[Bug 253848] panic: sackhint bytes rtx >= 0

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253848

--- Comment #3 from Richard Scheffenegger <rscheff at freebsd.org> ---
One change somewhat related to SACK loss recovery, which was added to
stable/13, was PRR (D18892).

Since early Dec, there was one reported instance of poor performance, and on
closer inspection, there are a few possibilities, where unexpected negative
values, or overflows can occur - but those "should" only expose themselves as
packets either not sent when expected, or in larger bursts (3 instead of 2)
packets, not a panic like you observed.

There are a few diffs to improve that behavior, which I would like to MFC to
stable/13 and 13.0 in the coming days:

D28780 - harness to ensure PRR doesn't end with fully closed window
(performance)
D28813 - fix an overflow and errors due to mixup of "min"/"imin" (implicit
typecast)
(D28816) uncritical really, use updated pipe calculation
D28819 - fix unintended double-accounting
(D28822) enhanced heuristic, per standards-track draft of PRR

I can not immediately see, how that may affect the sack_bytes_rexmit numbers,
but PRR is one change in the loss recovery space added recently - so the panic
may be somehow related.

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