[PATCH v6 0/3] make balloon pages movable by compaction

[PATCH v6 0/3] make balloon pages movable by compaction

[Rafael Aquini]

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-set follows the main idea discussed at 2012 LSFMMS session:
"Ballooning for transparent huge pages" -- http://lwn.net/Articles/490114/
to introduce the required changes to the virtio_balloon driver, as well as
the changes to the core compaction & migration bits, in order to make those
subsystems aware of ballooned pages and allow memory balloon pages become
movable within a guest, thus avoiding the aforementioned fragmentation issue

Rafael Aquini (3):
  mm: introduce compaction and migration for virtio ballooned pages
  virtio_balloon: introduce migration primitives to balloon pages
  mm: add vm event counters for balloon pages compaction

 drivers/virtio/virtio_balloon.c | 139 +++++++++++++++++++++++++++++++++++++---
 include/linux/mm.h              |  17 +++++
 include/linux/virtio_balloon.h  |   4 ++
 include/linux/vm_event_item.h   |   2 +
 mm/compaction.c                 | 132 ++++++++++++++++++++++++++++++++------
 mm/migrate.c                    |  32 ++++++++-
 mm/vmstat.c                     |   4 ++
 7 files changed, 302 insertions(+), 28 deletions(-)

Change log:
v6:
 * rename 'is_balloon_page()' to 'movable_balloon_page()' (Rik);
v5:
 * address Andrew Morton's review comments on the patch series;
 * address a couple extra nitpick suggestions on PATCH 01 (Minchan);
v4: 
 * address Rusty Russel's review comments on PATCH 02;
 * re-base virtio_balloon patch on 9c378abc5c0c6fc8e3acf5968924d274503819b3;
V3: 
 * address reviewers nitpick suggestions on PATCH 01 (Mel, Minchan);
V2: 
 * address Mel Gorman's review comments on PATCH 01;


Preliminary test results:
(2 VCPU 1024mB RAM KVM guest running 3.6.0_rc1+ -- after a reboot)

* 64mB balloon:
[root@localhost ~]# awk '/compact/ {print}' /proc/vmstat
compact_blocks_moved 0
compact_pages_moved 0
compact_pagemigrate_failed 0
compact_stall 0
compact_fail 0
compact_success 0
compact_balloon_migrated 0
compact_balloon_failed 0
compact_balloon_isolated 0
compact_balloon_freed 0
[root@localhost ~]#
[root@localhost ~]# for i in $(seq 1 6); do echo 1 > /proc/sys/vm/compact_memory & done &>/dev/null 
[1]   Done                    echo 1 > /proc/sys/vm/compact_memory
[2]   Done                    echo 1 > /proc/sys/vm/compact_memory
[3]   Done                    echo 1 > /proc/sys/vm/compact_memory
[4]   Done                    echo 1 > /proc/sys/vm/compact_memory
[5]-  Done                    echo 1 > /proc/sys/vm/compact_memory
[6]+  Done                    echo 1 > /proc/sys/vm/compact_memory
[root@localhost ~]# 
[root@localhost ~]# awk '/compact/ {print}' /proc/vmstat
compact_blocks_moved 3520
compact_pages_moved 47548
compact_pagemigrate_failed 120
compact_stall 0
compact_fail 0
compact_success 0
compact_balloon_migrated 16378
compact_balloon_failed 0
compact_balloon_isolated 16378
compact_balloon_freed 16378

* 128mB balloon:
[root@localhost ~]# awk '/compact/ {print}' /proc/vmstat
compact_blocks_moved 0
compact_pages_moved 0
compact_pagemigrate_failed 0
compact_stall 0
compact_fail 0
compact_success 0
compact_balloon_migrated 0
compact_balloon_failed 0
compact_balloon_isolated 0
compact_balloon_freed 0
[root@localhost ~]#
[root@localhost ~]# for i in $(seq 1 6); do echo 1 > /proc/sys/vm/compact_memory & done &>/dev/null 
[1]   Done                    echo 1 > /proc/sys/vm/compact_memory
[2]   Done                    echo 1 > /proc/sys/vm/compact_memory
[3]   Done                    echo 1 > /proc/sys/vm/compact_memory
[4]   Done                    echo 1 > /proc/sys/vm/compact_memory
[5]-  Done                    echo 1 > /proc/sys/vm/compact_memory
[6]+  Done                    echo 1 > /proc/sys/vm/compact_memory
[root@localhost ~]# 
[root@localhost ~]# awk '/compact/ {print}' /proc/vmstat
compact_blocks_moved 3356
compact_pages_moved 47099
compact_pagemigrate_failed 158
compact_stall 0
compact_fail 0
compact_success 0
compact_balloon_migrated 26275
compact_balloon_failed 42
compact_balloon_isolated 26317
compact_balloon_freed 26275

