btrfs: enable RAID1 balancing configuration via modprobe parameter

This update allows configuring the `raid1-balancing` methods using a
modprobe parameter when experimental mode CONFIG_BTRFS_EXPERIMENTAL
is enabled.

Examples:

- Set the RAID1 balancing method to round-robin with a custom
`min_contiguous_read` of 192k:
  $ modprobe btrfs raid1-balancing=round-robin:196608

- Set the round-robin balancing method with the default
`min_contiguous_read` of 256k:
  $ modprobe btrfs raid1-balancing=round-robin

- Set the `devid` balancing method, defaulting to the latest
device:
  $ modprobe btrfs raid1-balancing=devid

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -2527,6 +2527,11 @@ static const struct init_sequence mod_init_seq[] = {
 	}, {
 		.init_func = extent_map_init,
 		.exit_func = extent_map_exit,
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+	}, {
+		.init_func = btrfs_raid1_balancing_init,
+		.exit_func = NULL,
+#endif
 	}, {
 		.init_func = ordered_data_init,
 		.exit_func = ordered_data_exit,

fs/btrfs/sysfs.c

@@ -1313,7 +1313,21 @@ static const char *btrfs_read_policy_name[] = {
 #endif
 };
 
-static int btrfs_read_policy_to_enum(const char *str, s64 *value)
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+/* Global module configuration parameters */
+static char *raid1_balancing;
+char *btrfs_get_raid1_balancing(void)
+{
+	return raid1_balancing;
+}
+
+/* Set perm 0, disable sys/module/btrfs/parameter/raid1_balancing interface */
+module_param(raid1_balancing, charp, 0);
+MODULE_PARM_DESC(raid1_balancing,
+"Global read policy; pid (default), round-robin:[min_contiguous_read], devid:[[devid]|[latest-gen]|[oldest-gen]]");
+#endif
+
+int btrfs_read_policy_to_enum(const char *str, s64 *value)
 {
 	char param[32] = {'\0'};
 	char *__maybe_unused value_str;
@@ -1348,6 +1362,18 @@ static int btrfs_read_policy_to_enum(const char *str, s64 *value)
 	return -EINVAL;
 }
 
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+int __init btrfs_raid1_balancing_init(void)
+{
+	if (btrfs_read_policy_to_enum(raid1_balancing, NULL) == -EINVAL) {
+		btrfs_err(NULL, "Invalid raid1_balancing %s", raid1_balancing);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+#endif
+
 static ssize_t btrfs_read_policy_show(struct kobject *kobj,
 				      struct kobj_attribute *a, char *buf)
 {

fs/btrfs/sysfs.h

@@ -47,5 +47,10 @@ void btrfs_sysfs_del_qgroups(struct btrfs_fs_info *fs_info);
 int btrfs_sysfs_add_qgroups(struct btrfs_fs_info *fs_info);
 void btrfs_sysfs_del_one_qgroup(struct btrfs_fs_info *fs_info,
 				struct btrfs_qgroup *qgroup);
+int btrfs_read_policy_to_enum(const char *str, s64 *value);
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+int __init btrfs_raid1_balancing_init(void);
+char *btrfs_get_raid1_balancing(void);
+#endif
 
 #endif

fs/btrfs/volumes.c

@@ -1328,10 +1328,13 @@ static int open_fs_devices(struct btrfs_fs_devices *fs_devices,
 	fs_devices->latest_dev = latest_dev;
 	fs_devices->total_rw_bytes = 0;
 	fs_devices->chunk_alloc_policy = BTRFS_CHUNK_ALLOC_REGULAR;
-	fs_devices->read_policy = BTRFS_READ_POLICY_PID;
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
 	fs_devices->rr_min_contiguous_read = BTRFS_DEFAULT_RR_MIN_CONTIGUOUS_READ;
 	fs_devices->read_devid = latest_dev->devid;
+	fs_devices->read_policy =
+		btrfs_read_policy_to_enum(btrfs_get_raid1_balancing(), NULL);
+#else
+	fs_devices->read_policy = BTRFS_READ_POLICY_PID;
 #endif
 
 	return 0;