kern/155407: Exhausted net.inet.tcp.reass.maxsegments block recovering tcp session

[Slawa Olhovchenkov]

>Number:         155407
>Category:       kern
>Synopsis:       Exhausted net.inet.tcp.reass.maxsegments block recovering tcp session
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Wed Mar 09 12:30:11 UTC 2011
>Closed-Date:
>Last-Modified:
>Originator:     Slawa Olhovchenkov
>Release:        8.2
>Organization:
>Environment:
FreeBSD mail.haustus.ru 8.2-RC3 FreeBSD 8.2-RC3 #0: Sun Jan 30 06:28:31 UTC 2011     root at mason.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  amd64

>Description:
Exhausted net.inet.tcp.reass.maxsegments block recovering tcp session (for this socket and any other socket waiting for retransmited packets).
After exhausted net.inet.tcp.reass.maxsegments allocation new entry in tcp_reass failed (for this socket and any other socket waiting for retransmited packets).

        te = uma_zalloc(V_tcp_reass_zone, M_NOWAIT);
        if (te == NULL) {
                TCPSTAT_INC(tcps_rcvmemdrop);
                m_freem(m);
                *tlenp = 0;
                return (0);
        }

That is packet can be reduce reassemble queue dropped.
>How-To-Repeat:
Need high bandwidth-delay channel with high, but rare drop.
After lost incoming TCP segment and before got retranmited this segment net.inet.tcp.reass.maxsegments exhausted.

Software emulation:

Tuning by http://fasterdata.es.net/fasterdata/host-tuning/freebsd/
sysctl net.inet.tcp.recvspace=1048576
sysctl net.inet.ip.dummynet.pipe_slot_limit=3000
ipfw pipe 1 config delay 300 queue 3000
ipfw add 100 pipe 1 ip from test_hot to me

fetch http://test_host/big_file

After reach 7MB/s: ipfw pipe 1 config delay 300 queue 3000 plr 0.1

>Fix:
Workaround: set net.inet.tcp.reass.maxsegments in /boot/loader.conf to big value.

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