nfsd: use an xarray to store v4.1 session slots

Using an xarray to store session slots will make it easier to change the
number of active slots based on demand, and removes an unnecessary
limit.

To achieve good throughput with a high-latency server it can be helpful
to have hundreds of concurrent writes, which means hundreds of slots.
So increase the limit to 2048 (twice what the Linux client will
currently use).  This limit is only a sanity check, not a hard limit.

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

@@ -1915,8 +1915,11 @@ free_session_slots(struct nfsd4_session *ses)
 	int i;
 
 	for (i = 0; i < ses->se_fchannel.maxreqs; i++) {
-		free_svc_cred(&ses->se_slots[i]->sl_cred);
-		kfree(ses->se_slots[i]);
+		struct nfsd4_slot *slot = xa_load(&ses->se_slots, i);
+
+		xa_erase(&ses->se_slots, i);
+		free_svc_cred(&slot->sl_cred);
+		kfree(slot);
 	}
 }
 
@@ -1996,17 +1999,20 @@ static struct nfsd4_session *alloc_session(struct nfsd4_channel_attrs *fattrs,
 	struct nfsd4_session *new;
 	int i;
 
-	BUILD_BUG_ON(struct_size(new, se_slots, NFSD_MAX_SLOTS_PER_SESSION)
-		     > PAGE_SIZE);
-
-	new = kzalloc(struct_size(new, se_slots, numslots), GFP_KERNEL);
+	new = kzalloc(sizeof(*new), GFP_KERNEL);
 	if (!new)
 		return NULL;
+	xa_init(&new->se_slots);
 	/* allocate each struct nfsd4_slot and data cache in one piece */
 	for (i = 0; i < numslots; i++) {
-		new->se_slots[i] = kzalloc(slotsize, GFP_KERNEL);
-		if (!new->se_slots[i])
+		struct nfsd4_slot *slot;
+		slot = kzalloc(slotsize, GFP_KERNEL);
+		if (!slot)
 			goto out_free;
+		if (xa_is_err(xa_store(&new->se_slots, i, slot, GFP_KERNEL))) {
+			kfree(slot);
+			goto out_free;
+		}
 	}
 
 	memcpy(&new->se_fchannel, fattrs, sizeof(struct nfsd4_channel_attrs));
@@ -2017,7 +2023,8 @@ static struct nfsd4_session *alloc_session(struct nfsd4_channel_attrs *fattrs,
 	return new;
 out_free:
 	while (i--)
-		kfree(new->se_slots[i]);
+		kfree(xa_load(&new->se_slots, i));
+	xa_destroy(&new->se_slots);
 	kfree(new);
 	return NULL;
 }
@@ -2124,6 +2131,7 @@ static void nfsd4_del_conns(struct nfsd4_session *s)
 static void __free_session(struct nfsd4_session *ses)
 {
 	free_session_slots(ses);
+	xa_destroy(&ses->se_slots);
 	kfree(ses);
 }
 
@@ -4278,7 +4286,7 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	if (seq->slotid >= session->se_fchannel.maxreqs)
 		goto out_put_session;
 
-	slot = session->se_slots[seq->slotid];
+	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

fs/nfsd/state.h

@@ -227,8 +227,11 @@ static inline struct nfs4_delegation *delegstateid(struct nfs4_stid *s)
 	return container_of(s, struct nfs4_delegation, dl_stid);
 }
 
-/* Maximum number of slots per session. 160 is useful for long haul TCP */
-#define NFSD_MAX_SLOTS_PER_SESSION     160
+/* Maximum number of slots per session.  This is for sanity-check only.
+ * It could be increased if we had a mechanism to shutdown misbehaving clients.
+ * A large number can be needed to get good throughput on high-latency servers.
+ */
+#define NFSD_MAX_SLOTS_PER_SESSION	2048
 /* Maximum  session per slot cache size */
 #define NFSD_SLOT_CACHE_SIZE		2048
 /* Maximum number of NFSD_SLOT_CACHE_SIZE slots per session */
@@ -327,7 +330,7 @@ struct nfsd4_session {
 	struct nfsd4_cb_sec	se_cb_sec;
 	struct list_head	se_conns;
 	u32			se_cb_seq_nr[NFSD_BC_SLOT_TABLE_SIZE];
-	struct nfsd4_slot	*se_slots[];	/* forward channel slots */
+	struct xarray		se_slots;	/* forward channel slots */
 };
 
 /* formatted contents of nfs4_sessionid */