nfsd: allocate new session-based DRC slots on demand.

If a client ever uses the highest available slot for a given session,
attempt to allocate more slots so there is room for the client to use
them if wanted.  GFP_NOWAIT is used so if there is not plenty of
free memory, failure is expected - which is what we want.  It also
allows the allocation while holding a spinlock.

Each time we increase the number of slots by 20% (rounded up).  This
allows fairly quick growth while avoiding excessive over-shoot.

We would expect to stablise with around 10% more slots available than
the client actually uses.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4state.c

@@ -4235,11 +4235,6 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	slot = xa_load(&session->se_slots, seq->slotid);
 	dprintk("%s: slotid %d\n", __func__, seq->slotid);
 
-	/* We do not negotiate the number of slots yet, so set the
-	 * maxslots to the session maxreqs which is used to encode
-	 * sr_highest_slotid and the sr_target_slot id to maxslots */
-	seq->maxslots = session->se_fchannel.maxreqs;
-
 	trace_nfsd_slot_seqid_sequence(clp, seq, slot);
 	status = check_slot_seqid(seq->seqid, slot->sl_seqid,
 					slot->sl_flags & NFSD4_SLOT_INUSE);
@@ -4289,6 +4284,38 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	cstate->session = session;
 	cstate->clp = clp;
 
+	/*
+	 * If the client ever uses the highest available slot,
+	 * gently try to allocate another 20%.  This allows
+	 * fairly quick growth without grossly over-shooting what
+	 * the client might use.
+	 */
+	if (seq->slotid == session->se_fchannel.maxreqs - 1 &&
+	    session->se_fchannel.maxreqs < NFSD_MAX_SLOTS_PER_SESSION) {
+		int s = session->se_fchannel.maxreqs;
+		int cnt = DIV_ROUND_UP(s, 5);
+
+		do {
+			/*
+			 * GFP_NOWAIT both allows allocation under a
+			 * spinlock, and only succeeds if there is
+			 * plenty of memory.
+			 */
+			slot = kzalloc(slot_bytes(&session->se_fchannel),
+				       GFP_NOWAIT);
+			if (slot &&
+			    !xa_is_err(xa_store(&session->se_slots, s, slot,
+						GFP_NOWAIT))) {
+				s += 1;
+				session->se_fchannel.maxreqs = s;
+			} else {
+				kfree(slot);
+				slot = NULL;
+			}
+		} while (slot && --cnt > 0);
+	}
+	seq->maxslots = session->se_fchannel.maxreqs;
+
 out:
 	switch (clp->cl_cb_state) {
 	case NFSD4_CB_DOWN: