Subject: Re: [PATCH v1 06/23] vmw_balloon: stop using the balloon_dev_info lock

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 21 Oct 2025 16:57:28 -0400

On Tue, Oct 21, 2025 at 02:59:11PM +0200, David Hildenbrand wrote:
> Let's not piggy-back on the existing lock and use a separate lock for the
> huge page list.

which is fine but it is worth documenting that we are also
no longer disabling interrupts while taking this lock.

> This is a preparation for changing the locking used to protect
> balloon_dev_info.
> 
> While at it, talk about "page migration" instead of "page compaction".
> We'll change that in core code soon as well.
> 
> Signed-off-by: David Hildenbrand <[email-masked]>
> ---
>  drivers/misc/vmw_balloon.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/misc/vmw_balloon.c b/drivers/misc/vmw_balloon.c
> index 52b8c0f1eead7..53e9335b6718c 100644
> --- a/drivers/misc/vmw_balloon.c
> +++ b/drivers/misc/vmw_balloon.c
> @@ -354,10 +354,15 @@ struct vmballoon {
>  	/**
>  	 * @huge_pages - list of the inflated 2MB pages.
>  	 *
> -	 * Protected by @b_dev_info.pages_lock .
> +	 * Protected by @huge_pages_lock.
>  	 */
>  	struct list_head huge_pages;
>  
> +	/**
> +	 * @huge_pages_lock: lock for the list of inflated 2MB pages.
> +	 */
> +	spinlock_t huge_pages_lock;
> +
>  	/**
>  	 * @vmci_doorbell.
>  	 *
> @@ -987,7 +992,6 @@ static void vmballoon_enqueue_page_list(struct vmballoon *b,
>  					unsigned int *n_pages,
>  					enum vmballoon_page_size_type page_size)
>  {
> -	unsigned long flags;
>  	struct page *page;
>  
>  	if (page_size == VMW_BALLOON_4K_PAGE) {
> @@ -995,9 +999,9 @@ static void vmballoon_enqueue_page_list(struct vmballoon *b,
>  	} else {
>  		/*
>  		 * Keep the huge pages in a local list which is not available
> -		 * for the balloon compaction mechanism.
> +		 * for the balloon page migration.
>  		 */
> -		spin_lock_irqsave(&b->b_dev_info.pages_lock, flags);
> +		spin_lock(&b->huge_pages_lock);
>  
>  		list_for_each_entry(page, pages, lru) {
>  			vmballoon_mark_page_offline(page, VMW_BALLOON_2M_PAGE);
> @@ -1006,7 +1010,7 @@ static void vmballoon_enqueue_page_list(struct vmballoon *b,
>  		list_splice_init(pages, &b->huge_pages);
>  		__count_vm_events(BALLOON_INFLATE, *n_pages *
>  				  vmballoon_page_in_frames(VMW_BALLOON_2M_PAGE));
> -		spin_unlock_irqrestore(&b->b_dev_info.pages_lock, flags);
> +		spin_unlock(&b->huge_pages_lock);
>  	}
>  
>  	*n_pages = 0;
> @@ -1033,7 +1037,6 @@ static void vmballoon_dequeue_page_list(struct vmballoon *b,
>  {
>  	struct page *page, *tmp;
>  	unsigned int i = 0;
> -	unsigned long flags;
>  
>  	/* In the case of 4k pages, use the compaction infrastructure */
>  	if (page_size == VMW_BALLOON_4K_PAGE) {
> @@ -1043,7 +1046,7 @@ static void vmballoon_dequeue_page_list(struct vmballoon *b,
>  	}
>  
>  	/* 2MB pages */
> -	spin_lock_irqsave(&b->b_dev_info.pages_lock, flags);
> +	spin_lock(&b->huge_pages_lock);
>  	list_for_each_entry_safe(page, tmp, &b->huge_pages, lru) {
>  		vmballoon_mark_page_online(page, VMW_BALLOON_2M_PAGE);
>  
> @@ -1054,7 +1057,7 @@ static void vmballoon_dequeue_page_list(struct vmballoon *b,
>  
>  	__count_vm_events(BALLOON_DEFLATE,
>  			  i * vmballoon_page_in_frames(VMW_BALLOON_2M_PAGE));
> -	spin_unlock_irqrestore(&b->b_dev_info.pages_lock, flags);
> +	spin_unlock(&b->huge_pages_lock);
>  	*n_pages = i;
>  }
>  
> @@ -1828,6 +1831,7 @@ static int __init vmballoon_init(void)
>  		balloon.b_dev_info.migratepage = vmballoon_migratepage;
>  
>  	INIT_LIST_HEAD(&balloon.huge_pages);
> +	spin_lock_init(&balloon.huge_pages_lock);
>  	spin_lock_init(&balloon.comm_lock);
>  	init_rwsem(&balloon.conf_sem);
>  	balloon.vmci_doorbell = VMCI_INVALID_HANDLE;
> -- 
> 2.51.0