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

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@citrix.com]
> Sent: 28 February 2019 11:02
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: Igor Druzhinin <igor.druzhinin@citrix.com>; xen-devel@lists.xenproject.org;
> netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Wei Liu <wei.liu2@citrix.com>;
> davem@davemloft.net
> Subject: Re: [PATCH] xen-netback: fix occasional leak of grant ref mappings under memory pressure
> 
> On Thu, Feb 28, 2019 at 09:46:57AM +0000, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Igor Druzhinin [mailto:igor.druzhinin@citrix.com]
> > > Sent: 28 February 2019 02:03
> > > To: xen-devel@lists.xenproject.org; netdev@vger.kernel.org; linux-kernel@vger.kernel.org
> > > Cc: Wei Liu <wei.liu2@citrix.com>; Paul Durrant <Paul.Durrant@citrix.com>; davem@davemloft.net;
> Igor
> > > Druzhinin <igor.druzhinin@citrix.com>
> > > 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@citrix.com>
> > > ---
> > >  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?

No. nskb is the frag list... it is unlinked from skb by the call to skb_frag_list_init() and then it can be freed on its own. The skb is what we need to retain, because that now contains all the data.

  Cheers,

    Paul

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