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

[Yuan Liu <yuan1.liu@intel.com>]

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@intel.com>
Signed-off-by: Yuan Liu <yuan1.liu@intel.com>
---
 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;
 	}
 
 	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