On Wed, May 19, 2021 at 2:54 PM John Fastabend <john.fastabend@xxxxxxxxx> wrote: > > Cong Wang wrote: > > On Wed, May 19, 2021 at 12:06 PM John Fastabend > > <john.fastabend@xxxxxxxxx> wrote: > > > > > > Cong Wang wrote: > > > > On Tue, May 18, 2021 at 12:56 PM John Fastabend > > > > <john.fastabend@xxxxxxxxx> wrote: > > > > > > > > > > Cong Wang wrote: > > > > > > On Mon, May 17, 2021 at 10:36 PM John Fastabend > > > > > > <john.fastabend@xxxxxxxxx> wrote: > > > > > > > > > > > > > > Cong Wang wrote: > > > > > > > > From: Cong Wang <cong.wang@xxxxxxxxxxxxx> > > > > > > > > > > > > > > > > sk_psock_verdict_recv() clones the skb and uses the clone > > > > > > > > afterward, so udp_read_sock() should free the original skb after > > > > > > > > done using it. > > > > > > > > > > > > > > The clone only happens if sk_psock_verdict_recv() returns >0. > > > > > > > > > > > > Sure, in case of error, no one uses the original skb either, > > > > > > so still need to free it. > > > > > > > > > > But the data is going to be dropped then. I'm questioning if this > > > > > is the best we can do or not. Its simplest sure, but could we > > > > > do a bit more work and peek those skbs or requeue them? Otherwise > > > > > if you cross memory limits for a bit your likely to drop these > > > > > unnecessarily. > > > > > > > > What are the benefits of not dropping it? When sockmap takes > > > > over sk->sk_data_ready() it should have total control over the skb's > > > > in the receive queue. Otherwise user-space recvmsg() would race > > > > with sockmap when they try to read the first skb at the same time, > > > > therefore potentially user-space could get duplicated data (one via > > > > recvmsg(), one via sockmap). I don't see any benefits but races here. > > > > > > The benefit of _not_ dropping it is the packet gets to the receiver > > > side. We've spent a bit of effort to get a packet across the network, > > > received on the stack, and then we drop it at the last point is not > > > so friendly. > > > > Well, at least udp_recvmsg() could drop packets too in various > > scenarios, for example, a copy error. So, I do not think sockmap > > is special. > > OK I am at least convinced now that dropping packets is OK and likely > a useful performance/complexity compromise. > > But, at this point we wont have any visibility into these drops correct? > Looks like the pattern in UDP stack to handle this is to increment > sk_drops and UDP_MIB_INERRORS. How about we do that here as well? We are not dropping the packet, the packet is cloned and deliver to user-space via sk_psock_verdict_recv(), thus, we are simply leaking the original skb, regardless of any error. Maybe udp_read_sock() should check desc->error, but it has nothing to do with this path which only aims to address a memory leak. A separate patch is need to check desc->error, if really needed. Thanks.
