xen-balloon: Add interface to retrieve ballooned pages

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.

Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
[v2: Deal with rebasing on top of modified balloon code]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 67 insertions, 6 deletions

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 7497041d0631..8c81cd24ed83 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -128,14 +128,17 @@ 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)) {
@@ -233,7 +236,7 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
                 return BP_EAGAIN;
 
         for (i = 0; i < rc; i++) {
-                page = balloon_retrieve();
+                page = balloon_retrieve(false);
                 BUG_ON(page == NULL);
 
                 pfn = page_to_pfn(page);
@@ -263,7 +266,7 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
         return BP_DONE;
 }
 
-static enum bp_state decrease_reservation(unsigned long nr_pages)
+static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
 {
         enum bp_state state = BP_DONE;
         unsigned long  pfn, i;
@@ -279,7 +282,7 @@ static enum bp_state decrease_reservation(unsigned long nr_pages)
                 nr_pages = ARRAY_SIZE(frame_list);
 
         for (i = 0; i < nr_pages; i++) {
-                if ((page = alloc_page(GFP_BALLOON)) == NULL) {
+                if ((page = alloc_page(gfp)) == NULL) {
                         nr_pages = i;
                         state = BP_EAGAIN;
                         break;
@@ -340,7 +343,7 @@ static void balloon_process(struct work_struct *work)
                         state = increase_reservation(credit);
 
                 if (credit < 0)
-                        state = decrease_reservation(-credit);
+                        state = decrease_reservation(-credit, GFP_BALLOON);
 
                 state = update_schedule(state);
 
@@ -366,6 +369,64 @@ 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(true);
+                if (page) {
+                        pages[pgno++] = page;
+                } else {
+                        enum bp_state st;
+                        st = decrease_reservation(nr_pages - pgno, GFP_HIGHUSER);
+                        if (st != BP_DONE)
+                                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_delayed_work(&balloon_worker, 0);
+        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_delayed_work(&balloon_worker, 0);
+
+        mutex_unlock(&balloon_mutex);
+}
+EXPORT_SYMBOL(free_xenballooned_pages);
+
 static int __init balloon_init(void)
 {
         unsigned long pfn, extra_pfn_end;