[PATCHv4 6/8] nvme: export I/O failure count when no path is available via sysfs

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

When I/O is submitted to the NVMe namespace head and no available path
can handle the request, the driver fails the I/O immediately. Currently,
such failures are only reported via kernel log messages, which may be
lost over time since dmesg is a circular buffer.

Add a new ns-head sysfs counter io_fail_no_available_path_count, under
diag attribute group to expose the number of I/Os that failed due to the
absence of an available path. This provides persistent visibility into
path-related I/O failures and can help users diagnose the cause of I/O
errors. This counter is also writable and so user may reset its value,
if needed.

This counter can also be consumed by monitoring tools such as nvme-top.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/multipath.c | 30 ++++++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h      |  2 ++
 drivers/nvme/host/sysfs.c     |  5 +++++
 3 files changed, 37 insertions(+)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index f72a687daa8f..dce566aca748 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -527,6 +527,7 @@ static void nvme_ns_head_submit_bio(struct bio *bio)
 		dev_warn_ratelimited(dev, "no available path - failing I/O\n");
 
 		bio_io_error(bio);
+		atomic_long_inc(&head->io_fail_no_available_path_count);
 	}
 
 	srcu_read_unlock(&head->srcu, srcu_idx);
@@ -1208,6 +1209,35 @@ static ssize_t io_requeue_no_usable_path_count_store(struct device *dev,
 
 DEVICE_ATTR_RW(io_requeue_no_usable_path_count);
 
+static ssize_t io_fail_no_available_path_count_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct gendisk *disk = dev_to_disk(dev);
+	struct nvme_ns_head *head = disk->private_data;
+
+	return sysfs_emit(buf, "%lu\n",
+		    atomic_long_read(&head->io_fail_no_available_path_count));
+}
+
+static ssize_t io_fail_no_available_path_count_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t count)
+{
+	int err;
+	unsigned long fail_cnt;
+	struct gendisk *disk = dev_to_disk(dev);
+	struct nvme_ns_head *head = disk->private_data;
+
+	err = kstrtoul(buf, 0, &fail_cnt);
+	if (err)
+		return -EINVAL;
+
+	atomic_long_set(&head->io_fail_no_available_path_count, fail_cnt);
+
+	return count;
+}
+
+DEVICE_ATTR_RW(io_fail_no_available_path_count);
+
 static int nvme_lookup_ana_group_desc(struct nvme_ctrl *ctrl,
 		struct nvme_ana_group_desc *desc, void *data)
 {
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 845e338449ce..9434abf2659e 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -565,6 +565,7 @@ struct nvme_ns_head {
 	struct delayed_work	remove_work;
 	unsigned int		delayed_removal_secs;
 	atomic_long_t		io_requeue_no_usable_path_count;
+	atomic_long_t		io_fail_no_available_path_count;
 #define NVME_NSHEAD_DISK_LIVE		0
 #define NVME_NSHEAD_QUEUE_IF_NO_PATH	1
 	struct nvme_ns __rcu	*current_path[];
@@ -1069,6 +1070,7 @@ extern struct device_attribute dev_attr_numa_nodes;
 extern struct device_attribute dev_attr_delayed_removal_secs;
 extern struct device_attribute dev_attr_multipath_failover_count;
 extern struct device_attribute dev_attr_io_requeue_no_usable_path_count;
+extern struct device_attribute dev_attr_io_fail_no_available_path_count;
 extern struct device_attribute subsys_attr_iopolicy;
 
 static inline bool nvme_disk_is_ns_head(struct gendisk *disk)
diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
index 9fe3a74b2bef..01d771d85f31 100644
--- a/drivers/nvme/host/sysfs.c
+++ b/drivers/nvme/host/sysfs.c
@@ -411,6 +411,7 @@ static struct attribute *nvme_ns_diag_attrs[] = {
 #ifdef CONFIG_NVME_MULTIPATH
 	&dev_attr_multipath_failover_count.attr,
 	&dev_attr_io_requeue_no_usable_path_count.attr,
+	&dev_attr_io_fail_no_available_path_count.attr,
 #endif
 	NULL,
 };
@@ -439,6 +440,10 @@ static umode_t nvme_ns_diag_attrs_are_visible(struct kobject *kobj,
 		if (!nvme_disk_is_ns_head(dev_to_disk(dev)))
 			return 0;
 	}
+	if (a == &dev_attr_io_fail_no_available_path_count.attr) {
+		if (!nvme_disk_is_ns_head(dev_to_disk(dev)))
+			return 0;
+	}
 #endif
 	return a->mode;
 }
-- 
2.53.0