Re: [Xen-devel] [PATCHv1 2/2] xen-netback: unref frags when handling a from-guest skb with a frag list

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 03/03/15 16:26, David Vrabel wrote:
> Every time a VIF is destroyed up-to 256 pages may be leaked if packets
                                "up to" (and maybe a comma before it?)
> with more than MAX_SKB_FRAGS frags where transmitted from the guest.
                                     were
> Even worse, if another user of ballooned pages allocated one of these
> ballooned pages it would not handle the the unexpectedly non-zero page
                                          ^^^
One too many "the"
> count (e.g., gntdev would deadlock when unmapping a grant because the
> page count would never reach 1).
>
> When handling a from-guest skb with a frag list, unref the frags
> before releasing them so they are freed correctly when the VIF is
> destroyed.
>
> Also swap over to the new (local) frags /after/ calling the skb
> destructor.  This isn't strictly necessary but it's less confusing.
>
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Reviewed-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> ---
>   drivers/net/xen-netback/netback.c |   43 +++++++++++++++++++------------------
>   1 file changed, 22 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/net/xen-netback/netback.c 
> b/drivers/net/xen-netback/netback.c
> index f7a31d2..3d06eeb 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1343,7 +1343,7 @@ static int xenvif_handle_frag_list(struct xenvif_queue 
> *queue, struct sk_buff *s
>   {
>         unsigned int offset = skb_headlen(skb);
>         skb_frag_t frags[MAX_SKB_FRAGS];
> -       int i;
> +       int i, nr_frags;
>         struct ubuf_info *uarg;
>         struct sk_buff *nskb = skb_shinfo(skb)->frag_list;
>
> @@ -1357,17 +1357,16 @@ static int xenvif_handle_frag_list(struct xenvif_queue 
> *queue, struct sk_buff *s
>         skb->data_len += nskb->len;
>
>         /* create a brand new frags array and coalesce there */
> -       for (i = 0; offset < skb->len; i++) {
> +       for (nr_frags = 0; offset < skb->len; nr_frags++) {
>                 struct page *page;
>                 unsigned int len;
>
> -               BUG_ON(i >= MAX_SKB_FRAGS);
> +               BUG_ON(nr_frags >= MAX_SKB_FRAGS);
>                 page = alloc_page(GFP_ATOMIC);
>                 if (!page) {
> -                       int j;
>                         skb->truesize += skb->data_len;
> -                       for (j = 0; j < i; j++)
> -                               put_page(frags[j].page.p);
> +                       for (i = 0; i < nr_frags; i++)
> +                               put_page(frags[i].page.p);
>                         return -ENOMEM;
>                 }
>
> @@ -1379,27 +1378,29 @@ static int xenvif_handle_frag_list(struct xenvif_queue 
> *queue, struct sk_buff *s
>                         BUG();
>
>                 offset += len;
> -               frags[i].page.p = page;
> -               frags[i].page_offset = 0;
> -               skb_frag_size_set(&frags[i], len);
> +               frags[nr_frags].page.p = page;
> +               frags[nr_frags].page_offset = 0;
> +               skb_frag_size_set(&frags[nr_frags], len);
>         }
> -       /* swap out with old one */
> -       memcpy(skb_shinfo(skb)->frags,
> -              frags,
> -              i * sizeof(skb_frag_t));
> -       skb_shinfo(skb)->nr_frags = i;
> -       skb->truesize += i * PAGE_SIZE;
> -
> -       /* remove traces of mapped pages and frag_list */
> +
> +       /* Copied all the bits from the frag list -- free it. */
>         skb_frag_list_init(skb);
> -       uarg = skb_shinfo(skb)->destructor_arg;
> -       /* increase inflight counter to offset decrement in callback */
> +       xenvif_skb_zerocopy_prepare(queue, nskb);
> +       kfree_skb(nskb);
> +
> +       /* Release all the original (foreign) frags. */
> +       for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
> +               skb_frag_unref(skb, i);
>         atomic_inc(&queue->inflight_packets);
> +       uarg = skb_shinfo(skb)->destructor_arg;
>         uarg->callback(uarg, true);
>         skb_shinfo(skb)->destructor_arg = NULL;
>
> -       xenvif_skb_zerocopy_prepare(queue, nskb);
> -       kfree_skb(nskb);
> +       /* Fill the skb with the new (local) frags. */
> +       memcpy(skb_shinfo(skb)->frags, frags,
> +              nr_frags * sizeof(skb_frag_t));
> +       skb_shinfo(skb)->nr_frags = nr_frags;
> +       skb->truesize += nr_frags * PAGE_SIZE;
>
>         return 0;
>   }
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