kern/178079: Switching TCP CC algorithm panics on sparc64 with a complaint from the MMU

[Jukka Ukkonen]

>Number:         178079
>Category:       kern
>Synopsis:       Switching TCP CC algorithm panics on sparc64 with a complaint from the MMU
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Apr 23 11:10:01 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator:     Jukka Ukkonen
>Release:        9.1-STABLE
>Organization:
-----
>Environment:
FreeBSD mimir 9.1-STABLE FreeBSD 9.1-STABLE #0 r249763M: Tue Apr 23 05:47:38 EEST 2013     root at mimir:/usr/obj/usr/src/sys/GENERIC  sparc64
>Description:
An attempt to switch the TCP congestion control algorithm on a sparc64 system
causes an instant panic.
The message shown on the console mentions something about NewReno and then
something about the MMU. Though I do not have the exact text shown to quote
here, it looked to me quite a lot like one of those MMU complaints one gets
for improperly aligned data.

Notice:
I have never seen anything remotely similar happening on an amd64.
So, it is supposedly impossible to replicate the problem using a CPU
which does not care about the data alignment.

>How-To-Repeat:
On a sparc64 system follow these steps:

First make sure you have the alternate CC modules loaded...

kldload /boot/kernel/cc_*.ko

Then try this...

sysctl net.inet.tcp.cc.algorithm=htcp

At least in my case this has been a guaranteed trick to panic
the system with a complaint from the MMU.

>Fix:
Not the foggiest idea yet.

I will keep a crash dump around for a while just in case someone wishes to have
a chance to see it. I cannot promise to save it for very long, though.

OTOH with the consistency I have been getting new crash dumps when touching
the CC algorithm on sparc64 it should be easy enough for anyone to generate
ones own crash dump if needs be. ;-)


>Release-Note:
>Audit-Trail:
>Unformatted: