state is the same for all PFNs in a subsection.
>
>Reviewed-by: Jason Zeng <jason.zeng@xxxxxxxxx>
>Signed-off-by: Yuan Liu <yuan1.liu@xxxxxxxxx>
>---
> mm/memory_hotplug.c | 42 +++++++++++++++++++++++++++---------------
> 1 file changed, 27 insertions(+), 15 deletions(-)
>
>diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>index 4c699fd9c479..3a281d595207 100644
>--- a/mm/memory_hotplug.c
>+++ b/mm/memory_hotplug.c
>@@ -427,17 +427,24 @@ static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
> 				     unsigned long start_pfn,
> 				     unsigned long end_pfn)
> {
>-	for (; start_pfn < end_pfn; start_pfn += PAGES_PER_SUBSECTION) {
>-		if (unlikely(!pfn_to_online_page(start_pfn)))
>-			continue;
>+	unsigned long next_pfn;
> 
>-		if (unlikely(pfn_to_nid(start_pfn) != nid))
>-			continue;
>+	for (; start_pfn < end_pfn; start_pfn = next_pfn) {
>+		unsigned long tail_pfn;
> 
>-		if (zone != page_zone(pfn_to_page(start_pfn)))
>+		next_pfn = start_pfn + PAGES_PER_SUBSECTION;
>+		tail_pfn = next_pfn - 1;
>+
>+		if (unlikely(!pfn_to_online_page(start_pfn)))
> 			continue;
> 
>-		return start_pfn;
>+		if (likely(pfn_to_nid(start_pfn) == nid) &&
>+		    zone == page_zone(pfn_to_page(start_pfn)))
>+			return start_pfn;
>+
>+		if (likely(pfn_to_nid(tail_pfn) == nid) &&
>+		    zone == page_zone(pfn_to_page(tail_pfn)))
>+			return start_pfn;

Here we are checking range [start_pfn, tail_pfn]. When we come here, it means
start_pfn's nid or zone doesn't match our expectation. But if tail_pfn does,
why it still return start_pfn?

> 	}
> 
> 	return 0;
>@@ -448,21 +455,26 @@ static unsigned long find_biggest_section_pfn(int nid, struct zone *zone,
> 				    unsigned long start_pfn,
> 				    unsigned long end_pfn)
> {
>-	unsigned long pfn;
>+	unsigned long pfn, prev_pfn;
> 
> 	/* pfn is the end pfn of a memory section. */
> 	pfn = end_pfn - 1;
>-	for (; pfn >= start_pfn; pfn -= PAGES_PER_SUBSECTION) {
>-		if (unlikely(!pfn_to_online_page(pfn)))
>-			continue;
>+	for (; pfn >= start_pfn; pfn = prev_pfn) {
>+		unsigned long head_pfn;
> 
>-		if (unlikely(pfn_to_nid(pfn) != nid))
>-			continue;
>+		prev_pfn = pfn - PAGES_PER_SUBSECTION;
>+		head_pfn = prev_pfn + 1;
> 
>-		if (zone != page_zone(pfn_to_page(pfn)))
>+		if (unlikely(!pfn_to_online_page(pfn)))
> 			continue;
> 
>-		return pfn;
>+		if (likely(pfn_to_nid(pfn) == nid) &&
>+		    zone == page_zone(pfn_to_page(pfn)))
>+			return pfn;
>+
>+		if (likely(pfn_to_nid(head_pfn) == nid) &&
>+		    zone == page_zone(pfn_to_page(head_pfn)))
>+			return pfn;
> 	}
> 
> 	return 0;
>-- 
>2.47.3

-- 
Wei Yang
Help you, Help me