[PATCH 2/3 v4] xen-balloon: Add interface to retrieve ballooned pages

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

Pages that have been ballooned are useful for other Xen drivers doing
grant table actions, because these pages have valid struct page/PFNs but
have no valid MFN so are available for remapping.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
---
 drivers/xen/balloon.c |   65 ++++++++++++++++++++++++++++++++++++++++++++++--
 include/xen/balloon.h |    3 ++
 2 files changed, 65 insertions(+), 3 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index b0a7a92..d187fce 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -116,14 +116,17 @@ static void balloon_append(struct page *page)
 }
 
 /* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
-static struct page *balloon_retrieve(void)
+static struct page *balloon_retrieve(bool prefer_highmem)
 {
 	struct page *page;
 
 	if (list_empty(&ballooned_pages))
 		return NULL;
 
-	page = list_entry(ballooned_pages.next, struct page, lru);
+	if (prefer_highmem)
+		page = list_entry(ballooned_pages.prev, struct page, lru);
+	else
+		page = list_entry(ballooned_pages.next, struct page, lru);
 	list_del(&page->lru);
 
 	if (PageHighMem(page)) {
@@ -198,7 +201,7 @@ static int increase_reservation(unsigned long nr_pages)
 		goto out;
 
 	for (i = 0; i < rc; i++) {
-		page = balloon_retrieve();
+		page = balloon_retrieve(0);
 		BUG_ON(page == NULL);
 
 		pfn = page_to_pfn(page);
@@ -328,6 +331,62 @@ void balloon_set_new_target(unsigned long target)
 }
 EXPORT_SYMBOL_GPL(balloon_set_new_target);
 
+/**
+ * alloc_xenballooned_pages - get pages that have been ballooned out
+ * @nr_pages: Number of pages to get
+ * @pages: pages returned
+ * @return 0 on success, error otherwise
+ */
+int alloc_xenballooned_pages(int nr_pages, struct page** pages)
+{
+	int pgno = 0;
+	struct page* page;
+	mutex_lock(&balloon_mutex);
+	while (pgno < nr_pages) {
+		page = balloon_retrieve(1);
+		if (page) {
+			pages[pgno++] = page;
+		} else {
+			if (decrease_reservation(nr_pages - pgno))
+				goto out_undo;
+		}
+	}
+	mutex_unlock(&balloon_mutex);
+	return 0;
+ out_undo:
+	while (pgno)
+		balloon_append(pages[--pgno]);
+	/* Free the memory back to the kernel soon */
+	schedule_work(&balloon_worker);
+	mutex_unlock(&balloon_mutex);
+	return -ENOMEM;
+}
+EXPORT_SYMBOL(alloc_xenballooned_pages);
+
+/**
+ * free_xenballooned_pages - return pages retrieved with get_ballooned_pages
+ * @nr_pages: Number of pages
+ * @pages: pages to return
+ */
+void free_xenballooned_pages(int nr_pages, struct page** pages)
+{
+	int i;
+
+	mutex_lock(&balloon_mutex);
+
+	for (i = 0; i < nr_pages; i++) {
+		if (pages[i])
+			balloon_append(pages[i]);
+	}
+
+	/* The balloon may be too large now. Shrink it if needed. */
+	if (current_target() != balloon_stats.current_pages)
+		schedule_work(&balloon_worker);
+
+	mutex_unlock(&balloon_mutex);
+}
+EXPORT_SYMBOL(free_xenballooned_pages);
+
 static int __init balloon_init(void)
 {
  	unsigned long pfn, nr_pages, extra_pfn_end;
diff --git a/include/xen/balloon.h b/include/xen/balloon.h
index b2b7c21..72a6927 100644
--- a/include/xen/balloon.h
+++ b/include/xen/balloon.h
@@ -19,3 +19,6 @@ struct balloon_stats {
 extern struct balloon_stats balloon_stats;
 
 void balloon_set_new_target(unsigned long target);
+
+int alloc_xenballooned_pages(int nr_pages, struct page** pages);
+void free_xenballooned_pages(int nr_pages, struct page** pages);
-- 
1.7.3.4