Re: [PATCH v3 00/24] mm: balloon infrastructure cleanups

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 20 Jan 2026 00:01:08 +0100 "David Hildenbrand (Red Hat)" <david@kernel.org> wrote:

> I started with wanting to remove the dependency of the balloon
> infrastructure on the page lock, but ended up performing various other
> cleanups, some of which I had on my todo list for years.
> 
> This series heavily cleans up and simplifies our balloon infrastructure,
> including our balloon page migration functionality.

Updated, thanks.

fwiw, below is how v3 altered mm.git:

--- a/include/linux/balloon.h~b
+++ a/include/linux/balloon.h
@@ -22,9 +22,9 @@
  *
  * As the page isolation scanning step a compaction thread does is a lockless
  * procedure (from a page standpoint), it might bring some racy situations while
- * performing balloon page compaction. In order to sort out these racy scenarios
- * and safely perform balloon's page compaction and migration we must, always,
- * ensure following these simple rules:
+ * performing balloon page migration. In order to sort out these racy scenarios
+ * and safely perform balloon's page migration we must, always, ensure following
+ * these simple rules:
  *
  *   i. Inflation/deflation must set/clear page->private under the
  *      balloon_pages_lock
@@ -47,8 +47,8 @@
  * Balloon device information descriptor.
  * This struct is used to allow the common balloon page migration interface
  * procedures to find the proper balloon device holding memory pages they'll
- * have to cope for page compaction / migration, as well as it serves the
- * balloon driver as a page book-keeper for its registered balloon devices.
+ * have to cope for page migration, as well as it serves the balloon driver as
+ * a page book-keeper for its registered balloon devices.
  */
 struct balloon_dev_info {
 	unsigned long isolated_pages;	/* # of isolated pages for migration */
--- a/mm/balloon.c~b
+++ a/mm/balloon.c
@@ -16,7 +16,7 @@
  */
 static DEFINE_SPINLOCK(balloon_pages_lock);
 
-static inline struct balloon_dev_info *balloon_page_device(struct page *page)
+static struct balloon_dev_info *balloon_page_device(struct page *page)
 {
 	return (struct balloon_dev_info *)page_private(page);
 }
@@ -29,7 +29,7 @@ static inline struct balloon_dev_info *b
  *
  * Caller must ensure the balloon_pages_lock is held.
  */
-static inline void balloon_page_insert(struct balloon_dev_info *balloon,
+static void balloon_page_insert(struct balloon_dev_info *balloon,
 				       struct page *page)
 {
 	lockdep_assert_held(&balloon_pages_lock);
@@ -48,7 +48,7 @@ static inline void balloon_page_insert(s
  *
  * Caller must ensure the balloon_pages_lock is held.
  */
-static inline void balloon_page_finalize(struct page *page)
+static void balloon_page_finalize(struct page *page)
 {
 	lockdep_assert_held(&balloon_pages_lock);
 	if (IS_ENABLED(CONFIG_BALLOON_MIGRATION))
@@ -262,7 +262,11 @@ static void balloon_page_putback(struct
 	struct balloon_dev_info *b_dev_info = balloon_page_device(page);
 	unsigned long flags;
 
-	/* Isolated balloon pages cannot get deflated. */
+	/*
+	 * When we isolated the page, the page was still inflated in a balloon
+	 * device. As isolated balloon pages cannot get deflated, we still have
+	 * a balloon device here.
+	 */
 	if (WARN_ON_ONCE(!b_dev_info))
 		return;
 
@@ -279,18 +283,22 @@ static int balloon_page_migrate(struct p
 	unsigned long flags;
 	int rc;
 
-	/* Isolated balloon pages cannot get deflated. */
+	/*
+	 * When we isolated the page, the page was still inflated in a balloon
+	 * device. As isolated balloon pages cannot get deflated, we still have
+	 * a balloon device here.
+	 */
 	if (WARN_ON_ONCE(!b_dev_info))
 		return -EAGAIN;
 
 	rc = b_dev_info->migratepage(b_dev_info, newpage, page, mode);
-	switch (rc) {
-	case 0:
-		spin_lock_irqsave(&balloon_pages_lock, flags);
+	if (rc < 0 && rc != -ENOENT)
+		return rc;
 
+	spin_lock_irqsave(&balloon_pages_lock, flags);
+	if (!rc) {
 		/* Insert the new page into the balloon list. */
 		get_page(newpage);
-
 		balloon_page_insert(b_dev_info, newpage);
 		__count_vm_event(BALLOON_MIGRATE);
 
@@ -303,18 +311,12 @@ static int balloon_page_migrate(struct p
 			adjust_managed_page_count(page, 1);
 			adjust_managed_page_count(newpage, -1);
 		}
-		break;
-	case -ENOENT:
-		spin_lock_irqsave(&balloon_pages_lock, flags);
-
+	} else {
 		/* Old page was deflated but new page not inflated. */
 		__count_vm_event(BALLOON_DEFLATE);
 
 		if (b_dev_info->adjust_managed_page_count)
 			adjust_managed_page_count(page, 1);
-		break;
-	default:
-		return rc;
 	}
 
 	b_dev_info->isolated_pages--;
_