Re: [PATCH] xen-netback: fix occasional leak of grant ref mappings under memory pressure

[Wei Liu]

On Thu, Feb 28, 2019 at 09:46:57AM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Igor Druzhinin [mailto:igor.druzhinin@xxxxxxxxxx]
> > Sent: 28 February 2019 02:03
> > To: xen-devel@xxxxxxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; davem@xxxxxxxxxxxxx; Igor
> > Druzhinin <igor.druzhinin@xxxxxxxxxx>
> > Subject: [PATCH] xen-netback: fix occasional leak of grant ref mappings under memory pressure
> > 
> > Zero-copy callback flag is not yet set on frag list skb at the moment
> > xenvif_handle_frag_list() returns -ENOMEM. This eventually results in
> > leaking grant ref mappings since xenvif_zerocopy_callback() is never
> > called for these fragments. Those eventually build up and cause Xen
> > to kill Dom0 as the slots get reused for new mappings.
> > 
> > That behavior is observed under certain workloads where sudden spikes
> > of page cache usage for writes coexist with active atomic skb allocations.
> > 
> > Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> > ---
> >  drivers/net/xen-netback/netback.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> > index 80aae3a..2023317 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -1146,9 +1146,12 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
> > 
> >  		if (unlikely(skb_has_frag_list(skb))) {
> >  			if (xenvif_handle_frag_list(queue, skb)) {
> > +				struct sk_buff *nskb =
> > +						skb_shinfo(skb)->frag_list;
> >  				if (net_ratelimit())
> >  					netdev_err(queue->vif->dev,
> >  						   "Not enough memory to consolidate frag_list!\n");
> > +				xenvif_skb_zerocopy_prepare(queue, nskb);
> >  				xenvif_skb_zerocopy_prepare(queue, skb);
> >  				kfree_skb(skb);
> >  				continue;
> 
> Whilst this fix will do the job, I think it would be better to get rid of the kfree_skb() from inside xenvif_handle_frag_list() and always deal with it here rather than having it happen in two different places. Something like the following...

+1 for having only one place. 

> 
> ---8<---
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 80aae3a32c2a..093c7b860772 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1027,13 +1027,13 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
>  /* Consolidate skb with a frag_list into a brand new one with local pages on
>   * frags. Returns 0 or -ENOMEM if can't allocate new pages.
>   */
> -static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *skb)
> +static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 80aae3a32c2a..093c7b860772 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1027,13 +1027,13 @@ static void xenvif_tx_build_gops(struct xenvif_queue *qu
> eue,
>  /* Consolidate skb with a frag_list into a brand new one with local pages on
>   * frags. Returns 0 or -ENOMEM if can't allocate new pages.
>   */
> -static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *
> skb)
> +static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *
> skb,
> +                                  struct sk_buff *nskb)
>  {
>         unsigned int offset = skb_headlen(skb);
>         skb_frag_t frags[MAX_SKB_FRAGS];
>         int i, f;
>         struct ubuf_info *uarg;
> -       struct sk_buff *nskb = skb_shinfo(skb)->frag_list;
> 
>         queue->stats.tx_zerocopy_sent += 2;
>         queue->stats.tx_frag_overflow++;
> @@ -1072,11 +1072,6 @@ static int xenvif_handle_frag_list(struct xenvif_queue *q
> ueue, struct sk_buff *s
>                 skb_frag_size_set(&frags[i], len);
>         }
> 
> -       /* Copied all the bits from the frag list -- free it. */
> -       skb_frag_list_init(skb);
> -       xenvif_skb_zerocopy_prepare(queue, nskb);
> -       kfree_skb(nskb);
> -
>         /* Release all the original (foreign) frags. */
>         for (f = 0; f < skb_shinfo(skb)->nr_frags; f++)
>                 skb_frag_unref(skb, f);
> @@ -1145,7 +1140,11 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
>                 xenvif_fill_frags(queue, skb);
> 
>                 if (unlikely(skb_has_frag_list(skb))) {
> -                       if (xenvif_handle_frag_list(queue, skb)) {
> +                       struct sk_buff *nskb = skb_shinfo(skb)->frag_list;
> +
> +                       xenvif_skb_zerocopy_prepare(queue, nskb);
> +
> +                       if (xenvif_handle_frag_list(queue, skb, nskb)) {
>                                 if (net_ratelimit())
>                                         netdev_err(queue->vif->dev,
>                                                    "Not enough memory to consolidate frag_list!\n");
> @@ -1153,6 +1152,10 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
>                                 kfree_skb(skb);
>                                 continue;
>                         }
> +
> +                       /* Copied all the bits from the frag list. */
> +                       skb_frag_list_init(skb);
> +                       kfree(nskb);

I think you want kfree_skb here?

Wei.

>                 }
> 
>                 skb->dev      = queue->vif->dev;
> ---8<---
> 
> What do you think?
> 
>   Paul
> 
> > --
> > 2.7.4
>