On Mon, Jun 17, 2019 at 7:28 PM Eric Dumazet <eric.duma...@gmail.com> wrote:
>
>
>
> On 6/17/19 5:18 PM, Christoph Paasch wrote:
> >
> > Hi Eric, I now have a packetdrill test that started failing (see
> > below). Admittedly, a bit weird test with the SO_SNDBUF forced so low.
> >
> > Nevertheless, previously this test would pass, now it stalls after the
> > write() because tcp_fragment() returns -ENOMEM. Your commit-message
> > mentions that this could trigger when one sets SO_SNDBUF low. But,
> > here we have a complete stall of the connection and we never recover.
> >
> > I don't know if we care about this, but there it is :-)
>
> I guess it is WAI :)
>
> Honestly I am not sure we want to add code just to allow these degenerated
> use cases.
>
> Upstream kernels could check if rtx queue is empty or not, but this check
> will be not trivial to backport
>
>
> Can you test :
Yes, this does the trick for my packetdrill-test.
I wonder, is there a way we could end up in a situation where we can't
retransmit anymore?
For example, sk_wmem_queued has grown so much that the new test fails.
Then, if we legitimately need to fragment in __tcp_retransmit_skb() we
won't be able to do so. So we will never retransmit. And if no ACK
comes back in to make some room we are stuck, no?
Christoph
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index
> 00c01a01b547ec67c971dc25a74c9258563cf871..06576540133806222f43d4a9532c5a929a2965b0
> 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1296,7 +1296,8 @@ int tcp_fragment(struct sock *sk, enum tcp_queue
> tcp_queue,
> if (nsize < 0)
> nsize = 0;
>
> - if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf)) {
> + if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf &&
> + !tcp_rtx_queue_empty(sk))) {
> NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
> return -ENOMEM;
> }
