> On 21 Mar 2021, at 23:21, Rick Macklem <rmack...@uoguelph.ca> wrote: > > Youssef GHORBAL <youssef.ghor...@pasteur.fr> wrote: >> Hi Jason, >> >>> On 17 Mar 2021, at 18:17, Jason Breitman <jbreit...@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.
Indeed proprietary user space nfsd. >> 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. That was my exepection too. At the time the NFS server was not overloaded (and happely accepting new sessions from the same client and many other clients) Dell/EMC support was not willing to dig deeper, and just treated the symptoms (recycling more aggressively the FIN_WAIT_2 sockets) >> - 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 > > _______________________________________________ > freebsd-net@freebsd.org mailing list > https://urldefense.com/v3/__https://lists.freebsd.org/mailman/listinfo/freebsd-net__;!!JFdNOqOXpB6UZW0!-PPciMNEPwU1aJzMke0cDKBhodIOXM0p-Qzep5FfH_wuzKLbt3Q0bs68g5QeykOMotxUiU_i$ > > To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org" > <xprtdied.patch> _______________________________________________ freebsd-net@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org"