-- 
1.7.11.2

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

[Rafael Aquini]

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);
 	}
 }
 
@@ -778,6 +781,17 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		}
 	}
 
+	if (unlikely(movable_balloon_page(page))) {
+		/*
+		 * A ballooned page does not need any special attention from
+		 * physical to virtual reverse mapping procedures.
+		 * Skip any attempt to unmap PTEs or to remap swap cache,
+		 * in order to avoid burning cycles at rmap level.
+		 */
+		remap_swapcache = 0;
+		goto skip_unmap;
+	}
+
 	/*
 	 * Corner case handling:
 	 * 1. When a new swap-cache page is read into, it is added to the LRU
@@ -846,6 +860,20 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 			goto out;
 
 	rc = __unmap_and_move(page, newpage, force, offlining, mode);
+
+	if (unlikely(movable_balloon_page(newpage))) {
+		/*
+		 * A ballooned page has been migrated already. Now, it is the
+		 * time to wrap-up counters, handle the old page back to Buddy
+		 * and return.
+		 */
+		list_del(&page->lru);
+		dec_zone_page_state(page, NR_ISOLATED_ANON +
+				    page_is_file_cache(page));
+		put_page(page);
+		__free_page(page);
+		return rc;
+	}
 out:
 	if (rc != -EAGAIN) {
 		/*
-- 
1.7.11.2

[PATCH v6 2/3] virtio_balloon: introduce migration primitives to balloon pages

[Rafael Aquini]

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.

Besides making balloon pages movable at allocation time and introducing
the necessary primitives to perform balloon page migration/compaction,
this patch also introduces the following locking scheme to provide the
proper synchronization and protection for struct virtio_balloon elements
against concurrent accesses due to parallel operations introduced by
memory compaction / page migration.
 - balloon_lock (mutex) : synchronizes the access demand to elements of
			  struct virtio_balloon and its queue operations;
 - pages_lock (spinlock): special protection to balloon pages list against
			  concurrent list handling operations;

Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 drivers/virtio/virtio_balloon.c | 138 +++++++++++++++++++++++++++++++++++++---
 include/linux/virtio_balloon.h  |   4 ++
 2 files changed, 134 insertions(+), 8 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 0908e60..7c937a0 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -27,6 +27,7 @@
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/module.h>
+#include <linux/fs.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by
@@ -35,6 +36,12 @@
  */
 #define VIRTIO_BALLOON_PAGES_PER_PAGE (PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
 
+/* Synchronizes accesses/updates to the struct virtio_balloon elements */
+DEFINE_MUTEX(balloon_lock);
+
+/* Protects 'virtio_balloon->pages' list against concurrent handling */
+DEFINE_SPINLOCK(pages_lock);
+
 struct virtio_balloon
 {
 	struct virtio_device *vdev;
@@ -51,6 +58,7 @@ struct virtio_balloon
 
 	/* Number of balloon pages we've told the Host we're not using. */
 	unsigned int num_pages;
+
 	/*
 	 * The pages we've told the Host we're not using.
 	 * Each page on this list adds VIRTIO_BALLOON_PAGES_PER_PAGE
@@ -125,10 +133,12 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
 
+	mutex_lock(&balloon_lock);
 	for (vb->num_pfns = 0; vb->num_pfns < num;
 	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
-		struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY |
-					__GFP_NOMEMALLOC | __GFP_NOWARN);
+		struct page *page = alloc_page(GFP_HIGHUSER_MOVABLE |
+						__GFP_NORETRY | __GFP_NOWARN |
+						__GFP_NOMEMALLOC);
 		if (!page) {
 			if (printk_ratelimit())
 				dev_printk(KERN_INFO, &vb->vdev->dev,
@@ -141,7 +151,10 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
 		set_page_pfns(vb->pfns + vb->num_pfns, page);
 		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
 		totalram_pages--;
+		spin_lock(&pages_lock);
 		list_add(&page->lru, &vb->pages);
+		page->mapping = balloon_mapping;
+		spin_unlock(&pages_lock);
 	}
 
