[RFC PATCHv2 2/3] nvme: introduce multipath_head_always module param

[Nilay Shroff <nilay@linux.ibm.com>]

Currently, a multipath head disk node is not created for single-ported
NVMe adapters or private namespaces. However, creating a head node in
these cases can help transparently handle transient PCIe link failures.
Without a head node, features like delayed removal cannot be leveraged,
making it difficult to tolerate such link failures. To address this,
this commit introduces nvme_core module parameter multipath_head_always.

When this param is set to true, it forces the creation of a multipath
head node regardless NVMe disk or namespace type. So this option allows
the use of delayed removal of head node functionality even for single-
ported NVMe disks and private namespaces and thus helps transparently
handling transient PCIe link failures.

By default multipath_head_always is set to false, thus preserving the
existing behavior. Setting it to true enables improved fault tolerance
in PCIe setups. Moreover, please note that enabling this option would
also implicitly enable nvme_core.multipath.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/multipath.c | 70 +++++++++++++++++++++++++++++++----
 1 file changed, 63 insertions(+), 7 deletions(-)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 68318337c275..1acdbbddfe01 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -10,10 +10,59 @@
 #include "nvme.h"
 
 bool multipath = true;
-module_param(multipath, bool, 0444);
+bool multipath_head_always;	/* default is flase */
+
+static int multipath_param_set(const char *val, const struct kernel_param *kp)
+{
+	int ret;
+
+	ret = param_set_bool(val, kp);
+	if (ret)
+		return ret;
+
+	if (multipath_head_always && !*(bool *)kp->arg) {
+		pr_err("Can't disable multipath when multipath_head_always is configured.\n");
+		*(bool *)kp->arg = true;
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static const struct kernel_param_ops multipath_param_ops = {
+	.set = multipath_param_set,
+	.get = param_get_bool,
+};
+
+module_param_cb(multipath, &multipath_param_ops, &multipath, 0444);
 MODULE_PARM_DESC(multipath,
 	"turn on native support for multiple controllers per subsystem");
 
+static int multipath_head_always_set(const char *val,
+		const struct kernel_param *kp)
+{
+	int ret;
+
+	ret = param_set_bool(val, kp);
+	if (ret < 0)
+		return ret;
+
+	if (*(bool *)kp->arg)
+		multipath = true;
+
+	return 0;
+}
+
+static const struct kernel_param_ops multipath_head_always_param_ops = {
+	.set = multipath_head_always_set,
+	.get = param_get_bool,
+};
+
+module_param_cb(multipath_head_always, &multipath_head_always_param_ops,
+		&multipath_head_always, 0444);
+MODULE_PARM_DESC(multipath_head_always,
+	"create multipath head node always; note that this also implicitly enables native multipath support");
+
 static const char *nvme_iopolicy_names[] = {
 	[NVME_IOPOLICY_NUMA]	= "numa",
 	[NVME_IOPOLICY_RR]	= "round-robin",
@@ -681,13 +730,20 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
 	head->delayed_removal_secs = 0;
 
 	/*
-	 * Add a multipath node if the subsystems supports multiple controllers.
-	 * We also do this for private namespaces as the namespace sharing flag
-	 * could change after a rescan.
+	 * If multipath_head_always is configured then we add a multipath head
+	 * disk node irrespective of disk is single/multi ported or namespace is
+	 * shared/private.
 	 */
-	if (!(ctrl->subsys->cmic & NVME_CTRL_CMIC_MULTI_CTRL) ||
-	    !nvme_is_unique_nsid(ctrl, head) || !multipath)
-		return 0;
+	if (!multipath_head_always) {
+		/*
+		 * Add a multipath node if the subsystems supports multiple
+		 * controllers. We also do this for private namespaces as the
+		 * namespace sharing flag could change after a rescan.
+		 */
+		if (!(ctrl->subsys->cmic & NVME_CTRL_CMIC_MULTI_CTRL) ||
+		    !nvme_is_unique_nsid(ctrl, head) || !multipath)
+			return 0;
+	}
 
 	blk_set_stacking_limits(&lim);
 	lim.dma_alignment = 3;
-- 
2.49.0