[RFC PATCH 40/45] PM: hibernate: walk per-superpageblock free lists in mark_free_pages

[Rik van Riel <riel@surriel.com>]

From: Rik van Riel <riel@meta.com>

mark_free_pages() walks the buddy allocator's free lists and calls
swsusp_set_page_free() on each free page so it is omitted from the
hibernation image. After the SPB rework, free pages live on per-
superpageblock free lists rather than the zone-level free list, so
the existing list_for_each_entry() walk over zone->free_area[order]
.free_list[t] found nothing. The hibernation snapshot then treated
every free page as needing to be saved, wasting image space and risking
OOM during the snapshot.

Wrap the existing per-page walk in an SPB iteration loop. When the
zone has no SPBs (e.g. an unpopulated hotplug zone), fall back to the
zone-level free list. The whole function still runs under
spin_lock_irqsave(&zone->lock) without drops, so there are no lock-
order or hotplug concerns.

Note: kernel/power/snapshot.o currently fails to build with
-Werror=return-type due to an unrelated pre-existing warning in
enough_free_mem(). This patch was reviewed but not build-tested in
isolation; the change itself is mechanical.

Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Pavel Machek <pavel@kernel.org>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Rik van Riel <riel@surriel.com>
Assisted-by: Claude:claude-opus-4.7 syzkaller
---
 kernel/power/snapshot.c | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
index 6e1321837c66..682c3bf2ba8b 100644
--- a/kernel/power/snapshot.c
+++ b/kernel/power/snapshot.c
@@ -1270,17 +1270,32 @@ static void mark_free_pages(struct zone *zone)
 	}
 
 	for_each_migratetype_order(order, t) {
-		list_for_each_entry(page,
-				&zone->free_area[order].free_list[t], buddy_list) {
-			unsigned long i;
-
-			pfn = page_to_pfn(page);
-			for (i = 0; i < (1UL << order); i++) {
-				if (!--page_count) {
-					touch_nmi_watchdog();
-					page_count = WD_PAGE_COUNT;
+		unsigned long sb_idx;
+		unsigned long nr_lists = zone->nr_superpageblocks ? : 1;
+
+		/*
+		 * After the SPB rework, free pages live on per-superpageblock
+		 * free lists. Walk every SPB's list for this (order, mt) cell.
+		 * If the zone has no SPBs (unpopulated zone), fall back to the
+		 * zone-level list head so that any pre-SPB pages are still
+		 * marked.
+		 */
+		for (sb_idx = 0; sb_idx < nr_lists; sb_idx++) {
+			struct list_head *list = zone->nr_superpageblocks ?
+				&zone->superpageblocks[sb_idx].free_area[order].free_list[t] :
+				&zone->free_area[order].free_list[t];
+
+			list_for_each_entry(page, list, buddy_list) {
+				unsigned long i;
+
+				pfn = page_to_pfn(page);
+				for (i = 0; i < (1UL << order); i++) {
+					if (!--page_count) {
+						touch_nmi_watchdog();
+						page_count = WD_PAGE_COUNT;
+					}
+					swsusp_set_page_free(pfn_to_page(pfn + i));
 				}
-				swsusp_set_page_free(pfn_to_page(pfn + i));
 			}
 		}
 	}
-- 
2.52.0