 	/* Didn't get any?  Oh well. */
@@ -149,6 +162,7 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
 		return;
 
 	tell_host(vb, vb->inflate_vq);
+	mutex_unlock(&balloon_lock);
 }
 
 static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
@@ -169,10 +183,22 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
 
+	mutex_lock(&balloon_lock);
 	for (vb->num_pfns = 0; vb->num_pfns < num;
 	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+		/*
+		 * We can race against virtballoon_isolatepage() and end up
+		 * stumbling across a _temporarily_ empty 'pages' list.
+		 */
+		spin_lock(&pages_lock);
+		if (unlikely(list_empty(&vb->pages))) {
+			spin_unlock(&pages_lock);
+			break;
+		}
 		page = list_first_entry(&vb->pages, struct page, lru);
+		page->mapping = NULL;
 		list_del(&page->lru);
+		spin_unlock(&pages_lock);
 		set_page_pfns(vb->pfns + vb->num_pfns, page);
 		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}
@@ -182,8 +208,11 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
 	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
 	 * is true, we *have* to do it in this order
 	 */
-	tell_host(vb, vb->deflate_vq);
-	release_pages_by_pfn(vb->pfns, vb->num_pfns);
+	if (vb->num_pfns > 0) {
+		tell_host(vb, vb->deflate_vq);
+		release_pages_by_pfn(vb->pfns, vb->num_pfns);
+	}
+	mutex_unlock(&balloon_lock);
 }
 
 static inline void update_stat(struct virtio_balloon *vb, int idx,
@@ -239,6 +268,7 @@ static void stats_handle_request(struct virtio_balloon *vb)
 	struct scatterlist sg;
 	unsigned int len;
 
+	mutex_lock(&balloon_lock);
 	vb->need_stats_update = 0;
 	update_balloon_stats(vb);
 
@@ -249,6 +279,7 @@ static void stats_handle_request(struct virtio_balloon *vb)
 	if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
 		BUG();
 	virtqueue_kick(vq);
+	mutex_unlock(&balloon_lock);
 }
 
 static void virtballoon_changed(struct virtio_device *vdev)
@@ -261,22 +292,27 @@ static void virtballoon_changed(struct virtio_device *vdev)
 static inline s64 towards_target(struct virtio_balloon *vb)
 {
 	__le32 v;
-	s64 target;
+	s64 target, actual;
 
+	mutex_lock(&balloon_lock);
+	actual = vb->num_pages;
 	vb->vdev->config->get(vb->vdev,
 			      offsetof(struct virtio_balloon_config, num_pages),
 			      &v, sizeof(v));
 	target = le32_to_cpu(v);
-	return target - vb->num_pages;
+	mutex_unlock(&balloon_lock);
+	return target - actual;
 }
 
 static void update_balloon_size(struct virtio_balloon *vb)
 {
-	__le32 actual = cpu_to_le32(vb->num_pages);
-
+	__le32 actual;
+	mutex_lock(&balloon_lock);
+	actual = cpu_to_le32(vb->num_pages);
 	vb->vdev->config->set(vb->vdev,
 			      offsetof(struct virtio_balloon_config, actual),
 			      &actual, sizeof(actual));
+	mutex_unlock(&balloon_lock);
 }
 
 static int balloon(void *_vballoon)
@@ -339,6 +375,76 @@ static int init_vqs(struct virtio_balloon *vb)
 	return 0;
 }
 
