Re: [PATCH v1 13/29] mm/balloon_compaction: stop using __ClearPageMovable()

[Lorenzo Stoakes]

On Mon, Jun 30, 2025 at 02:59:54PM +0200, David Hildenbrand wrote:
> We can just look at the balloon device (stored in page->private), to see
> if the page is still part of the balloon.
>
> As isolated balloon pages cannot get released (they are taken off the
> balloon list while isolated), we don't have to worry about this case in
> the putback and migration callback. Add a WARN_ON_ONCE for now.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Seems reasonable, one comment below re: comment.

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  include/linux/balloon_compaction.h |  4 +---
>  mm/balloon_compaction.c            | 11 +++++++++++
>  2 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index bfc6e50bd004b..9bce8e9f5018c 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -136,10 +136,8 @@ static inline gfp_t balloon_mapping_gfp_mask(void)
>   */
>  static inline void balloon_page_finalize(struct page *page)
>  {
> -	if (IS_ENABLED(CONFIG_BALLOON_COMPACTION)) {
> -		__ClearPageMovable(page);
> +	if (IS_ENABLED(CONFIG_BALLOON_COMPACTION))
>  		set_page_private(page, 0);
> -	}
>  	/* PageOffline is sticky until the page is freed to the buddy. */
>  }
>
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index ec176bdb8a78b..e4f1a122d786b 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -206,6 +206,9 @@ static bool balloon_page_isolate(struct page *page, isolate_mode_t mode)
>  	struct balloon_dev_info *b_dev_info = balloon_page_device(page);
>  	unsigned long flags;
>
> +	if (!b_dev_info)
> +		return false;
> +
>  	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>  	list_del(&page->lru);
>  	b_dev_info->isolated_pages++;
> @@ -219,6 +222,10 @@ static void balloon_page_putback(struct page *page)
>  	struct balloon_dev_info *b_dev_info = balloon_page_device(page);
>  	unsigned long flags;
>
> +	/* Isolated balloon pages cannot get deflated. */
> +	if (WARN_ON_ONCE(!b_dev_info))
> +		return;
> +
>  	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>  	list_add(&page->lru, &b_dev_info->pages);
>  	b_dev_info->isolated_pages--;
> @@ -234,6 +241,10 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
>
> +	/* Isolated balloon pages cannot get deflated. */

Hm do you mean migrated?

> +	if (WARN_ON_ONCE(!balloon))
> +		return -EAGAIN;
> +
>  	return balloon->migratepage(balloon, newpage, page, mode);
>  }
>
> --
> 2.49.0
>