Re: [PATCH v6 1/3] mm: introduce compaction and migration for virtio ballooned pages

[Minchan Kim <minchan@kernel.org>]

Hi Rafael,

On Wed, Aug 08, 2012 at 07:53:19PM -0300, Rafael Aquini wrote:
> Memory fragmentation introduced by ballooning might reduce significantly
> the number of 2MB contiguous memory blocks that can be used within a guest,
> thus imposing performance penalties associated with the reduced number of
> transparent huge pages that could be used by the guest workload.
> 
> 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.
> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---
>  include/linux/mm.h |  17 +++++++
>  mm/compaction.c    | 131 +++++++++++++++++++++++++++++++++++++++++++++--------
>  mm/migrate.c       |  30 +++++++++++-
>  3 files changed, 158 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 311be90..18f978b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1662,5 +1662,22 @@ 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 isolate_balloon_page(struct page *);
> +extern bool putback_balloon_page(struct page *);
> +extern struct address_space *balloon_mapping;
> +
> +static inline bool movable_balloon_page(struct page *page)
> +{
> +	return (page->mapping && page->mapping == balloon_mapping);
> +}
> +
> +#else
> +static inline bool isolate_balloon_page(struct page *page) { return false; }
> +static inline bool putback_balloon_page(struct page *page) { return false; }
> +static inline bool movable_balloon_page(struct page *page) { return false; }
> +#endif /* (VIRTIO_BALLOON || VIRTIO_BALLOON_MODULE) && CONFIG_COMPACTION */
> +
>  #endif /* __KERNEL__ */
>  #endif /* _LINUX_MM_H */
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e78cb96..7372592 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -14,6 +14,7 @@
>  #include <linux/backing-dev.h>
>  #include <linux/sysctl.h>
>  #include <linux/sysfs.h>
> +#include <linux/export.h>
>  #include "internal.h"
>  
>  #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> @@ -21,6 +22,90 @@
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/compaction.h>
>  
> +#if defined(CONFIG_VIRTIO_BALLOON) || defined(CONFIG_VIRTIO_BALLOON_MODULE)
> +/*
> + * Balloon pages special page->mapping.
> + * Users must properly allocate and initialize an instance of balloon_mapping,
> + * and set it as the page->mapping for balloon enlisted page instances.
> + * There is no need on utilizing struct address_space locking schemes for
> + * balloon_mapping as, once it gets initialized at balloon driver, it will
> + * remain just like a static reference that helps us on identifying a guest
> + * ballooned page by its mapping, as well as it will keep the 'a_ops' callback
> + * pointers to the functions that will execute the balloon page mobility tasks.
> + *
> + * address_space_operations necessary methods for ballooned pages:
> + *   .migratepage    - used to perform balloon's page migration (as is)
> + *   .invalidatepage - used to isolate a page from balloon's page list
> + *   .freepage       - used to reinsert an isolated page to balloon's page list
> + */
> +struct address_space *balloon_mapping;
> +EXPORT_SYMBOL_GPL(balloon_mapping);
> +
> +static inline void __isolate_balloon_page(struct page *page)
> +{
> +	page->mapping->a_ops->invalidatepage(page, 0);
> +}
> +
> +static inline void __putback_balloon_page(struct page *page)
> +{
> +	page->mapping->a_ops->freepage(page);
> +}
> +
> +/* __isolate_lru_page() counterpart for a ballooned page */
> +bool isolate_balloon_page(struct page *page)
> +{
> +	if (WARN_ON(!movable_balloon_page(page)))
> +		return false;
> +
> +	if (likely(get_page_unless_zero(page))) {
> +		/*
> +		 * As balloon pages are not isolated from LRU lists, concurrent
> +		 * compaction threads can race against page migration functions
> +		 * move_to_new_page() & __unmap_and_move().
> +		 * In order to avoid having an already isolated balloon page
> +		 * being (wrongly) re-isolated while it is under migration,
