Subject: Re: [PATCH v1 07/23] mm/balloon_compaction: use a device-independent balloon (list) lock

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 21 Oct 2025 16:52:33 -0400

On Tue, Oct 21, 2025 at 02:59:12PM +0200, David Hildenbrand wrote:
> 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.
> 
> Signed-off-by: David Hildenbrand <[email-masked]>
> ---
>  include/linux/balloon_compaction.h |  6 ++---
>  mm/balloon_compaction.c            | 36 +++++++++++++++++-------------
>  2 files changed, 23 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 3109d3c43d306..e2d9eb40e1fbb 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

Using &balloon_pages_lock with an & is kinda weird here.

>   *
>   * 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 fd9ec47cf4670..97e838795354d 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;
>  }
> @@ -230,10 +236,10 @@ 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,
> @@ -253,7 +259,7 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
>  	rc = b_dev_info->migratepage(b_dev_info, newpage, page, mode);
>  	switch (rc) {
>  	case 0:
> -		spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> +		spin_lock_irqsave(&balloon_pages_lock, flags);
>  
>  		/* Insert the new page into the balloon list. */
>  		get_page(newpage);
> @@ -272,7 +278,7 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
>  		}
>  		break;
>  	case -ENOENT:
> -		spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> +		spin_lock_irqsave(&balloon_pages_lock, flags);
>  
>  		/* Old page was deflated but new page not inflated. */
>  		__count_vm_event(BALLOON_DEFLATE);
> @@ -285,7 +291,7 @@ 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);
> -- 
> 2.51.0