On Fri, 2020-04-17 at 14:08 -0400, Olga Kornievskaia wrote:
> On Fri, Apr 17, 2020 at 12:53 PM Trond Myklebust
> <trondmy@xxxxxxxxxxxxxxx> wrote:
> > On Fri, 2020-04-17 at 12:46 -0400, Olga Kornievskaia wrote:
> > > On Fri, Apr 17, 2020 at 12:20 PM Trond Myklebust
> > > <trondmy@xxxxxxxxxxxxxxx> wrote:
> > > > On Fri, 2020-04-17 at 11:43 -0400, Olga Kornievskaia wrote:
> > > > > Hi Trond,
> > > > >
> > > > > On Fri, Apr 17, 2020 at 11:31 AM Trond Myklebust
> > > > > <trondmy@xxxxxxxxxxxxxxx> wrote:
> > > > > > Hi Olga,
> > > > > >
> > > > > > On Fri, 2020-04-17 at 11:15 -0400, Olga Kornievskaia wrote:
> > > > > > > When nconnect is used, SEQUENCE operation currently isn't
> > > > > > > bound
> > > > > > > to
> > > > > > > a particular transport. The problem is created on an idle
> > > > > > > mount,
> > > > > > > where SEQUENCE is the only operation being sent and
> > > > > > > opened
> > > > > > > TPC
> > > > > > > connections are slowly being close from the lack of use.
> > > > > > > If
> > > > > > > SEQUENCE
> > > > > > > is not assigned to the main connection, the main
> > > > > > > connection
> > > > > > > can
> > > > > > > be closed and with that so is the back channel bound to
> > > > > > > that
> > > > > > > connection.
> > > > > > >
> > > > > > > Since the only way client handles callback_path down is
> > > > > > > by
> > > > > > > sending
> > > > > > > BIND_CONN_TO_SESSION requesting to bind both backchannel
> > > > > > > and
> > > > > > > fore
> > > > > > > channel on the connection that was left going, but that
> > > > > > > connection
> > > > > > > was already bound to only forechannel. According to the
> > > > > > > spec,
> > > > > > > it's
> > > > > > > not allowed to change channel binding after they are
> > > > > > > done.
> > > > > > >
> > > > > > > The fix is to make sure that a lone SEQUENCE always goes
> > > > > > > on
> > > > > > > the
> > > > > > > main connection, keeping backchannel alive.
> > > > > > >
> > > > > > > Fixes: 5a0c257f8 ("NFS: send state management on a single
> > > > > > > connection")
> > > > > > > Signed-off-by: Olga Kornievskaia <kolga@xxxxxxxxxx>
> > > > > > > ---
> > > > > > > fs/nfs/nfs4proc.c | 2 +-
> > > > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > >
> > > > > > > diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> > > > > > > index 99e9f2e..461f85d 100644
> > > > > > > --- a/fs/nfs/nfs4proc.c
> > > > > > > +++ b/fs/nfs/nfs4proc.c
> > > > > > > @@ -8857,7 +8857,7 @@ static struct rpc_task
> > > > > > > *_nfs41_proc_sequence(struct nfs_client *clp,
> > > > > > > .rpc_client = clp->cl_rpcclient,
> > > > > > > .rpc_message = &msg,
> > > > > > > .callback_ops = &nfs41_sequence_ops,
> > > > > > > - .flags = RPC_TASK_ASYNC | RPC_TASK_TIMEOUT,
> > > > > > > + .flags = RPC_TASK_ASYNC | RPC_TASK_TIMEOUT
> > > > > > > |
> > > > > > > RPC_TASK_NO_ROUND_ROBIN,
> > > > > > > };
> > > > > > > struct rpc_task *ret;
> > > > > > >
> > > > > >
> > > > > > This works only in the case where the client is only
> > > > > > sending
> > > > > > SEQUENCE
> > > > > > instructions. There are other cases where it could be
> > > > > > sending
> > > > > > out
> > > > > > other
> > > > > > operations that also renew the lease, but is doing it very
> > > > > > infrequently. Won't that also run into the same problem?
