Add zvol_inhibit_dev module option.

ZoL can create more zvols at runtime than can be configured during
system start, which hangs the init stack at reboot.

When a slow system has more than a few hundred zvols, udev will
fork bomb during system start and spend too much time in device
detection routines, so upstart kills it.

The zfs_inhibit_dev option allows an affected system to be rescued
by skipping /dev/zd* creation and thereby avoiding the udev
overload. All zvols are made inaccessible if this option is set, but
the `zfs destroy` and `zfs send` commands still work, and ZFS
filesystems can be mounted.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/zvol.c

@@ -46,6 +46,7 @@
 #include <sys/zvol.h>
 #include <linux/blkdev_compat.h>
 
+unsigned int zvol_inhibit_dev = 0;
 unsigned int zvol_major = ZVOL_MAJOR;
 unsigned int zvol_threads = 32;
 
@@ -1341,6 +1342,9 @@ zvol_create_minors(const char *pool)
 	spa_t *spa = NULL;
 	int error = 0;
 
+	if (zvol_inhibit_dev)
+		return (0);
+
 	mutex_enter(&zvol_state_lock);
 	if (pool) {
 		error = dmu_objset_find_spa(NULL, pool, zvol_create_minors_cb,
@@ -1370,6 +1374,9 @@ zvol_remove_minors(const char *pool)
 	zvol_state_t *zv, *zv_next;
 	char *str;
 
+	if (zvol_inhibit_dev)
+		return;
+
 	str = kmem_zalloc(MAXNAMELEN, KM_SLEEP);
 	if (pool) {
 		(void) strncpy(str, pool, strlen(pool));
@@ -1431,6 +1438,9 @@ zvol_fini(void)
 	list_destroy(&zvol_state_list);
 }
 
+module_param(zvol_inhibit_dev, uint, 0644);
+MODULE_PARM_DESC(zvol_inhibit_dev, "Do not create zvol device nodes");
+
 module_param(zvol_major, uint, 0444);
 MODULE_PARM_DESC(zvol_major, "Major number for zvol device");