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

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxxx>
---
 drivers/xen/balloon.c |   73 +++++++++++++++++++++++++++++++++++++++++++++----
 include/xen/balloon.h |    3 ++
 2 files changed, 70 insertions(+), 6 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index f2fa9ec..9c35cec 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)) {
@@ -240,7 +243,7 @@ static enum bp_state increase_reservation(unsigned long 
nr_pages)
        }
 
        for (i = 0; i < rc; i++) {
-               page = balloon_retrieve();
+               page = balloon_retrieve(false);
                BUG_ON(page == NULL);
 
                pfn = page_to_pfn(page);
@@ -270,7 +273,7 @@ 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;
@@ -286,7 +289,7 @@ 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) {
                        pr_info("xen_balloon: %s: Cannot allocate memory\n", 
__func__);
                        nr_pages = i;
                        state = BP_EAGAIN;
@@ -348,7 +351,7 @@ 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);
 
@@ -374,6 +377,64 @@ 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, nr_pages, extra_pfn_end;
diff --git a/include/xen/balloon.h b/include/xen/balloon.h
index f72e479..a2b22f0 100644
--- a/include/xen/balloon.h
+++ b/include/xen/balloon.h
@@ -20,3 +20,6 @@ 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


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