> > > > >
> > > > > Hm... I see so main channel can still go idle and close, when
> > > > > infrequent operations are happening on the other connections
> > > > > before
> > > > > round-robin-ing to the main connection....
> > > > >
> > > > > > Is the fundamental problem here that we're not handling the
> > > > > > SEQ4_STATUS_CB_PATH_DOWN / SEQ4_STATUS_CB_PATH_DOWN_SESSION
> > > > > > flags
> > > > > > correctly or is there something else going on?
> > > > >
> > > > > Yes the client doesn't recover properly. But the fix wasn't
> > > > > trivial
> > > > > to
> > > > > me (so I thought my patch was enough but I see it's not). Say
> > > > > client
> > > > > shuts down the main connection because it was idle. Now
> > > > > whatever
> > > > > operations goes on a different connection is going to get
> > > > > callback
> > > > > down. The only way the client can create a new backchannel
> > > > > (according
> > > > > to the spec) is if it creates a brand new connection and
> > > > > sends
> > > > > BIND_CONN_TO_SESSION there (all existing connections are
> > > > > already
> > > > > bound
> > > > > to fore channel and according to the spec you can't modify
> > > > > the
> > > > > existing binding). But then we'd need to make sure that it's
> > > > > the
> > > > > first
> > > > > one in the list of connections we iterate thru (as i think
> > > > > 1st
> > > > > marks
> > > > > the main connection?) as the other operations that supposed
> > > > > to
> > > > > only
> > > > > go
> > > > > on main connection need to know which connection to pick.
> > > > >
> > > > > The reason it's not seen against linux is because it doesn't
> > > > > follow
> > > > > the spec is doesn't reject attempts to bind a backchannel to
> > > > > an
> > > > > already existing connection that was only bound for fore
> > > > > channel.
> > > > >
> > > >
> > > > Oh, I see. So the server is replying NFS4ERR_INVAL in order to
> > > > let
> > > > the
> > > > client know that it is trying to change the channel bindings
> > > > for
> > > > that
> > > > connection.
> > >
> > > Well server isn't failing because client is asking for
> > > FORE_OR_BOTH
> > > and it's a choice so server is returning FORE. I'm not sure we
> > > can
> > > ask
> > > the server to fail the request with ERR_INVAL.... (rather I can
> > > ask
> > > but ) rather can we expect the server to do that always?
> >
> > In RFC5661, Section 18.34.3 I found the following normative text:
> >
> > Invoking BIND_CONN_TO_SESSION on a connection already associated
> > with
> > the specified session has no effect, and the server MUST respond
> > with
> > NFS4_OK, unless the client is demanding changes to the set of
> > channels the connection is associated with. If so, the server
> > MUST
> > return NFS4ERR_INVAL.
> >
> >
> > IOW: it sounds like your server isn't following the spec either.
>
> Thank you thank you! I missed that. Ok so we'll assume it does and I
> will try to come up with what you are suggesting...
>
> Do we need to do reorder the connection list? After we reset the
> connection, it will be on some random connection on the list of
> connections. But for the NO_ROBIN connections we pick the first
> connection in the list (which will not be the backchannel
> connection).
> Does it matter?
That's why I suggested the rpc_task_close_connection() wrapper below.
It should ensure that we close the same connection that got the
NFS4ERR_INVAL, and then when we restart the RPC call (using
rpc_restart_call_prepare()) it will just automatically create a new
connection on that same struct xprt...
>
> > > > Hmm... Is there any reason why we can't just add a handler to
> > > > nfs4_bind_one_conn_to_session_done() that intercepts
> > > > NFS4ERR_INVAL,
> > > > and
> > > > disconnects the xprt before retrying?
> > > > We should probably add a wrapper to xprt_force_disconnect() in
> > > > include/linux/sunrpc/clnt.h. Something like the following?
> > > >
> > > >
> > > > static inline void rpc_task_close_connection(struct rpc_task
> > > > *task)
> > > > {
> > > > if (task->tk_xprt)
> > > > xprt_force_disconnect(task->tk_xprt);
> > > > }
> > > >
> >
--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx
