NFS Mount Hangs

Sat Mar 27 12:20:14 UTC 2021

The issue happened again so we can say that disabling TSO and LRO on the NIC did not resolve this issue.
# ifconfig lagg0 -rxcsum -rxcsum6 -txcsum -txcsum6 -lro -tso -vlanhwtso
# ifconfig lagg0
lagg0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
	options=8100b8<VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,VLAN_HWFILTER>

We can also say that the sysctl settings did not resolve this issue.

# sysctl net.inet.tcp.fast_finwait2_recycle=1
net.inet.tcp.fast_finwait2_recycle: 0 -> 1

# sysctl net.inet.tcp.finwait2_timeout=1000
net.inet.tcp.finwait2_timeout: 60000 -> 1000

* I have not rebooted the NFS Server nor have I restarted nfsd, but do not believe that is required as these settings are at the TCP level and I would expect new sessions to use the updated settings.

The issue occurred after 5 days following a reboot of the client machines.
I ran the capture information again to make use of the situation.

#!/bin/sh

while true
do
  /bin/date >> /tmp/nfs-hang.log
  /bin/ps axHl | grep nfsd | grep -v grep >> /tmp/nfs-hang.log
  /usr/bin/procstat -kk 2947 >> /tmp/nfs-hang.log
  /usr/bin/procstat -kk 2944 >> /tmp/nfs-hang.log
  /bin/sleep 60
done

On the NFS Server
Active Internet connections (including servers)
Proto Recv-Q Send-Q Local Address          Foreign Address        (state)
tcp4       0      0 NFS.Server.IP.X.2049      NFS.Client.IP.X.48286     CLOSE_WAIT 

On the NFS Client
tcp        0      0 NFS.Client.IP.X:48286      NFS.Server.IP.X:2049       FIN_WAIT2

-------------- next part --------------

You had also asked for the output below.

# nfsstat -E -s
BackChannelCtBindConnToSes   
            0            0

# sysctl vfs.nfsd.request_space_throttle_count
vfs.nfsd.request_space_throttle_count: 0

I see that you are testing a patch and I look forward to seeing the results.

Jason Breitman

On Mar 21, 2021, at 6:21 PM, Rick Macklem <rmacklem at uoguelph.ca> wrote:

Youssef GHORBAL <youssef.ghorbal at pasteur.fr> wrote:
>Hi Jason,
>
>> On 17 Mar 2021, at 18:17, Jason Breitman <jbreitman at tildenparkcapital.com> wrote:
>>
>> Please review the details below and let me know if there is a setting that I should apply to my FreeBSD NFS Server or if there is a bug fix that I can apply to resolve my issue.
>> I shared this information with the linux-nfs mailing list and they believe the issue is on the server side.
>>
>> Issue
>> NFSv4 mounts periodically hang on the NFS Client.
>>
>> During this time, it is possible to manually mount from another NFS Server on the NFS Client having issues.
>> Also, other NFS Clients are successfully mounting from the NFS Server in question.
>> Rebooting the NFS Client appears to be the only solution.
>
>I had experienced a similar weird situation with periodically stuck Linux NFS clients >mounting Isilon NFS servers (Isilon is FreeBSD based but they seem to have there >own nfsd)
Yes, my understanding is that Isilon uses a proprietary user space nfsd and
not the kernel based RPC and nfsd in FreeBSD.

>We’ve had better luck and we did manage to have packet captures on both sides >during the issue. The gist of it goes like follows:
>
>- Data flows correctly between SERVER and the CLIENT
>- At some point SERVER starts decreasing it's TCP Receive Window until it reachs 0
>- The client (eager to send data) can only ack data sent by SERVER.
>- When SERVER was done sending data, the client starts sending TCP Window >Probes hoping that the TCP Window opens again so he can flush its buffers.
>- SERVER responds with a TCP Zero Window to those probes.
Having the window size drop to zero is not necessarily incorrect.
If the server is overloaded (has a backlog of NFS requests), it can stop doing
soreceive() on the socket (so the socket rcv buffer can fill up and the TCP window
closes). This results in "backpressure" to stop the NFS client from flooding the
NFS server with requests.
--> However, once the backlog is handled, the nfsd should start to soreceive()
again and this shouls cause the window to open back up.
--> Maybe this is broken in the socket/TCP code. I quickly got lost in
tcp_output() when it decides what to do about the rcvwin.