+/*
+ * '*vb_ptr' allows virtballoon_migratepage() & virtballoon_putbackpage() to
+ * access pertinent elements from struct virtio_balloon
+ */
+struct virtio_balloon *vb_ptr;
+
+/*
+ * Populate balloon_mapping->a_ops->migratepage method to perform the balloon
+ * page migration task.
+ *
+ * After a ballooned page gets isolated by compaction procedures, this is the
+ * function that performs the page migration on behalf of move_to_new_page(),
+ * when the last calls (page)->mapping->a_ops->migratepage.
+ *
+ * Page migration for virtio balloon is done in a simple swap fashion which
+ * follows these two steps:
+ *  1) insert newpage into vb->pages list and update the host about it;
+ *  2) update the host about the removed old page from vb->pages list;
+ */
+int virtballoon_migratepage(struct address_space *mapping,
+		struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+	mutex_lock(&balloon_lock);
+
+	/* balloon's page migration 1st step */
+	vb_ptr->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
+	spin_lock(&pages_lock);
+	list_add(&newpage->lru, &vb_ptr->pages);
+	spin_unlock(&pages_lock);
+	set_page_pfns(vb_ptr->pfns, newpage);
+	tell_host(vb_ptr, vb_ptr->inflate_vq);
+
+	/* balloon's page migration 2nd step */
+	vb_ptr->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
+	set_page_pfns(vb_ptr->pfns, page);
+	tell_host(vb_ptr, vb_ptr->deflate_vq);
+
+	mutex_unlock(&balloon_lock);
+
+	return 0;
+}
+
+/*
+ * Populate balloon_mapping->a_ops->invalidatepage method to help compaction on
+ * isolating a page from the balloon page list.
+ */
+void virtballoon_isolatepage(struct page *page, unsigned long mode)
+{
+	spin_lock(&pages_lock);
+	list_del(&page->lru);
+	spin_unlock(&pages_lock);
+}
+
+/*
+ * Populate balloon_mapping->a_ops->freepage method to help compaction on
+ * re-inserting an isolated page into the balloon page list.
+ */
+void virtballoon_putbackpage(struct page *page)
+{
+	spin_lock(&pages_lock);
+	list_add(&page->lru, &vb_ptr->pages);
+	spin_unlock(&pages_lock);
+}
+
+static const struct address_space_operations virtio_balloon_aops = {
+	.migratepage = virtballoon_migratepage,
+	.invalidatepage = virtballoon_isolatepage,
+	.freepage = virtballoon_putbackpage,
+};
+
 static int virtballoon_probe(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb;
@@ -351,11 +457,25 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	}
 
 	INIT_LIST_HEAD(&vb->pages);
+
 	vb->num_pages = 0;
 	init_waitqueue_head(&vb->config_change);
 	init_waitqueue_head(&vb->acked);
 	vb->vdev = vdev;
 	vb->need_stats_update = 0;
+	vb_ptr = vb;
+
+	/* Init the ballooned page->mapping special balloon_mapping */
+	balloon_mapping = kmalloc(sizeof(*balloon_mapping), GFP_KERNEL);
+	if (!balloon_mapping) {
+		err = -ENOMEM;
+		goto out_free_vb;
+	}
+
+	INIT_RADIX_TREE(&balloon_mapping->page_tree, GFP_ATOMIC | __GFP_NOWARN);
+	INIT_LIST_HEAD(&balloon_mapping->i_mmap_nonlinear);
+	spin_lock_init(&balloon_mapping->tree_lock);
+	balloon_mapping->a_ops = &virtio_balloon_aops;
 
 	err = init_vqs(vb);
 	if (err)
@@ -373,6 +493,7 @@ out_del_vqs:
 	vdev->config->del_vqs(vdev);
 out_free_vb:
 	kfree(vb);
+	kfree(balloon_mapping);
 out:
 	return err;
 }
@@ -397,6 +518,7 @@ static void __devexit virtballoon_remove(struct virtio_device *vdev)
 	kthread_stop(vb->thread);
 	remove_common(vb);
 	kfree(vb);
+	kfree(balloon_mapping);
 }
 
 #ifdef CONFIG_PM
diff --git a/include/linux/virtio_balloon.h b/include/linux/virtio_balloon.h
index 652dc8b..930f1b7 100644
--- a/include/linux/virtio_balloon.h
+++ b/include/linux/virtio_balloon.h
@@ -56,4 +56,8 @@ struct virtio_balloon_stat {
 	u64 val;
 } __attribute__((packed));
 
+#if !defined(CONFIG_COMPACTION)
+struct address_space *balloon_mapping;
+#endif
+
 #endif /* _LINUX_VIRTIO_BALLOON_H */
-- 
1.7.11.2

[PATCH v6 3/3] mm: add vm event counters for balloon pages compaction

[Rafael Aquini]

This patch is only for testing report purposes and shall be dropped in case of
the rest of this patchset getting accepted for merging.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 drivers/virtio/virtio_balloon.c | 1 +
 include/linux/vm_event_item.h   | 2 ++
 mm/compaction.c                 | 1 +
 mm/migrate.c                    | 6 ++++--
 mm/vmstat.c                     | 4 ++++
 5 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 7c937a0..b8f7ea5 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -414,6 +414,7 @@ int virtballoon_migratepage(struct address_space *mapping,
 
 	mutex_unlock(&balloon_lock);
 
