[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] xen-netfront: Fix handling packets on compound pages with skb_linearize
On Mon, Dec 01, 2014 at 09:55:24AM +0100, Stefan Bader wrote: > On 11.08.2014 19:32, Zoltan Kiss wrote: > > There is a long known problem with the netfront/netback interface: if the > > guest > > tries to send a packet which constitues more than MAX_SKB_FRAGS + 1 ring > > slots, > > it gets dropped. The reason is that netback maps these slots to a frag in > > the > > frags array, which is limited by size. Having so many slots can occur since > > compound pages were introduced, as the ring protocol slice them up into > > individual (non-compound) page aligned slots. The theoretical worst case > > scenario looks like this (note, skbs are limited to 64 Kb here): > > linear buffer: at most PAGE_SIZE - 17 * 2 bytes, overlapping page boundary, > > using 2 slots > > first 15 frags: 1 + PAGE_SIZE + 1 bytes long, first and last bytes are at > > the > > end and the beginning of a page, therefore they use 3 * 15 = 45 slots > > last 2 frags: 1 + 1 bytes, overlapping page boundary, 2 * 2 = 4 slots > > Although I don't think this 51 slots skb can really happen, we need a > > solution > > which can deal with every scenario. In real life there is only a few slots > > overdue, but usually it causes the TCP stream to be blocked, as the retry > > will > > most likely have the same buffer layout. > > This patch solves this problem by linearizing the packet. This is not the > > fastest way, and it can fail much easier as it tries to allocate a big > > linear > > area for the whole packet, but probably easier by an order of magnitude than > > anything else. Probably this code path is not touched very frequently > > anyway. > > > > Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx> > > Cc: Wei Liu <wei.liu2@xxxxxxxxxx> > > Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxx> > > Cc: Paul Durrant <paul.durrant@xxxxxxxxxx> > > Cc: netdev@xxxxxxxxxxxxxxx > > Cc: linux-kernel@xxxxxxxxxxxxxxx > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx > > This does not seem to be marked explicitly as stable. Has someone already > asked > David Miller to put it on his stable queue? IMO it qualifies quite well and > the > actual change should be simple to pick/backport. > Thank you Stefan, I'm queuing this for the next 3.16 kernel release. Cheers, -- Luís > -Stefan > > > > > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c > > index 055222b..23359ae 100644 > > --- a/drivers/net/xen-netfront.c > > +++ b/drivers/net/xen-netfront.c > > @@ -628,9 +628,10 @@ static int xennet_start_xmit(struct sk_buff *skb, > > struct net_device *dev) > > slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) + > > xennet_count_skb_frag_slots(skb); > > if (unlikely(slots > MAX_SKB_FRAGS + 1)) { > > - net_alert_ratelimited( > > - "xennet: skb rides the rocket: %d slots\n", slots); > > - goto drop; > > + net_dbg_ratelimited("xennet: skb rides the rocket: %d slots, %d > > bytes\n", > > + slots, skb->len); > > + if (skb_linearize(skb)) > > + goto drop; > > } > > > > spin_lock_irqsave(&queue->tx_lock, flags); > > > > _______________________________________________ > > Xen-devel mailing list > > Xen-devel@xxxxxxxxxxxxx > > http://lists.xen.org/xen-devel > > > > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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