Re: [PATCH net 1/4] xen-netback: Fix handling frag_list on grant op error path

[Zoltan Kiss]

On 18/07/14 16:24, Wei Liu wrote:
> On Thu, Jul 17, 2014 at 08:09:49PM +0100, Zoltan Kiss wrote:
> > The error handling for skb's with frag_list was completely wrong, it caused
> > double unmap attempts to happen if the error was on the first skb. Move it to
> > the right place in the loop.
> >
> > 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 1844a47..604ff71 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -1030,10 +1030,16 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
> >   {
> >   	struct gnttab_map_grant_ref *gop_map = *gopp_map;
> >   	u16 pending_idx = XENVIF_TX_CB(skb)->pending_idx;
> > +	/* This points to the shinfo of the actually checked skb, which could be
> > +	 * either the first or the one on the frag_list
> > +	 */
>
> I think "checked skb" should be "skb being checked". Feel free to
> disagree as I'm not native English speaker. :-/
>
> >   	struct skb_shared_info *shinfo = skb_shinfo(skb);
> > +	/* If this is non-NULL, we are currently checking the frag_list skb, and
> > +	 * this points to the shinfo of the first one
> > +	 */
> > +	struct skb_shared_info *first_shinfo = NULL;
> >   	int nr_frags = shinfo->nr_frags;
> >   	int i, err;
> > -	struct sk_buff *first_skb = NULL;
> >
> >   	/* Check status of header. */
> >   	err = (*gopp_copy)->status;
> > @@ -1086,31 +1092,28 @@ check_frags:
> >   			xenvif_idx_unmap(queue, pending_idx);
> >   		}
> >
> > +		/* And if we found the error while checking the frag_list, unmap
> > +		 * the first skb's frags
> > +		 */
> > +		if (first_shinfo) {
> > +			for (j = 0; j < first_shinfo->nr_frags; j++) {
> > +				pending_idx = frag_get_pending_idx(&first_shinfo->frags[j]);
> > +				xenvif_idx_unmap(queue, pending_idx);
> > +			}
> > +		}
> > +
> >   		/* Remember the error: invalidate all subsequent fragments. */
> >   		err = newerr;
> >   	}
> >
> > -	if (skb_has_frag_list(skb)) {
> > -		first_skb = skb;
> > -		skb = shinfo->frag_list;
> > -		shinfo = skb_shinfo(skb);
> > +	if (skb_has_frag_list(skb) && !first_shinfo) {
>
> Will it ever come to the point that we have another skb in this skb's
> frag list? Is there any reason prevents you from looping over the
> (possible) subsequent skbs? I guess if the error is deep in the list
> it's a bit hard to bookkeep...
>
> > +		first_shinfo = skb_shinfo(skb);
> > +		shinfo = skb_shinfo(skb_shinfo(skb)->frag_list);
>
> In that case I would suggest you add
> BUG_ON(skb_has_frag_list(skb_shinfo(skb)->frag_list)). I think having
> more nested frag_list should be a bug in current design.

There are already 3 things which prevents this
- in count_requests we drop the packet if it has more than 
XEN_NETBK_LEGACY_SLOTS_MAX slots
- in get_requests there is a BUG_ON(frag_overflow > MAX_SKB_FRAGS), 
which shouldn't really due to the prev point
- in the same funciont we create a frag_list skb exactly once
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