[PATCH] mm/memory_hotplug: make shrink_zone_span() more robust

[David Hildenbrand (Arm)]

Let's make shrink_zone_span() more robust by checking in
find_smallest_section_pfn() / find_biggest_section_pfn() that start+end
of the subsection.

While at it, clean up the function heavily, factoring the core check
out into subsection_overlaps_zone().

There likely is no need to check for the nid first: we require
SPARSEMEM_VMEMMAP_ENABLE where pfn_to_page() is cheap, and the
pfn_to_nid() on CONFIG_NUMA would do a pfn_to_page() either way. So
let's just drop that for now.

Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
---
 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 226ab9cb078ad..79937f9672d01 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -425,49 +425,39 @@ 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;
+	/* All pages in a subsection are either online or offline. */
+	if (unlikely(!pfn_to_online_page(start_pfn)))
+		return false;
 
-		if (zone != page_zone(pfn_to_page(start_pfn)))
-			continue;
+	/* Checking start+end is sufficient. */
+	return zone == page_zone(pfn_to_page(start_pfn)) ||
+	       zone == page_zone(pfn_to_page(end_pfn));
+}
 
-		return start_pfn;
+/* find the smallest valid pfn in the range [start_pfn, end_pfn) */
+static unsigned long find_smallest_section_pfn(struct zone *zone,
+		unsigned long start_pfn, unsigned long end_pfn)
+{
+	for (; start_pfn < end_pfn; start_pfn += PAGES_PER_SUBSECTION) {
+		if (subsection_overlaps_zone(start_pfn, zone))
+			return start_pfn;
 	}
-
 	return 0;
 }
 
 /* find the biggest valid pfn in the range [start_pfn, end_pfn). */
-static unsigned long find_biggest_section_pfn(int nid, struct zone *zone,
-				    unsigned long start_pfn,
-				    unsigned long end_pfn)
+static unsigned long find_biggest_section_pfn(struct zone *zone,
+		unsigned long start_pfn, unsigned long end_pfn)
 {
-	unsigned long 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;
-
-		if (unlikely(pfn_to_nid(pfn) != nid))
-			continue;
-
-		if (zone != page_zone(pfn_to_page(pfn)))
-			continue;
-
-		return pfn;
+	for (; end_pfn >= start_pfn; end_pfn -= PAGES_PER_SUBSECTION) {
+		if (subsection_overlaps_zone(end_pfn - 1, zone))
+			return end_pfn - 1;
 	}
-
 	return 0;
 }
 
@@ -475,7 +465,6 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
 			     unsigned long end_pfn)
 {
 	unsigned long pfn;
-	int nid = zone_to_nid(zone);
 
 	if (zone->zone_start_pfn == start_pfn) {
 		/*
@@ -484,7 +473,7 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
 		 * In this case, we find second smallest valid mem_section
 		 * for shrinking zone.
 		 */
-		pfn = find_smallest_section_pfn(nid, zone, end_pfn,
+		pfn = find_smallest_section_pfn(zone, end_pfn,
 						zone_end_pfn(zone));
 		if (pfn) {
 			zone->spanned_pages = zone_end_pfn(zone) - pfn;
@@ -500,7 +489,7 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
 		 * In this case, we find second biggest valid mem_section for
 		 * shrinking zone.
 		 */
-		pfn = find_biggest_section_pfn(nid, zone, zone->zone_start_pfn,
+		pfn = find_biggest_section_pfn(zone, zone->zone_start_pfn,
 					       start_pfn);
 		if (pfn)
 			zone->spanned_pages = pfn - zone->zone_start_pfn + 1;
-- 
2.43.0


-- 
Cheers,

David