[Bug 263445] Fatal trap 12: page fault while in kernel mode // supervisor read data, page not present // 13.1-RC3

From: <bugzilla-noreply_at_freebsd.org>
Date: Tue, 31 May 2022 11:26:15 UTC

--- Comment #8 from Richard Scheffenegger <rscheff@freebsd.org> ---
Hi Victor,

sorry for the delay. Can you verify, that in all your cores the tcp t_state (p
*tp in the first frame of tcp_do_segment) is TCPS_LAST_ACK (8)?

And that you have some unacknowledged SACK information, where one byte is

p *tp->sackhint.nexthole

f 10
p tp->t_state
p *tp->sackhint.nexthole
$4 = {start = 3327712881, end = 3327714341, rxmit = 3327714340, scblink =
{tqe_next = 0x0, tqe_prev = 0xfffffe044ba66578}}

It seems as if there is a little bit of data being sent, and before all of the
data is fully acked by the client, the application closes the socket, but only
receives a SACK for the FIN (?), while the two prior data packets are still
outstanding - and probably an off-by-one error happens during SACK

As a stopgap measure, you can disable SACK (net.inet.tcp.sack.enable=0), or
disable PRR - this is one new SACK-related feature, but should only affect
timing (when to send, NOT what to send) with net.inet.tcp.do_prr=0.

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