btrfs: configure read policy via module parameter

For testing purposes allow to configure the read policy via module
parameter from the beginning. Available only with CONFIG_BTRFS_EXPERIMENTAL

Examples:

- Set the RAID1 balancing method to round-robin with a custom
  min_contig_read of 4k:
  $ modprobe btrfs read_policy=round-robin:4096

- Set the round-robin balancing method with the default
  min_contiguous_read:
  $ modprobe btrfs read_policy=round-robin

- Set the "devid" balancing method, defaulting to the latest device:
  $ modprobe btrfs read_policy=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_read_policy_init,
+		.exit_func = NULL,
+#endif
 	}, {
 		.init_func = ordered_data_init,
 		.exit_func = ordered_data_exit,

fs/btrfs/sysfs.c

@@ -1313,7 +1313,22 @@ static const char *btrfs_read_policy_name[] = {
 #endif
 };
 
-static int btrfs_read_policy_to_enum(const char *str, s64 *value_ret)
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+
+/* Global module configuration parameters. */
+static char *read_policy;
+char *btrfs_get_mod_read_policy(void)
+{
+	return read_policy;
+}
+
+/* Set perms to 0, disable /sys/module/btrfs/parameter/read_policy interface. */
+module_param(read_policy, charp, 0);
+MODULE_PARM_DESC(read_policy,
+"Global read policy: pid (default), round-robin[:<min_contig_read>], devid[:<devid>]");
+#endif
+
+int btrfs_read_policy_to_enum(const char *str, s64 *value_ret)
 {
 	char param[32] = { 0 };
 	char __maybe_unused *value_str;
@@ -1344,6 +1359,20 @@ static int btrfs_read_policy_to_enum(const char *str, s64 *value_ret)
 	return sysfs_match_string(btrfs_read_policy_name, param);
 }
 
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+int __init btrfs_read_policy_init(void)
+{
+	s64 value;
+
+	if (btrfs_read_policy_to_enum(read_policy, &value) == -EINVAL) {
+		btrfs_err(NULL, "invalid read policy or value %s", read_policy);
+		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 +47,11 @@ 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_read_policy_init(void);
+char *btrfs_get_mod_read_policy(void);
+#endif
 
 #endif

fs/btrfs/volumes.c

@@ -1299,6 +1299,7 @@ static int open_fs_devices(struct btrfs_fs_devices *fs_devices,
 	struct btrfs_device *device;
 	struct btrfs_device *latest_dev = NULL;
 	struct btrfs_device *tmp_device;
+	s64 __maybe_unused value = 0;
 	int ret = 0;
 
 	list_for_each_entry_safe(device, tmp_device, &fs_devices->devices,
@@ -1328,10 +1329,22 @@ 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_contig_read = BTRFS_DEFAULT_RR_MIN_CONTIG_READ;
 	fs_devices->read_devid = latest_dev->devid;
+	fs_devices->read_policy = btrfs_read_policy_to_enum(btrfs_get_mod_read_policy(),
+							    &value);
+	if (fs_devices->read_policy == BTRFS_READ_POLICY_RR)
+		fs_devices->collect_fs_stats = true;
+
+	if (value) {
+		if (fs_devices->read_policy == BTRFS_READ_POLICY_RR)
+			fs_devices->rr_min_contig_read = value;
+		if (fs_devices->read_policy == BTRFS_READ_POLICY_DEVID)
+			fs_devices->read_devid = value;
+	}
+#else
+	fs_devices->read_policy = BTRFS_READ_POLICY_PID;
 #endif
 
 	return 0;