+	count_vm_event(COMPACTBALLOONMIGRATED);
 	return 0;
 }
 
diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 57f7b10..a632a5d 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -41,6 +41,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 #ifdef CONFIG_COMPACTION
 		COMPACTBLOCKS, COMPACTPAGES, COMPACTPAGEFAILED,
 		COMPACTSTALL, COMPACTFAIL, COMPACTSUCCESS,
+		COMPACTBALLOONMIGRATED, COMPACTBALLOONFAILED,
+		COMPACTBALLOONISOLATED, COMPACTBALLOONFREED,
 #endif
 #ifdef CONFIG_HUGETLB_PAGE
 		HTLB_BUDDY_PGALLOC, HTLB_BUDDY_PGALLOC_FAIL,
diff --git a/mm/compaction.c b/mm/compaction.c
index 7372592..5d6a344 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -77,6 +77,7 @@ bool isolate_balloon_page(struct page *page)
 			    (page_count(page) == 2)) {
 				__isolate_balloon_page(page);
 				unlock_page(page);
+				count_vm_event(COMPACTBALLOONISOLATED);
 				return true;
 			}
 			unlock_page(page);
diff --git a/mm/migrate.c b/mm/migrate.c
index 871a304..4115875 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -79,9 +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));
-		if (unlikely(movable_balloon_page(page)))
+		if (unlikely(movable_balloon_page(page))) {
+			count_vm_event(COMPACTBALLOONFAILED);
 			WARN_ON(!putback_balloon_page(page));
-		else
+		} else
 			putback_lru_page(page);
 	}
 }
@@ -872,6 +873,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 				    page_is_file_cache(page));
 		put_page(page);
 		__free_page(page);
+		count_vm_event(COMPACTBALLOONFREED);
 		return rc;
 	}
 out:
diff --git a/mm/vmstat.c b/mm/vmstat.c
index df7a674..8d80f60 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -768,6 +768,10 @@ const char * const vmstat_text[] = {
 	"compact_stall",
 	"compact_fail",
 	"compact_success",
+	"compact_balloon_migrated",
+	"compact_balloon_failed",
+	"compact_balloon_isolated",
+	"compact_balloon_freed",
 #endif
 
 #ifdef CONFIG_HUGETLB_PAGE
-- 
1.7.11.2

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

[Rik van Riel]

On 08/08/2012 06:53 PM, 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>

Reviewed-by: Rik van Riel <riel@redhat.com>

-- 
All rights reversed

Re: [PATCH v6 2/3] virtio_balloon: introduce migration primitives to balloon pages

[Rik van Riel]

On 08/08/2012 06:53 PM, 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.
>
> Besides making balloon pages movable at allocation time and introducing
> the necessary primitives to perform balloon page migration/compaction,
> this patch also introduces the following locking scheme to provide the
> proper synchronization and protection for struct virtio_balloon elements
> against concurrent accesses due to parallel operations introduced by
> memory compaction / page migration.
>   - balloon_lock (mutex) : synchronizes the access demand to elements of
> 			  struct virtio_balloon and its queue operations;
>   - pages_lock (spinlock): special protection to balloon pages list against
> 			  concurrent list handling operations;
>
> Signed-off-by: Rafael Aquini<aquini@redhat.com>

Reviewed-by: Rik van Riel <riel@redhat.com>

-- 
All rights reversed

Re: [PATCH v6 3/3] mm: add vm event counters for balloon pages compaction

[Rik van Riel]

On 08/08/2012 06:53 PM, Rafael Aquini wrote:
> This patch is only for testing report purposes and shall be dropped in case of
> the rest of this patchset getting accepted for merging.

I wonder if it would make sense to just keep these statistics, so
if a change breaks balloon migration in the future, we'll be able
to see it...

> Signed-off-by: Rafael Aquini<aquini@redhat.com>

Acked-by: Rik van Riel <riel@redhat.com>

-- 
All rights reversed

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

[Minchan Kim]

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

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

[Mel Gorman]

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>

Mostly looks ok but I have one question;

> <SNIP>
>
> +/* 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);
> +	}

You might have answered this already as I skipped over a few revisions
and if you have, sorry about that and please add a comment :)

This trylock_page looks risky as it looks like it can fail if another
process running compaction tries to isolate this page. It locks the page,
finds it cant and releases the lock but in the meantime this trylock can
fail. It triggers a WARN_ON so we'll get a bug report but it leaves the
reference count elevated and this page has now leaked.

