Re: [PATCH v10 3/5] virtio_balloon: introduce migration primitives to balloon pages

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, in order to
enhance the syncronization methods for accessing elements of struct
virtio_balloon, thus providing protection against concurrent access
introduced by parallel memory compaction threads.

 - balloon_lock (mutex) : synchronizes the access demand to elements of
                          struct virtio_balloon and its queue operations;
 - pages_lock (spinlock): special protection to balloon's pages bookmarking
                          elements (list and atomic counters) against the
                          potential memory compaction concurrency;


...

 struct virtio_balloon
 {

@@ -46,11 +48,24 @@ struct virtio_balloon
 	/* The thread servicing the balloon. */
 	struct task_struct *thread;
 
+	/* balloon special page->mapping */
+	struct address_space *mapping;
+
+	/* Synchronize access/update to this struct virtio_balloon elements */
+	struct mutex balloon_lock;
+
 	/* Waiting for host to ack the pages we released. */
 	wait_queue_head_t acked;
 
+	/* Protect pages list, and pages bookeeping counters */
+	spinlock_t pages_lock;
+
+	/* Number of balloon pages isolated from 'pages' list for compaction */
+	unsigned int num_isolated_pages;

 	/* 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

@@ -60,7 +75,7 @@ struct virtio_balloon
 
 	/* The array of pfns we tell the Host about. */
 	unsigned int num_pfns;
-	u32 pfns[256];
+	u32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
 
 	/* Memory statistics */
 	int need_stats_update;

@@ -122,13 +137,17 @@ static void set_page_pfns(u32 pfns[], struct page *page)
 
 static void fill_balloon(struct virtio_balloon *vb, size_t num)
 {
+	/* Get the proper GFP alloc mask from vb->mapping flags */
+	gfp_t vb_gfp_mask = mapping_gfp_mask(vb->mapping);
+
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
 
+	mutex_lock(&vb->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(vb_gfp_mask | __GFP_NORETRY |
+					       __GFP_NOWARN | __GFP_NOMEMALLOC);

@@ -139,9 +158,15 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
 			break;
 		}
 		set_page_pfns(vb->pfns + vb->num_pfns, page);
-		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
 		totalram_pages--;
+
+		BUG_ON(!trylock_page(page));
+		spin_lock(&vb->pages_lock);
 		list_add(&page->lru, &vb->pages);
+		assign_balloon_mapping(page, vb->mapping);
+		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
+		spin_unlock(&vb->pages_lock);
+		unlock_page(page);
 	}

...