Re: [Xen-devel] [PATCH net 3/4] xen-netback: Fix releasing header slot on error path

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 18/07/14 16:25, Wei Liu wrote:
> On Thu, Jul 17, 2014 at 08:09:51PM +0100, Zoltan Kiss wrote:
> > This patch makes this function aware that the first frag and the header might
> > share the same ring slot. That could happen if the first slot is bigger than
> > MAX_SKB_LEN. Due to this the error path might release that slot twice or never,
> I guess you mean PKT_PROT_LEN.
Yes
> Just one question, how come that we didn't come across this with copying
> backend? Comparing txreq.size against PKT_PROT_LEN is not new in mapping
> backend.
We had one grant copy for the header and the first frag in that case, 
and we skipped the first frag:
        /* Skip first skb fragment if it is on same page as header fragment. */
        start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);


> > depending on the error scenario.
> > xenvif_idx_release is also removed from xenvif_idx_unmap, and called separately.
> >
> > Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> > Reported-by: Armin Zentai <armin.zentai@xxxxxxx>
> > Cc: netdev@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > ---
> > diff --git a/drivers/net/xen-netback/netback.c 
> > b/drivers/net/xen-netback/netback.c
> > index e9ffb05..938d5a9 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -1039,6 +1039,8 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
> >          */
> >         struct skb_shared_info *first_shinfo = NULL;
> >         int nr_frags = shinfo->nr_frags;
> > +       const bool sharedslot = nr_frags &&
> > +                               frag_get_pending_idx(&shinfo->frags[0]) == 
> > pending_idx;
> >         int i, err;
> >
> >         /* Check status of header. */
> > @@ -1051,7 +1053,10 @@ static int xenvif_tx_check_gop(struct xenvif_queue 
> > *queue,
> >                                    (*gopp_copy)->status,
> >                                    pending_idx,
> >                                    (*gopp_copy)->source.u.ref);
> > -               xenvif_idx_release(queue, pending_idx, XEN_NETIF_RSP_ERROR);
> > +               /* The first frag might still have this slot mapped */
> > +               if (!sharedslot)
> > +                       xenvif_idx_release(queue, pending_idx,
> > +                                          XEN_NETIF_RSP_ERROR);
> >         }
> >
> >   check_frags:
> > @@ -1068,8 +1073,19 @@ check_frags:
> >                                                 pending_idx,
> >                                                 gop_map->handle);
> >                         /* Had a previous error? Invalidate this fragment. */
> > -                       if (unlikely(err))
> > +                       if (unlikely(err)) {
> >                                 xenvif_idx_unmap(queue, pending_idx);
> > +                               /* If the mapping of the first frag was OK, but
> > +                                * the header's copy failed, and they are
> > +                                * sharing a slot, send an error
> > +                                */
> > +                               if (i == 0 && sharedslot)
> > +                                       xenvif_idx_release(queue, pending_idx,
> > +                                                          XEN_NETIF_RSP_ERROR);
> > +                               else
> > +                                       xenvif_idx_release(queue, pending_idx,
> > +                                                          XEN_NETIF_RSP_OKAY);
> I guess this is sort of OK, just a bit fragile. Couldn't think of a
> better way to refactor this function. :-(
I was thinking a lot about how to refactor this whole thing, but I gave 
up too ...
> > +                       }
> >                         continue;
> >                 }
> >
> > @@ -1081,15 +1097,27 @@ check_frags:
> >                                    gop_map->status,
> >                                    pending_idx,
> >                                    gop_map->ref);
> > +
> Stray blank line.
>
> And did you miss a callsite of xenvif_idx_unmap in this function which
> is added in your first patch?
Nope, the xenvif_idx_release is there
> Wei.

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