>- After 6 minutes (the NFS server default Idle timeout) SERVER racefully closes the >TCP connection sending a FIN Packet (and still a TCP Window 0)
This probably does not happen for Jason's case, since the 6minute timeout
is disabled when the TCP connection is assigned as a backchannel (most likely
the case for NFSv4.1).

>- CLIENT ACK that FIN.
>- SERVER goes in FIN_WAIT_2 state
>- CLIENT closes its half part part of the socket and goes in LAST_ACK state.
>- FIN is never sent by the client since there still data in its SendQ and receiver TCP >Window is still 0. At this stage the client starts sending TCP Window Probes again >and again hoping that the server opens its TCP Window so it can flush it's buffers >and terminate its side of the socket.
>- SERVER keeps responding with a TCP Zero Window to those probes.
>=> The last two steps goes on and on for hours/days freezing the NFS mount bound >to that TCP session.
>
>If we had a situation where CLIENT was responsible for closing the TCP Window (and >initiating the TCP FIN first) and server wanting to send data we’ll end up in the same >state as you I think.
>
>We’ve never had the root cause of why the SERVER decided to close the TCP >Window and no more acccept data, the fix on the Isilon part was to recycle more >aggressively the FIN_WAIT_2 sockets (net.inet.tcp.fast_finwait2_recycle=1 & >net.inet.tcp.finwait2_timeout=5000). Once the socket recycled and at the next >occurence of CLIENT TCP Window probe, SERVER sends a RST, triggering the >teardown of the session on the client side, a new TCP handchake, etc and traffic >flows again (NFS starts responding)
>
>To avoid rebooting the client (and before the aggressive FIN_WAIT_2 was >implemented on the Isilon side) we’ve added a check script on the client that detects >LAST_ACK sockets on the client and through iptables rule enforces a TCP RST, >Something like: -A OUTPUT -p tcp -d $nfs_server_addr --sport $local_port -j REJECT >--reject-with tcp-reset (the script removes this iptables rule as soon as the LAST_ACK >disappears)
>
>The bottom line would be to have a packet capture during the outage (client and/or >server side), it will show you at least the shape of the TCP exchange when NFS is >stuck.
Interesting story and good work w.r.t. sluething, Youssef, thanks.

I looked at Jason's log and it shows everything is ok w.r.t the nfsd threads.
(They're just waiting for RPC requests.)
However, I do now think I know why the soclose() does not happen.
When the TCP connection is assigned as a backchannel, that takes a reference
cnt on the structure. This refcnt won't be released until the connection is
replaced by a BindConnectiotoSession operation from the client. But that won't
happen until the client creates a new TCP connection.
--> No refcnt release-->no refcnt of 0-->no soclose().

I've created the attached patch (completely different from the previous one)
that adds soshutdown(SHUT_WR) calls in the three places where the TCP
connection is going away. This seems to get it past CLOSE_WAIT without a
soclose().
--> I know you are not comfortable with patching your server, but I do think
this change will get the socket shutdown to complete.

There are a couple more things you can check on the server...
# nfsstat -E -s
--> Look for the count under "BindConnToSes".
--> If non-zero, backchannels have been assigned
# sysctl -a | fgrep request_space_throttle_count
--> If non-zero, the server has been overloaded at some point.

I think the attached patch might work around the problem.
The code that should open up the receive window needs to be checked.
I am also looking at enabling the 6minute timeout when a backchannel is
assigned.

rick

Youssef

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