btrfs: sysfs: add per-fs attribute for read policy

Add

 /sys/fs/btrfs/UUID/read_policy

attribute so that the read policy for the raid1, raid1c34 and raid10 can
be tuned.

When this attribute is read, it will show all available policies, with
active policy in [ ]. The read_policy attribute can be written using one
of the items listed in there.

For example:
  $ cat /sys/fs/btrfs/UUID/read_policy
  [pid]
  $ echo pid > /sys/fs/btrfs/UUID/read_policy
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -907,6 +907,54 @@ static bool strmatch(const char *buffer, const char *string)
 	return false;
 }
 
+static const char * const btrfs_read_policy_name[] = { "pid" };
+
+static ssize_t btrfs_read_policy_show(struct kobject *kobj,
+				      struct kobj_attribute *a, char *buf)
+{
+	struct btrfs_fs_devices *fs_devices = to_fs_devs(kobj);
+	ssize_t ret = 0;
+	int i;
+
+	for (i = 0; i < BTRFS_NR_READ_POLICY; i++) {
+		if (fs_devices->read_policy == i)
+			ret += scnprintf(buf + ret, PAGE_SIZE - ret, "%s[%s]",
+					 (ret == 0 ? "" : " "),
+					 btrfs_read_policy_name[i]);
+		else
+			ret += scnprintf(buf + ret, PAGE_SIZE - ret, "%s%s",
+					 (ret == 0 ? "" : " "),
+					 btrfs_read_policy_name[i]);
+	}
+
+	ret += scnprintf(buf + ret, PAGE_SIZE - ret, "\n");
+
+	return ret;
+}
+
+static ssize_t btrfs_read_policy_store(struct kobject *kobj,
+				       struct kobj_attribute *a,
+				       const char *buf, size_t len)
+{
+	struct btrfs_fs_devices *fs_devices = to_fs_devs(kobj);
+	int i;
+
+	for (i = 0; i < BTRFS_NR_READ_POLICY; i++) {
+		if (strmatch(buf, btrfs_read_policy_name[i])) {
+			if (i != fs_devices->read_policy) {
+				fs_devices->read_policy = i;
+				btrfs_info(fs_devices->fs_info,
+					   "read policy set to '%s'",
+					   btrfs_read_policy_name[i]);
+			}
+			return len;
+		}
+	}
+
+	return -EINVAL;
+}
+BTRFS_ATTR_RW(, read_policy, btrfs_read_policy_show, btrfs_read_policy_store);
+
 static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, label),
 	BTRFS_ATTR_PTR(, nodesize),
@@ -917,6 +965,7 @@ static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, checksum),
 	BTRFS_ATTR_PTR(, exclusive_operation),
 	BTRFS_ATTR_PTR(, generation),
+	BTRFS_ATTR_PTR(, read_policy),
 	NULL,
 };