[PATCH v9 03/37] mm: page_alloc: propagate PageReported flag across buddy splits

[Michael S. Tsirkin]

When a reported free page is split via expand() to satisfy a
smaller allocation, the sub-pages placed back on the free lists
lose the PageReported flag.  This means they will be unnecessarily
re-reported to the hypervisor in the next reporting cycle, wasting
work.

While I was unable to quantify the performance difference, it is
an obvious waste, even if small.

Propagate the PageReported flag to sub-pages during expand(),
both in page_del_and_expand() and try_to_claim_block(), so
that they are recognized as already-reported.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
---
 mm/page_alloc.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 227d58dc3de6..1192c4e47da4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1510,6 +1510,7 @@ static void split_large_buddy(struct zone *zone, struct page *page,
 			      unsigned long pfn, int order, fpi_t fpi)
 {
 	unsigned long end = pfn + (1 << order);
+	bool reported = PageReported(page);
 
 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn, 1 << order));
 	/* Caller removed page from freelist, buddy info cleared! */
@@ -1521,6 +1522,8 @@ static void split_large_buddy(struct zone *zone, struct page *page,
 	do {
 		int mt = get_pfnblock_migratetype(page, pfn);
 
+		if (reported)
+			__SetPageReported(page);
 		__free_one_page(page, pfn, zone, order, mt, fpi);
 		pfn += 1 << order;
 		if (pfn == end)
@@ -1699,7 +1702,7 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
  * -- nyc
  */
 static inline unsigned int expand(struct zone *zone, struct page *page, int low,
-				  int high, int migratetype)
+				  int high, int migratetype, bool reported)
 {
 	unsigned int size = 1 << high;
 	unsigned int nr_added = 0;
@@ -1721,6 +1724,15 @@ static inline unsigned int expand(struct zone *zone, struct page *page, int low,
 		__add_to_free_list(&page[size], zone, high, migratetype, false);
 		set_buddy_order(&page[size], high);
 		nr_added += size;
+
+		/*
+		 * The parent page has been reported to the host.  The
+		 * sub-pages are part of the same reported block, so mark
+		 * them reported too.  This avoids re-reporting pages that
+		 * the host already knows about.
+		 */
+		if (reported)
+			__SetPageReported(&page[size]);
 	}
 
 	return nr_added;
@@ -1731,9 +1743,10 @@ static __always_inline void page_del_and_expand(struct zone *zone,
 						int high, int migratetype)
 {
 	int nr_pages = 1 << high;
+	bool was_reported = page_reported(page);
 
 	__del_page_from_free_list(page, zone, high, migratetype);
-	nr_pages -= expand(zone, page, low, high, migratetype);
+	nr_pages -= expand(zone, page, low, high, migratetype, was_reported);
 	account_freepages(zone, -nr_pages, migratetype);
 }
 
@@ -2288,10 +2301,12 @@ try_to_claim_block(struct zone *zone, struct page *page,
 	/* Take ownership for orders >= pageblock_order */
 	if (current_order >= pageblock_order) {
 		unsigned int nr_added;
+		bool was_reported = page_reported(page);
 
 		del_page_from_free_list(page, zone, current_order, block_type);
 		change_pageblock_range(page, current_order, start_type);
-		nr_added = expand(zone, page, order, current_order, start_type);
+		nr_added = expand(zone, page, order, current_order, start_type,
+				  was_reported);
 		account_freepages(zone, nr_added, start_type);
 		return page;
 	}
-- 
MST