[PATCH RFC 3/4] nfsd: add a fairq module parameter

Add nfsd.fairq (bool, default off).  When set, nfsd enables the sunrpc
per-client fair-queue dispatcher for its service via svc_set_fairq() at
service creation, so that a client cannot increase its share of nfsd
threads simply by opening more connections.  The parameter is read when
the service is created, so a change takes effect at the next nfsd start
(thread count 0 -> N).

With fair queueing enabled and no upper-layer identity stamped on a
transport, clients are distinguished by source address, which already
gives per-client fairness for NFSv3.

Signed-off-by: Benjamin Coddington <bcodding-F/[email protected]>
Co-Authored-By: Claude Opus 4.8 <noreply-IarDGxEC4Up8UrSeD/[email protected]>
---
 fs/nfsd/nfssvc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 551d3cf51036..3abe4d2aec5a 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -36,6 +36,18 @@
 #define NFSDDBG_FACILITY	NFSDDBG_SVC
 
 atomic_t			nfsd_th_cnt = ATOMIC_INIT(0);
+
+/*
+ * Per-client fair-queue dispatch.  When set, ready connections are scheduled
+ * round-robin per client (NFSv4.1 clientid, else source address) rather than
+ * per transport, so a client cannot grab a larger share of nfsd threads by
+ * opening more connections.  Read at service creation, so a change takes
+ * effect at the next nfsd start (thread count 0 -> N).
+ */
+static bool			nfsd_fairq;
+module_param_named(fairq, nfsd_fairq, bool, 0644);
+MODULE_PARM_DESC(fairq, "schedule nfsd service fairly per client (default off)");
+
 static int			nfsd(void *vrqstp);
 #if defined(CONFIG_NFSD_V2_ACL) || defined(CONFIG_NFSD_V3_ACL)
 static int			nfsd_acl_rpcbind_set(struct net *,
@@ -635,6 +647,13 @@ int nfsd_create_serv(struct net *net)
 		return -ENOMEM;
 	}
 
+	error = svc_set_fairq(serv, nfsd_fairq);
+	if (error) {
+		svc_destroy(&serv);
+		percpu_ref_exit(&nn->nfsd_net_ref);
+		return error;
+	}
+
 	error = svc_bind(serv, net);
 	if (error < 0) {
 		svc_destroy(&serv);
-- 
2.53.0