Re: [Qemu-devel] [PATCH v4] virtio-balloon: free pbp more aggressively

[David Hildenbrand <david@redhat.com>]

On 25.07.19 17:18, Michael S. Tsirkin wrote:
> Previous patches switched to a temporary pbp but that does not go far
> enough: after device uses a buffer, guest is free to reuse it, so
> tracking the page and freeing it later is wrong.
> 
> Free and reset the pbp after we push each element.
> 
> Fixes: ed48c59875b6 ("virtio-balloon: Safely handle BALLOON_PAGE_SIZE < host page size")
> Cc: qemu-stable@nongnu.org #v4.0.0
> Cc: David Hildenbrand <david@redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> 
> changes from v2, v3:
>     v3 was same as v2, sent by mistake
>     v4 fixes a typo
> 
>  hw/virtio/virtio-balloon.c | 37 ++++++++++++++++---------------------
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index fe9664e42c..25de154307 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -41,22 +41,19 @@ typedef struct PartiallyBalloonedPage {
>  
>  static void virtio_balloon_pbp_free(PartiallyBalloonedPage *pbp)
>  {
> -    if (!pbp) {
> +    if (!pbp->bitmap) {
>          return;
>      }
>      g_free(pbp->bitmap);
> -    g_free(pbp);
> +    pbp->bitmap = NULL;
>  }
>  
> -static PartiallyBalloonedPage *virtio_balloon_pbp_alloc(ram_addr_t base_gpa,
> -                                                        long subpages)
> +static void virtio_balloon_pbp_alloc(PartiallyBalloonedPage *pbp,
> +                                     ram_addr_t base_gpa,
> +                                     long subpages)
>  {
> -    PartiallyBalloonedPage *pbp = g_new0(PartiallyBalloonedPage, 1);
> -
>      pbp->base_gpa = base_gpa;
>      pbp->bitmap = bitmap_new(subpages);
> -
> -    return pbp;
>  }
>  
>  static bool virtio_balloon_pbp_matches(PartiallyBalloonedPage *pbp,
> @@ -67,7 +64,7 @@ static bool virtio_balloon_pbp_matches(PartiallyBalloonedPage *pbp,
>  
>  static void balloon_inflate_page(VirtIOBalloon *balloon,
>                                   MemoryRegion *mr, hwaddr mr_offset,
> -                                 PartiallyBalloonedPage **pbp)
> +                                 PartiallyBalloonedPage *pbp)
>  {
>      void *addr = memory_region_get_ram_ptr(mr) + mr_offset;
>      ram_addr_t rb_offset, rb_aligned_offset, base_gpa;
> @@ -104,22 +101,21 @@ static void balloon_inflate_page(VirtIOBalloon *balloon,
>      base_gpa = memory_region_get_ram_addr(mr) + mr_offset -
>                 (rb_offset - rb_aligned_offset);
>  
> -    if (*pbp && !virtio_balloon_pbp_matches(*pbp, base_gpa)) {
> +    if (pbp->bitmap && !virtio_balloon_pbp_matches(pbp, base_gpa)) {
>          /* We've partially ballooned part of a host page, but now
>           * we're trying to balloon part of a different one.  Too hard,
>           * give up on the old partial page */
> -        virtio_balloon_pbp_free(*pbp);
> -        *pbp = NULL;
> +        virtio_balloon_pbp_free(pbp);
>      }
>  
> -    if (!*pbp) {
> -        *pbp = virtio_balloon_pbp_alloc(base_gpa, subpages);
> +    if (!pbp->bitmap) {
> +        virtio_balloon_pbp_alloc(pbp, base_gpa, subpages);
>      }
>  
>      set_bit((rb_offset - rb_aligned_offset) / BALLOON_PAGE_SIZE,
> -            (*pbp)->bitmap);
> +            pbp->bitmap);
>  
> -    if (bitmap_full((*pbp)->bitmap, subpages)) {
> +    if (bitmap_full(pbp->bitmap, subpages)) {
>          /* We've accumulated a full host page, we can actually discard
>           * it now */
>  
> @@ -127,8 +123,7 @@ static void balloon_inflate_page(VirtIOBalloon *balloon,
>          /* We ignore errors from ram_block_discard_range(), because it
>           * has already reported them, and failing to discard a balloon
>           * page is not fatal */
> -        virtio_balloon_pbp_free(*pbp);
> -        *pbp = NULL;
> +        virtio_balloon_pbp_free(pbp);
>      }
>  }
>  
> @@ -328,13 +323,14 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
>  static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>  {
>      VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
> -    PartiallyBalloonedPage *pbp = NULL;
>      VirtQueueElement *elem;
>      MemoryRegionSection section;
>  
>      for (;;) {
> +        PartiallyBalloonedPage pbp = {};
>          size_t offset = 0;
>          uint32_t pfn;
> +
>          elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
>          if (!elem) {
>              break;
> @@ -379,9 +375,8 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>          virtqueue_push(vq, elem, offset);
>          virtio_notify(vdev, vq);
>          g_free(elem);
> +        virtio_balloon_pbp_free(&pbp);
>      }
> -
> -    virtio_balloon_pbp_free(pbp);
>  }
>  
>  static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)
> 

Works with hugetlbfs on x86 and LGTM:

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 

Thanks,

David / dhildenb