> +		 * lets be sure we have the page lock before proceeding with
> +		 * the balloon page isolation steps.
> +		 */
> +		if (likely(trylock_page(page))) {
> +			/*
> +			 * A ballooned page, by default, has just one refcount.
> +			 * Prevent concurrent compaction threads from isolating
> +			 * an already isolated balloon page.
> +			 */
> +			if (movable_balloon_page(page) &&
> +			    (page_count(page) == 2)) {
> +				__isolate_balloon_page(page);
> +				unlock_page(page);
> +				return true;
> +			}
> +			unlock_page(page);
> +		}
> +		/* Drop refcount taken for this already isolated page */
> +		put_page(page);
> +	}
> +	return false;
> +}
> +
> +/* putback_lru_page() counterpart for a ballooned page */
> +bool putback_balloon_page(struct page *page)
> +{
> +	if (WARN_ON(!movable_balloon_page(page)))
> +		return false;
> +
> +	if (likely(trylock_page(page))) {
> +		if (movable_balloon_page(page)) {
> +			__putback_balloon_page(page);
> +			put_page(page);
> +			unlock_page(page);
> +			return true;
> +		}
> +		unlock_page(page);
> +	}
> +	return false;
> +}
> +#endif /* CONFIG_VIRTIO_BALLOON || CONFIG_VIRTIO_BALLOON_MODULE */
> +
>  static unsigned long release_freepages(struct list_head *freelist)
>  {
>  	struct page *page, *next;
> @@ -312,32 +397,40 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  			continue;
>  		}
>  
> -		if (!PageLRU(page))
> -			continue;
> -
>  		/*
> -		 * PageLRU is set, and lru_lock excludes isolation,
> -		 * splitting and collapsing (collapsing has already
> -		 * happened if PageLRU is set).
> +		 * It is possible to migrate LRU pages and balloon pages.
> +		 * Skip any other type of page.
>  		 */
> -		if (PageTransHuge(page)) {
> -			low_pfn += (1 << compound_order(page)) - 1;
> -			continue;
> -		}
> +		if (PageLRU(page)) {
> +			/*
> +			 * PageLRU is set, and lru_lock excludes isolation,
> +			 * splitting and collapsing (collapsing has already
> +			 * happened if PageLRU is set).
> +			 */
> +			if (PageTransHuge(page)) {
> +				low_pfn += (1 << compound_order(page)) - 1;
> +				continue;
> +			}
>  
> -		if (!cc->sync)
> -			mode |= ISOLATE_ASYNC_MIGRATE;
> +			if (!cc->sync)
> +				mode |= ISOLATE_ASYNC_MIGRATE;
>  
> -		lruvec = mem_cgroup_page_lruvec(page, zone);
> +			lruvec = mem_cgroup_page_lruvec(page, zone);
>  
> -		/* Try isolate the page */
> -		if (__isolate_lru_page(page, mode) != 0)
> -			continue;
> +			/* Try isolate the page */
> +			if (__isolate_lru_page(page, mode) != 0)
> +				continue;
> +
> +			VM_BUG_ON(PageTransCompound(page));
>  
> -		VM_BUG_ON(PageTransCompound(page));
> +			/* Successfully isolated */
> +			del_page_from_lru_list(page, lruvec, page_lru(page));
> +		} else if (unlikely(movable_balloon_page(page))) {
> +			if (!isolate_balloon_page(page))
> +				continue;
> +		} else
> +			continue;
>  
> -		/* Successfully isolated */
> -		del_page_from_lru_list(page, lruvec, page_lru(page));
>  		list_add(&page->lru, migratelist);
>  		cc->nr_migratepages++;
>  		nr_isolated++;
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 77ed2d7..871a304 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -79,7 +79,10 @@ void putback_lru_pages(struct list_head *l)
>  		list_del(&page->lru);
>  		dec_zone_page_state(page, NR_ISOLATED_ANON +
>  				page_is_file_cache(page));
> -		putback_lru_page(page);
> +		if (unlikely(movable_balloon_page(page)))
> +			WARN_ON(!putback_balloon_page(page));
> +		else
> +			putback_lru_page(page);
>  	}

Don't hack putback_lru_pages. It's a function for handling LRU pages
and is used by several places.
Plz, don't add complexity to unrelavant parts.

You can define a new function putback_migratepages should be used as pair with
isolate_migratepages_range so that both functions are aware of balloon page.
IMHO, it's better abstraction rather than hook of generic function.

Otherwise, Looks good to me.

Thanks.

-- 
Kind regards,
Minchan Kim

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