[Bug 209471] Listen queue overflow due to too many sockets stuck in CLOSED state

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=209471

--- Comment #15 from Robert Blayzor <rblayzor at inoc.net> ---
I believe I have stumbled onto this problem in it's early stages. I notice this
by catching a process that seems to have a connection in a "CLOSED" state for
what seems like forever.

If I look at it closer...

dev at mta2 [~] netstat -an | grep CLOSED
tcp6       0      0 2607:f058:110:2:.25    2607:f058:110:2:.40858 CLOSED

dev at mta2 [~] sockstat | grep 40858
mailnull exim       35876 7  tcp6   2607:f058:110:2::1:2:25
2607:f058:110:2::f:0:40858
mailnull exim       35876 9  tcp6   2607:f058:110:2::1:2:25
2607:f058:110:2::f:0:40858

dev at mta2 [~] sudo tcpdrop 2607:f058:110:2::1:2 25 2607:f058:110:2::f:0 40858
tcpdrop: 2607:f058:110:2::1:2 25 2607:f058:110:2::f:0 40858: No such process


If I kill the process and restart it... the old process sticks around and the
socket never clears.


dev at mta2 [~] sudo killall -9 exim
dev at mta2 [~] ps auxww | grep exim
mailnull 35876   0.0  0.4  41708   8184  -  DL    9:21PM     0:09.40
/usr/local/sbin/exim -bd -q10m


dev at mta2 [~] netstat -an | grep CLOSED
tcp6       0      0 2607:f058:110:2:.25    2607:f058:110:2:.40858 CLOSED
dev at mta2 [~] sudo service exim start
Starting exim.
dev at mta2 [~] gps exim
root     39120   0.2  0.4  41708   8364  -  Ss   10:15PM     0:00.00
/usr/local/sbin/exim -bd -q10m
mailnull 35876   0.0  0.4  41708   8184  -  DL    9:21PM     0:09.48
/usr/local/sbin/exim -bd -q10m


sockstat | grep 40858
mailnull exim       35876 7  tcp6   2607:f058:110:2::1:2:25
2607:f058:110:2::f:0:40858
mailnull exim       35876 9  tcp6   2607:f058:110:2::1:2:25
2607:f058:110:2::f:0:40858


I'm not expert on kernel VM, but think stinks as related to previous bug. 

Top state shows:  vmf_de ?

35876 mailnull      1  20    0 41708K  8184K vmf_de  2   0:10   0.00% exim

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