Re: [PATCH v4 1/3] mm: introduce compaction and migration for virtio ballooned pages

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

On Tue, 17 Jul 2012 13:50:41 -0300
Rafael Aquini <aquini@redhat.com> wrote:

> This patch introduces the helper functions as well as the necessary changes
> to teach compaction and migration bits how to cope with pages which are
> part of a guest memory balloon, in order to make them movable by memory
> compaction procedures.
> 
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1629,5 +1629,20 @@ static inline unsigned int debug_guardpage_minorder(void) { return 0; }
>  static inline bool page_is_guard(struct page *page) { return false; }
>  #endif /* CONFIG_DEBUG_PAGEALLOC */
>  
> +#if (defined(CONFIG_VIRTIO_BALLOON) || \
> +	defined(CONFIG_VIRTIO_BALLOON_MODULE)) && defined(CONFIG_COMPACTION)
> +extern bool putback_balloon_page(struct page *);
> +extern struct address_space *balloon_mapping;
> +
> +static inline bool is_balloon_page(struct page *page)
> +{
> +	return (page->mapping == balloon_mapping) ? true : false;

You can simply do

	return page->mapping == balloon_mapping;

> +}
> +#else
> +static inline bool is_balloon_page(struct page *page)       { return false; }
> +static inline bool isolate_balloon_page(struct page *page)  { return false; }
> +static inline bool putback_balloon_page(struct page *page)  { return false; }
> +#endif /* (VIRTIO_BALLOON || VIRTIO_BALLOON_MODULE) && COMPACTION */

This means that if CONFIG_VIRTIO_BALLOON=y and CONFIG_COMPACTION=n,
is_balloon_page() will always return NULL.  IOW, no pages are balloon
pages!  This is wrong.

I'm not sure what to do about this, apart from renaming the function to
is_compactible_balloon_page() or something similarly aawkward.