xfs: expose the number of open zones in sysfs

Add a sysfs attribute for the current number of open zones so that it
can be trivially read from userspace in monitoring or testing software.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Christoph Hellwig

Documentation/admin-guide/xfs.rst

@@ -550,6 +550,10 @@ For zoned file systems, the following attributes are exposed in:
 	is limited by the capabilities of the backing zoned device, file system
 	size and the max_open_zones mount option.
 
+  nr_open_zones			(Min:  0  Default:  Varies  Max:  UINTMAX)
+	This read-only attribute exposes the current number of open zones
+	used by the file system.
+
   zonegc_low_space		(Min:  0  Default:  0  Max:  100)
 	Define a percentage for how much of the unused space that GC should keep
 	available for writing. A high value will reclaim more of the space

fs/xfs/xfs_sysfs.c

@@ -13,6 +13,7 @@
 #include "xfs_log.h"
 #include "xfs_log_priv.h"
 #include "xfs_mount.h"
+#include "xfs_zone_priv.h"
 #include "xfs_zones.h"
 #include "xfs_zone_alloc.h"
 
@@ -719,6 +720,17 @@ max_open_zones_show(
 }
 XFS_SYSFS_ATTR_RO(max_open_zones);
 
+static ssize_t
+nr_open_zones_show(
+	struct kobject		*kobj,
+	char			*buf)
+{
+	struct xfs_zone_info	*zi = zoned_to_mp(kobj)->m_zone_info;
+
+	return sysfs_emit(buf, "%u\n", READ_ONCE(zi->zi_nr_open_zones));
+}
+XFS_SYSFS_ATTR_RO(nr_open_zones);
+
 static ssize_t
 zonegc_low_space_store(
 	struct kobject		*kobj,
@@ -756,6 +768,7 @@ XFS_SYSFS_ATTR_RW(zonegc_low_space);
 
 static struct attribute *xfs_zoned_attrs[] = {
 	ATTR_LIST(max_open_zones),
+	ATTR_LIST(nr_open_zones),
 	ATTR_LIST(zonegc_low_space),
 	NULL,
 };