Why not just lock_page(page)? As you have already isolated this page you
know that the lock is only going to be held by a parallel compacting
process checking the reference count and the delay will be short. As a
bonus you can drop the WARN_ON check in the caller and make this void as
the WARN_ON check in the caller becomes redundant.

-- 
Mel Gorman
SUSE Labs

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

[Rafael Aquini]

On Thu, Aug 09, 2012 at 10:00:19AM +0100, Mel Gorman wrote:
> 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>
> 
> Mostly looks ok but I have one question;
> 
> > <SNIP>
> >
> > +/* 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);
> > +	}
> 
> You might have answered this already as I skipped over a few revisions
> and if you have, sorry about that and please add a comment :)
> 
> This trylock_page looks risky as it looks like it can fail if another
> process running compaction tries to isolate this page. It locks the page,
> finds it cant and releases the lock but in the meantime this trylock can
> fail. It triggers a WARN_ON so we'll get a bug report but it leaves the
> reference count elevated and this page has now leaked.
>
Good catch!
I had those bits changed to follow the same logics you had suggested for
isolate_balloon_page(), but I ended up completely missing this potential page
leak case you spotted. Thanks a lot!
 
> Why not just lock_page(page)? As you have already isolated this page you
> know that the lock is only going to be held by a parallel compacting
> process checking the reference count and the delay will be short. As a
> bonus you can drop the WARN_ON check in the caller and make this void as
> the WARN_ON check in the caller becomes redundant.
> 
Sure! 
what do you think of:

+/* putback_lru_page() counterpart for a ballooned page */
+void putback_balloon_page(struct page *page)
+{
+   lock_page(page);
+   if (!WARN_ON(!movable_balloon_page(page))) {
+           __putback_balloon_page(page);
+           put_page(page);
+   }
+   unlock_page(page);
+}

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

[Rafael Aquini]

On Thu, Aug 09, 2012 at 11:48:36AM -0300, Rafael Aquini wrote:
> Sure! 
> what do you think of:
> 
> +/* putback_lru_page() counterpart for a ballooned page */
> +void putback_balloon_page(struct page *page)
> +{
> +   lock_page(page);
> +   if (!WARN_ON(!movable_balloon_page(page))) {
> +           __putback_balloon_page(page);
> +           put_page(page);
> +   }
> +   unlock_page(page);
> +}
>
Or perhaps
 
+/* putback_lru_page() counterpart for a ballooned page */
+void putback_balloon_page(struct page *page)
+{
+   if (!WARN_ON(!movable_balloon_page(page))) {
+           lock_page(page);
+           __putback_balloon_page(page);
+           put_page(page);
+           unlock_page(page);
+   }
+}

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

[Mel Gorman]

On Thu, Aug 09, 2012 at 12:12:19PM -0300, Rafael Aquini wrote:
> On Thu, Aug 09, 2012 at 11:48:36AM -0300, Rafael Aquini wrote:
> > Sure! 
> > what do you think of:
> > 
> > +/* putback_lru_page() counterpart for a ballooned page */
> > +void putback_balloon_page(struct page *page)
> > +{
> > +   lock_page(page);
> > +   if (!WARN_ON(!movable_balloon_page(page))) {
> > +           __putback_balloon_page(page);
> > +           put_page(page);
> > +   }
> > +   unlock_page(page);
> > +}
> >
> Or perhaps
>  
> +/* putback_lru_page() counterpart for a ballooned page */
> +void putback_balloon_page(struct page *page)
> +{
> +   if (!WARN_ON(!movable_balloon_page(page))) {
> +           lock_page(page);
> +           __putback_balloon_page(page);
> +           put_page(page);
> +           unlock_page(page);
> +   }
> +}

That should be fine. I find the WARN_ON construct odd to read but only
because it's unusual. It is more typical to see something like

if (WARN_ON(!movable_balooon_page(page)))
	return;

lock_page(page);
__putback_balloon_page(page);
put_page(page);
unlock_page(page);

but either works. Do not forget to update the caller of course.

Thanks.


-- 
Mel Gorman
SUSE Labs