[qemu-xen stable-4.14] virtio-balloon: always indicate S_DONE when migration fails

[patchbot@xxxxxxx]

commit 6261aa4ff5e705a0f5ddd15c53d7b7051bace46d
Author:     David Hildenbrand <david@xxxxxxxxxx>
AuthorDate: Mon Jun 29 10:06:15 2020 +0200
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Wed Sep 2 19:06:19 2020 -0500

    virtio-balloon: always indicate S_DONE when migration fails
    
    If something goes wrong during precopy, before stopping the VM, we will
    never send a S_DONE indication to the VM, resulting in the hinted pages
    not getting released to be used by the guest OS (e.g., Linux).
    
    Easy to reproduce:
    1. Start migration (e.g., HMP "migrate -d 'exec:gzip -c > STATEFILE.gz'")
    2. Cancel migration (e.g., HMP "migrate_cancel")
    3. Oberve in the guest (e.g., cat /proc/meminfo) that there is basically
       no free memory left.
    
    While at it, add similar locking to virtio_balloon_free_page_done() as
    done in virtio_balloon_free_page_stop. Locking is still weird, but that
    has to be sorted out separately.
    
    There is nothing to do in the PRECOPY_NOTIFY_COMPLETE case. Add some
    comments regarding S_DONE handling.
    
    Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
    Reviewed-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
    Cc: Wei Wang <wei.w.wang@xxxxxxxxx>
    Cc: Alexander Duyck <alexander.duyck@xxxxxxxxx>
    Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
    Message-Id: <20200629080615.26022-1-david@xxxxxxxxxx>
    Reviewed-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
    Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
    (cherry picked from commit dd8eeb9671fc881e613008bd20035b85fe45383d)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 hw/virtio/virtio-balloon.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index d6e31de1d9..6c75db123e 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -572,8 +572,13 @@ static void virtio_balloon_free_page_done(VirtIOBalloon *s)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(s);
 
-    s->free_page_report_status = FREE_PAGE_REPORT_S_DONE;
-    virtio_notify_config(vdev);
+    if (s->free_page_report_status != FREE_PAGE_REPORT_S_DONE) {
+        /* See virtio_balloon_free_page_stop() */
+        qemu_mutex_lock(&s->free_page_lock);
+        s->free_page_report_status = FREE_PAGE_REPORT_S_DONE;
+        qemu_mutex_unlock(&s->free_page_lock);
+        virtio_notify_config(vdev);
+    }
 }
 
 static int
@@ -597,17 +602,26 @@ virtio_balloon_free_page_report_notify(NotifierWithReturn 
*n, void *data)
     case PRECOPY_NOTIFY_SETUP:
         precopy_enable_free_page_optimization();
         break;
-    case PRECOPY_NOTIFY_COMPLETE:
-    case PRECOPY_NOTIFY_CLEANUP:
     case PRECOPY_NOTIFY_BEFORE_BITMAP_SYNC:
         virtio_balloon_free_page_stop(dev);
         break;
     case PRECOPY_NOTIFY_AFTER_BITMAP_SYNC:
         if (vdev->vm_running) {
             virtio_balloon_free_page_start(dev);
-        } else {
-            virtio_balloon_free_page_done(dev);
+            break;
         }
+        /*
+         * Set S_DONE before migrating the vmstate, so the guest will reuse
+         * all hinted pages once running on the destination. Fall through.
+         */
+    case PRECOPY_NOTIFY_CLEANUP:
+        /*
+         * Especially, if something goes wrong during precopy or if migration
+         * is canceled, we have to properly communicate S_DONE to the VM.
+         */
+        virtio_balloon_free_page_done(dev);
+        break;
+    case PRECOPY_NOTIFY_COMPLETE:
         break;
     default:
         virtio_error(vdev, "%s: %d reason unknown", __func__, pnd->reason);
--
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