From: Scott Mayhew <smayhew@redhat.com>
To: linux-nfs@vger.kernel.org
Subject: [PATCH RFC 0/3] Add notifier blocks to close transport sockets when an
Date: Wed,  2 Dec 2015 17:04:51 -0500
Message-Id: <1449093894-41591-1-git-send-email-smayhew@redhat.com>
Sender: linux-nfs-owner@vger.kernel.org

A somewhat common configuration for highly available NFS v3 is to have nfsd and
lockd running at all times on the cluster nodes, and move the floating ip,
export configuration, and exported filesystem from one node to another when a
service failover or relocation occurs.

A problem arises in this sort of configuration though when an NFS service is
moved to another node and then moved back to the original node 'too quickly'
(i.e. before the original transport socket is closed on the first node).  When
this occurs, clients can experience delays that can last almost 15 minutes (2 *
svc_conn_age_period + time spent waiting in FIN_WAIT_1).  What happens is that
once the client reconnects to the original socket, the sequence numbers no
longer match up and bedlam ensues.
 
This isn't a new phenomenon -- slide 16 of this old presentation illustrates
the same scenario:
 
http://www.nfsv4bat.org/Documents/ConnectAThon/1996/nfstcp.pdf
 
One historical workaround was to set timeo=1 in the client's mount options.  The
reason the workaround worked is because once the client reconnects to the
original transport socket and the data stops moving,
we would start retransmitting at the RPC layer.  With the timeout set to 1/10 of
a second instead of the normal 60 seconds, the client's transport socket's send
buffer *much* more quickly, and once it filled up
there would a very good chance that an incomplete send would occur (from the
standpoint of the RPC layer -- at the network layer both sides are just spraying
ACKs at each other as fast as possible).  Once that happens, we would wind up
setting XPRT_CLOSE_WAIT in the client's rpc_xprt->state field in
xs_tcp_release_xprt() and on the next transmit the client would try to close the
connection.  Actually the FIN would get ignored by the server, again because the
sequence numbers were out of whack, so the client would wait for the FIN timeout
to expire, after which it would delete the socket, and upon receipt of the next
packet from the server to that port the client the client would respond with a
RST and things finally go back to normal.
 
That workaround used to work up until commit a9a6b52 (sunrpc: Dont start the
retransmission timer when out of socket space).  Now the client just waits for
its send buffer to empty out, which isn't going to happen in this scenario... so
we're back to waiting for the server's svc_serv->sv_temptimer aka
svc_age_temp_xprts() to do its thing.

These patches try to help that situation.  The first patch adds a function to
close temporary transports whose xpt_local matches the address passed in
server_addr immediately instead of waiting for them to be closed by the
svc_serv->sv_temptimer function.  The idea here is that if the ip address was
yanked out from under the service, then those transports are doomed and there's
no point in waiting up to 12 minutes to start cleaning them up.  The second
patch adds notifier_blocks (one for IPv4 and one for IPv6) to call that 
function to nfsd.  The third patch does the same thing, but for lockd.

I've been testing these patches on a RHEL 6 rgmanager cluster as well as a
Fedora 23 pacemaker cluster.  Note that the resource agents in pacemaker do not
behave the way I initially described... the pacemaker resource agents actually
do a full tear-down & bring up of the nfsd's as part of a service relocation, so
I hacked them up to behave like the older rgmanager agents in order to test.  I
tested with cthon and xfstests while moving the NFS service from one node to the
other every 60 seconds.  I also did more basic testing like taking & holding a
lock using the flock command from util-linux and making sure that the client was
able to reclaim the lock as I moved the service back and forth among the cluster
nodes.

For this to be effective, the clients still need to mount with a lower timeout,
but it doesn't need to be as aggressive as 1/10 of a second.

Also, for all this to work when the cluster nodes are running a firewall, it's
necessary to add a rule to trigger a RST.  The rule would need to be after the
rule that allows new NFS connections and before the catch-all rule that rejects
everyting else with ICMP-HOST-PROHIBITED.  For a Fedora server running
firewalld, the following commands accomplish that:

firewall-cmd --direct --add-passthrough ipv4 -A IN_FedoraServer_allow \
	-m tcp -p tcp --dport 2049 -j REJECT --reject-with tcp-reset
firewall-cmd --runtime-to-permanent

A similar rule would need to be added for whatever port lockd is running on as
well.

Scott Mayhew (3):
  sunrpc: Add a function to close temporary transports immediately
  nfsd: Register callbacks on the inetaddr_chain and inet6addr_chain
  lockd: Register callbacks on the inetaddr_chain and inet6addr_chain

 fs/lockd/svc.c                  | 74 +++++++++++++++++++++++++++++++++++++++--
 fs/nfsd/nfssvc.c                | 68 +++++++++++++++++++++++++++++++++++++
 include/linux/sunrpc/svc_xprt.h |  1 +
 net/sunrpc/svc_xprt.c           | 45 +++++++++++++++++++++++++
 4 files changed, 186 insertions(+), 2 deletions(-)

-- 
2.4.3