Re: [PATCH 2/8] nfsd: fix CB_SEQUENCE error handling of NFS4ERR_{BADSLOT,BADSESSION,SEQ_MISORDERED}

Jeff Layton
2025-01-24T16:04:55Z

On Fri, 2025-01-24 at 10:31 -0500, Chuck Lever wrote:
> On 1/24/25 9:46 AM, Jeff Layton wrote:
> > On Fri, 2025-01-24 at 09:32 -0500, Chuck Lever wrote:
> > > On 1/23/25 3:25 PM, Jeff Layton wrote:
> > > > The current error handling has some problems:
> > > >
> > > > BADSLOT and BADSESSION: don't release the slot before retrying the call
> > > >
> > > > SEQ_MISORDERED: does some sketchy resetting of the seqid? I can't find any
> > > > recommendation about doing that in the spec, and it seems wrong.
> > >
> > > Random thought: You might use the Linux NFS client's forechannel session
> > > implementation as a code reference.
> > >
> > >
> > > > Handle all three errors the same way: release the slot, but then handle
> > > > it just like we would as if we hadn't gotten a reply; mark the session
> > > > as faulty, and retry the call.
> > >
> > > Some questions:
> > >
> > > Why does it matter whether NFSD keeps the slot if both sides plan to
> > > destroy the session?
> > >
> >
> > It may not be required, but there is no reason to hold onto the slot in
> > these cases.
>
> In the BADSLOT case, if the slot is released, then another session
> consumer on the NFS server can use it and will encounter the same error.
> Best to keep it in the penalty box, IMO.
>
> If there are other slots, they are likely still usable. An
> implementation can choose to continue using the session rather than
> scuttling it immediately. In the past, with a single backchannel slot,
> NFSD had no choice but to replace the session. But now it can be more
> conservative.
>

It may still be usable, but if we got BADSLOT, then it's still
indicative of a faulty session. Best to mark it as such, IMO.

It's worth noting that these things should really _never_ happen, so
being somewhat heavy-handed in this situation seems reasonable.

>
> > Also, at this point, only nfsd has declared that it needs
> > a new session (see below).
>
> If the client's backchannel service has returned BADSESSION, then the
> client already knows this session is unusable.
>

Fair point. I still think it's reasonable to mark it CB_FAULT in that
case. That status is also displayed via the client_info file, so that's
helpful info.

>
> > > Also, AFAICT marking CB_FAULT does not destroy the session, it simply
> > > tries to recreate backchannel's rpc_clnt. Perhaps NFSD's callback code
> > > should actively destroy the session and let the client drive a fresh
> > > CREATE_SESSION to recover?
> > >
> >
> > Marking it with a fault just sets the cl_cb_state to NFSD4_CB_FAULT.
> > Then, on the next SEQUENCE call, that makes nfsd set
> > SEQ4_STATUS_BACKCHANNEL_FAULT, which should make the client recreate
> > the session. Obviously, there is some delay involved there since we
> > might have to wait for the client to do a lease renewal before this
> > happens.
> >
> > >
> > > > Fixes: 7ba6cad6c88f ("nfsd: New helper nfsd4_cb_sequence_done() for processing more cb errors")
> > > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > > ---
> > > > fs/nfsd/nfs4callback.c | 27 +++++++++++----------------
> > > > 1 file changed, 11 insertions(+), 16 deletions(-)
> > > >
> > > > diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
> > > > index e12205ef16ca932ffbcc86d67b0817aec2436c89..bfc9de1fcb67b4f05ed2f7a28038cd8290809c17 100644
> > > > --- a/fs/nfsd/nfs4callback.c
> > > > +++ b/fs/nfsd/nfs4callback.c
> > > > @@ -1371,17 +1371,24 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> > > > nfsd4_mark_cb_fault(cb->cb_clp);
> > > > ret = false;
> > > > break;
> > > > + case -NFS4ERR_BADSESSION:
> > > > + case -NFS4ERR_BADSLOT:
> > > > + case -NFS4ERR_SEQ_MISORDERED:
> > > > + /*
> > > > + * These errors indicate that something has gone wrong
> > > > + * with the server and client's synchronization. Release
> > > > + * the slot, but handle it as if we hadn't gotten a reply.
> > > > + */
> > > > + nfsd41_cb_release_slot(cb);
> > > > + fallthrough;
> > > > case 1:
> > > > /*
> > > > * cb_seq_status remains 1 if an RPC Reply was never
> > > > * received. NFSD can't know if the client processed
> > > > * the CB_SEQUENCE operation. Ask the client to send a
> > > > - * DESTROY_SESSION to recover.
> > > > + * DESTROY_SESSION to recover, but keep the slot.
> > > > */
> > > > - fallthrough;
> > > > - case -NFS4ERR_BADSESSION:
> > > > nfsd4_mark_cb_fault(cb->cb_clp);
> > > > - ret = false;
> > > > goto need_restart;
> > > > case -NFS4ERR_DELAY:
> > > > cb->cb_seq_status = 1;
> > > > @@ -1390,14 +1397,6 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> > > >
> > > > rpc_delay(task, 2 * HZ);
> > > > return false;
> > > > - case -NFS4ERR_BADSLOT:
> > > > - goto retry_nowait;
> > > > - case -NFS4ERR_SEQ_MISORDERED:
> > > > - if (session->se_cb_seq_nr[cb->cb_held_slot] != 1) {
> > > > - session->se_cb_seq_nr[cb->cb_held_slot] = 1;
> > > > - goto retry_nowait;
> > > > - }
> > > > - break;
> > > > default:
> > > > nfsd4_mark_cb_fault(cb->cb_clp);
> > > > }
> > > > @@ -1405,10 +1404,6 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> > > > nfsd41_cb_release_slot(cb);
> > > > out:
> > > > return ret;
> > > > -retry_nowait:
> > > > - if (rpc_restart_call_prepare(task))
> > > > - ret = false;
> > > > - goto out;
> > > > need_restart:
> > > > if (!test_bit(NFSD4_CLIENT_CB_KILL, &clp->cl_flags)) {
> > > > trace_nfsd_cb_restart(clp, cb);
> > > >
> > >
> > >
> >
>
>

--
Jeff Layton <jlayton@kernel.org>