Subject: [PATCH v3 08/24] mm/balloon_compaction: use a device-independent balloon (list) lock

From: "David Hildenbrand (Red Hat)" <[email-masked]>
Date: Tue, 20 Jan 2026 00:01:16 +0100

In order to remove the dependency on the page lock for balloon
pages, we need a lock that is independent of the page.

It's crucial that we can handle the scenario where balloon deflation
(clearing page->private) can race with page isolation (using
page->private to obtain the balloon_dev_info where the lock currently
resides).

The current lock in balloon_dev_info is therefore not suitable.

Fortunately, we never really have more than a single balloon device
per VM, so we can just keep it simple and use a static lock to protect
all balloon devices.

Based on this change we will remove the dependency on the page lock
next.

Acked-by: Michael S. Tsirkin <[email-masked]>
Signed-off-by: David Hildenbrand (Red Hat) <[email-masked]>
---
 include/linux/balloon_compaction.h |  6 ++----
 mm/balloon_compaction.c            | 34 ++++++++++++++++++------------
 2 files changed, 22 insertions(+), 18 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 3109d3c43d306..9a8568fcd477d 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -21,10 +21,10 @@
  *   i. Setting the PG_movable_ops flag and page->private with the following
  *	lock order
  *	    +-page_lock(page);
- *	      +--spin_lock_irq(&b_dev_info->pages_lock);
+ *	      +--spin_lock_irq(&balloon_pages_lock);
  *
  *  ii. isolation or dequeueing procedure must remove the page from balloon
- *      device page list under b_dev_info->pages_lock.
+ *      device page list under balloon_pages_lock
  *
  * The functions provided by this interface are placed to help on coping with
  * the aforementioned balloon page corner case, as well as to ensure the simple
@@ -52,7 +52,6 @@
  */
 struct balloon_dev_info {
 	unsigned long isolated_pages;	/* # of isolated pages for migration */
-	spinlock_t pages_lock;		/* Protection to pages list */
 	struct list_head pages;		/* Pages enqueued & handled to Host */
 	int (*migratepage)(struct balloon_dev_info *, struct page *newpage,
 			struct page *page, enum migrate_mode mode);
@@ -71,7 +70,6 @@ extern size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
 static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
 {
 	balloon->isolated_pages = 0;
-	spin_lock_init(&balloon->pages_lock);
 	INIT_LIST_HEAD(&balloon->pages);
 	balloon->migratepage = NULL;
 	balloon->adjust_managed_page_count = false;
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index 4fe2a0cff69ec..a0fd779bbd012 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -11,6 +11,12 @@
 #include <linux/export.h>
 #include <linux/balloon_compaction.h>
 
+/*
+ * Lock protecting the balloon_dev_info of all devices. We don't really
+ * expect more than one device.
+ */
+static DEFINE_SPINLOCK(balloon_pages_lock);
+
 static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
 				     struct page *page)
 {
@@ -47,13 +53,13 @@ size_t balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info,
 	unsigned long flags;
 	size_t n_pages = 0;
 
-	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	spin_lock_irqsave(&balloon_pages_lock, flags);
 	list_for_each_entry_safe(page, tmp, pages, lru) {
 		list_del(&page->lru);
 		balloon_page_enqueue_one(b_dev_info, page);
 		n_pages++;
 	}
-	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+	spin_unlock_irqrestore(&balloon_pages_lock, flags);
 	return n_pages;
 }
 EXPORT_SYMBOL_GPL(balloon_page_list_enqueue);
@@ -83,7 +89,7 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
 	unsigned long flags;
 	size_t n_pages = 0;
 
-	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	spin_lock_irqsave(&balloon_pages_lock, flags);
 	list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
 		if (n_pages == n_req_pages)
 			break;
@@ -106,7 +112,7 @@ size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
 		dec_node_page_state(page, NR_BALLOON_PAGES);
 		n_pages++;
 	}
-	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+	spin_unlock_irqrestore(&balloon_pages_lock, flags);
 
 	return n_pages;
 }
@@ -149,9 +155,9 @@ void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	spin_lock_irqsave(&balloon_pages_lock, flags);
 	balloon_page_enqueue_one(b_dev_info, page);
-	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+	spin_unlock_irqrestore(&balloon_pages_lock, flags);
 }
 EXPORT_SYMBOL_GPL(balloon_page_enqueue);
 
@@ -191,11 +197,11 @@ struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
 		 * BUG() here, otherwise the balloon driver may get stuck in
 		 * an infinite loop while attempting to release all its pages.
 		 */
-		spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+		spin_lock_irqsave(&balloon_pages_lock, flags);
 		if (unlikely(list_empty(&b_dev_info->pages) &&
 			     !b_dev_info->isolated_pages))
 			BUG();
-		spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+		spin_unlock_irqrestore(&balloon_pages_lock, flags);
 		return NULL;
 	}
 	return list_first_entry(&pages, struct page, lru);
@@ -213,10 +219,10 @@ static bool balloon_page_isolate(struct page *page, isolate_mode_t mode)
 	if (!b_dev_info)
 		return false;
 
-	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	spin_lock_irqsave(&balloon_pages_lock, flags);
 	list_del(&page->lru);
 	b_dev_info->isolated_pages++;
-	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+	spin_unlock_irqrestore(&balloon_pages_lock, flags);
 
 	return true;
 }
@@ -234,10 +240,10 @@ static void balloon_page_putback(struct page *page)
 	if (WARN_ON_ONCE(!b_dev_info))
 		return;
 
-	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	spin_lock_irqsave(&balloon_pages_lock, flags);
 	list_add(&page->lru, &b_dev_info->pages);
 	b_dev_info->isolated_pages--;
-	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+	spin_unlock_irqrestore(&balloon_pages_lock, flags);
 }
 
 static int balloon_page_migrate(struct page *newpage, struct page *page,
@@ -262,7 +268,7 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
 	if (rc < 0 && rc != -ENOENT)
 		return rc;
 
-	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+	spin_lock_irqsave(&balloon_pages_lock, flags);
 	if (!rc) {
 		/* Insert the new page into the balloon list. */
 		get_page(newpage);
@@ -287,7 +293,7 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
 	}
 
 	b_dev_info->isolated_pages--;
-	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+	spin_unlock_irqrestore(&balloon_pages_lock, flags);
 
 	/* Free the now-deflated page we isolated in balloon_page_isolate(). */
 	balloon_page_finalize(page);
-- 
2.52.0