Re: [PATCH 0/6] tcp: Fixes for issues uncovered by tests with 6.17-rc1 kernels

[Paul Holzinger <pholzing@redhat.com>]

Hi,

On 15/08/2025 18:10, Stefano Brivio wrote:
> Starting from Linux kernel commit 1d2fbaad7cd8 ("tcp: stronger
> sk_rcvbuf checks"), window limits are enforced more aggressively with
> a bigger amount of zero-window updates compared to what happened with
> e2142825c120 ("net: tcp: send zero-window ACK when no memory") alone,
> and occasional duplicate ACKs can now be seen also for local transfers
> with default (208 KiB) socket buffer sizes.
>
> Paul reports that, with 6.17-rc1-ish kernels, Podman tests for the
> pasta integration occasionally fail on the "TCP/IPv4 large transfer,
> tap" case.
>
> While playing with a reproducer that seems to be matching those
> failures:
>
>    while true; do ./pasta --trace -l /tmp/pasta.log -p /tmp/pasta.pcap --config-net -t 5555 -- socat TCP-LISTEN:5555 OPEN:/tmp/large.rcv,trunc & (sleep 0.3; socat -T2 OPEN:large.bin TCP:88.198.0.164:5555; ); wait; diff large.bin /tmp/large.rcv || break; done
>
> and a kernel including that commit, I hit a few different failures,
> that should be fixed by this series.
>
> Stefano Brivio (6):
>    tcp: FIN flags have to be retransmitted as well
>    tcp: Factor sequence rewind for retransmissions into a new function
>    tcp: Rewind sequence when guest shrinks window to zero
>    tcp: Fix closing logic for half-closed connections
>    tcp: Don't try to transmit right after the peer shrank the window to
>      zero
>    tcp: Fast re-transmit if half-closed, make TAP_FIN_RCVD path
>      consistent
>
>   tcp.c | 170 ++++++++++++++++++++++++++++++++++++++++------------------
>   1 file changed, 118 insertions(+), 52 deletions(-)
I applied the series in my test VM and run the reproducer from the 
podman tests where I noticed the flake.

It failed after about 10mins and then when I enabled pasta trace logs 
and tcpdump captures then it took 208 mins for me to reproduce so it 
doesn't seem to fix the issue I am seeing.

I did send the pasta log/pcap files in private to Stefano for further 
debugging.

-- 
Paul Holzinger