On 01/07/2022 11.57, Eric Dumazet wrote:
On Fri, Jul 1, 2022 at 11:43 AM Johan Almbladh
<johan.almbladh@xxxxxxxxxxxxxxxxx> wrote:
The byte queue limits (BQL) mechanism is intended to move queuing from
the driver to the network stack in order to reduce latency caused by
excessive queuing in hardware. However, when transmitting or redirecting
a packet with XDP, the qdisc layer is bypassed and there are no
additional queues. Since netif_xmit_stopped() also takes BQL limits into
account, but without having any alternative queuing, packets are
silently dropped.
This patch modifies the drop condition to only consider cases when the
driver itself cannot accept any more packets. This is analogous to the
condition in __dev_direct_xmit(). Dropped packets are also counted on
the device.
This means XDP packets are able to starve other packets going through a qdisc,
DDOS attacks will be more effective.
in-driver-XDP use dedicated TX queues, so they do not have this
starvation issue.
Good point. This happen in XDP-generic path, because XDP share the TX
queue with normal network stack.
This should be mentioned somewhere I guess.
I want to mention that (even for in-driver-XDP) not having a queuing
mechanism for XDP redirect is a general problem (and huge foot gun).
E.g. doing XDP-redirect between interfaces with different link rates
quickly result in issues.
We have Toke + PhD student (Frey Cc) working[1] on "XDQ" to address this
generically. I urge them to look at the code for the push-back
mechanism that netif_xmit_frozen_or_drv_stopped() and BQL provides and
somehow integrated XDQ with this...
--Jesper
[1] https://youtu.be/tthG9LP5GFk
Signed-off-by: Johan Almbladh <johan.almbladh@xxxxxxxxxxxxxxxxx>
---
net/core/dev.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/net/core/dev.c b/net/core/dev.c
index 8e6f22961206..41b5d7ac5ec5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4875,10 +4875,12 @@ void generic_xdp_tx(struct sk_buff *skb, struct bpf_prog *xdp_prog)
txq = netdev_core_pick_tx(dev, skb, NULL);
cpu = smp_processor_id();
HARD_TX_LOCK(dev, txq, cpu);
- if (!netif_xmit_stopped(txq)) {
+ if (!netif_xmit_frozen_or_drv_stopped(txq)) {
rc = netdev_start_xmit(skb, dev, txq, 0);
if (dev_xmit_complete(rc))
free_skb = false;
+ } else {
+ dev_core_stats_tx_dropped_inc(dev);
}
HARD_TX_UNLOCK(dev, txq);
if (free_skb) {
--
2.30.2
