On Tue, May 05, 2026 at 06:11:13PM +0200, Stefano Garzarella wrote: > On Tue, May 05, 2026 at 07:14:36AM -0700, Eric Dumazet wrote: > > On Tue, May 5, 2026 at 6:52 AM Stefano Garzarella <[email protected]> > > wrote: > > > > > > On Thu, Apr 30, 2026 at 12:26:52PM +0000, Eric Dumazet wrote: > > > >virtio_transport_inc_rx_pkt() checks vvs->rx_bytes + len > > > > >vvs->buf_alloc. > > > > > > > >virtio_transport_recv_enqueue() skips coalescing for packets > > > >with VIRTIO_VSOCK_SEQ_EOM. > > > > > > > >If fed with packets with len == 0 and VIRTIO_VSOCK_SEQ_EOM, > > > >a very large number of packets can be queued > > > >because vvs->rx_bytes stays at 0. > > > > > > > >Fix this by estimating the skb metadata size: > > > > > > > > (Number of skbs in the queue) * SKB_TRUESIZE(0) > > > > > > > >Fixes: 077706165717 ("virtio/vsock: don't use skbuff state to account > > > >credit") > > > >Signed-off-by: Eric Dumazet <[email protected]> > > > >Cc: Arseniy Krasnov <[email protected]> > > > >Cc: Stefan Hajnoczi <[email protected]> > > > >Cc: Stefano Garzarella <[email protected]> > > > >Cc: "Michael S. Tsirkin" <[email protected]> > > > >Cc: Jason Wang <[email protected]> > > > >Cc: Xuan Zhuo <[email protected]> > > > >Cc: "Eugenio Pérez" <[email protected]> > > > >Cc: [email protected] > > > >Cc: [email protected] > > > >--- > > > > net/vmw_vsock/virtio_transport_common.c | 4 +++- > > > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > > > > >diff --git a/net/vmw_vsock/virtio_transport_common.c > > > >b/net/vmw_vsock/virtio_transport_common.c > > > >index > > > >416d533f493d7b07e9c77c43f741d28cfcd0953e..9b8014516f4fb1130ae184635fbba4dfee58bd64 > > > > 100644 > > > >--- a/net/vmw_vsock/virtio_transport_common.c > > > >+++ b/net/vmw_vsock/virtio_transport_common.c > > > >@@ -447,7 +447,9 @@ static int virtio_transport_send_pkt_info(struct > > > >vsock_sock *vsk, > > > > static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs, > > > > u32 len) > > > > { > > > >- if (vvs->buf_used + len > vvs->buf_alloc) > > > >+ u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * > > > >SKB_TRUESIZE(0); > > > >+ > > > >+ if (skb_overhead + vvs->buf_used + len > vvs->buf_alloc) > > > > return false; > > > > > > I'm not sure about this fix, I mean that maybe this is incomplete. > > > In virtio-vsock, there is a credit mechanism between the two peers: > > > https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-4850003 > > > > > > This takes only the payload into account, so it’s true that this problem > > > exists; however, perhaps we should also inform the other peer of a lower > > > credit balance, otherwise the other peer will believe it has much more > > > credit than it actually does, send a large payload, and then the packet > > > will be discarded and the data lost (there are no retransmissions, > > > etc.). > > > > I dunno, perhaps revert 077706165717 ("virtio/vsock: don't use skbuff > > state to account credit") > > and find a better fix then? > > IIRC the same issue was there before the commit fixed by that one (commit > 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")), so > not sure about reverting it TBH. > > CCing Arseniy and Bobby. > > > > > There is always a discrepancy between skb->len and skb->truesize. > > You will not be able to announce a 1MB window, and accept one milliion > > skb of 1-byte each. > > > > This kind of contract is broken. > > > > Yep, I agree, but before we start discarding data (and losing it), IMHO we > should at least inform the other peer that we're out of space. > > @Stefan, @Michael, do you think we can do something in the spec to avoid > this issue and in some way take into account also the metadata in the > credit. I mean to avoid the 1-byte packets flooding. > > Thanks, > Stefano > >
Indeed the old pre-fix skb code would have the same issue. I can't think of any way around this without extending the spec. Best, Bobby
