[PATCH 4/6] nfsd: allocate new session-based DRC slots on demand.

[NeilBrown <neilb@suse.de>]

If a client ever uses the highest available slot for a given session,
attempt to allocate another slot so there is room for the client to use
more slots 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.

We would expect to stablise with one more slot available than the client
actually uses.

Now that we grow the slot table on demand we can start with a smaller
allocation.  Define NFSD_MAX_INITIAL_SLOTS and allocate at most that
many when session is created.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfsd/nfs4state.c | 32 ++++++++++++++++++++++++++------
 fs/nfsd/state.h     |  2 ++
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 31ff9f92a895..fb522165b376 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1956,7 +1956,7 @@ static struct nfsd4_session *alloc_session(struct nfsd4_channel_attrs *fattrs,
 	if (!slot || xa_is_err(xa_store(&new->se_slots, 0, slot, GFP_KERNEL)))
 		goto out_free;
 
-	for (i = 1; i < numslots; i++) {
+	for (i = 1; i < numslots && i < NFSD_MAX_INITIAL_SLOTS; i++) {
 		slot = kzalloc(slotsize, GFP_KERNEL | __GFP_NORETRY);
 		if (!slot)
 			break;
@@ -4248,11 +4248,6 @@ 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);
@@ -4302,6 +4297,31 @@ 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 one.
+	 */
+	if (seq->slotid == session->se_fchannel.maxreqs - 1 &&
+	    session->se_fchannel.maxreqs < NFSD_MAX_SLOTS_PER_SESSION) {
+		int s = session->se_fchannel.maxreqs;
+
+		/*
+		 * GFP_NOWAIT is a low-priority non-blocking allocation
+		 * which can be used under client_lock and only succeeds
+		 * if there is plenty of memory.
+		 * Use GFP_ATOMIC which is higher priority for xa_store()
+		 * so we are less likely to waste the effort of the first
+		 * allocation.
+		 */
+		slot = kzalloc(slot_bytes(&session->se_fchannel), GFP_NOWAIT);
+		if (slot && !xa_is_err(xa_store(&session->se_slots, s, slot,
+						GFP_ATOMIC)))
+			session->se_fchannel.maxreqs += 1;
+		else
+			kfree(slot);
+	}
+	seq->maxslots = session->se_fchannel.maxreqs;
+
 out:
 	switch (clp->cl_cb_state) {
 	case NFSD4_CB_DOWN:
diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index e97626916a68..a14a823670e9 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -249,6 +249,8 @@ static inline struct nfs4_delegation *delegstateid(struct nfs4_stid *s)
  * get good throughput on high-latency servers.
  */
 #define NFSD_MAX_SLOTS_PER_SESSION	2048
+/* Maximum number of slots per session to allocate for CREATE_SESSION */
+#define NFSD_MAX_INITIAL_SLOTS		32
 /* Maximum  session per slot cache size */
 #define NFSD_SLOT_CACHE_SIZE		2048
 /* Maximum number of NFSD_SLOT_CACHE_SIZE slots per session */
-- 
2.47.0