btrfs: sysfs: add bdi link to the fsid directory

Since BTRFS uses a private bdi it makes sense to create a link to this
bdi under /sys/fs/btrfs/<UUID>/bdi. This allows size of read ahead to
be controlled. Without this patch it's not possible to uniquely identify
which bdi pertains to which btrfs filesystem in the case of multiple
btrfs filesystems.

It's fine to simply call sysfs_remove_link without checking if the
link indeed has been created. The call path

sysfs_remove_link
 kernfs_remove_by_name
  kernfs_remove_by_name_ns

will simply return -ENOENT in case it doesn't exist.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -937,8 +937,12 @@ void btrfs_sysfs_remove_fsid(struct btrfs_fs_devices *fs_devs)
 
 void btrfs_sysfs_remove_mounted(struct btrfs_fs_info *fs_info)
 {
+	struct kobject *fsid_kobj = &fs_info->fs_devices->fsid_kobj;
+
 	btrfs_reset_fs_info_ptr(fs_info);
 
+	sysfs_remove_link(fsid_kobj, "bdi");
+
 	if (fs_info->space_info_kobj) {
 		sysfs_remove_files(fs_info->space_info_kobj, allocation_attrs);
 		kobject_del(fs_info->space_info_kobj);
@@ -958,8 +962,8 @@ void btrfs_sysfs_remove_mounted(struct btrfs_fs_info *fs_info)
 	}
 #endif
 	addrm_unknown_feature_attrs(fs_info, false);
-	sysfs_remove_group(&fs_info->fs_devices->fsid_kobj, &btrfs_feature_attr_group);
-	sysfs_remove_files(&fs_info->fs_devices->fsid_kobj, btrfs_attrs);
+	sysfs_remove_group(fsid_kobj, &btrfs_feature_attr_group);
+	sysfs_remove_files(fsid_kobj, btrfs_attrs);
 	btrfs_sysfs_remove_devices_dir(fs_info->fs_devices, NULL);
 }
 
@@ -1439,6 +1443,10 @@ int btrfs_sysfs_add_mounted(struct btrfs_fs_info *fs_info)
 	if (error)
 		goto failure;
 
+	error = sysfs_create_link(fsid_kobj, &fs_info->sb->s_bdi->dev->kobj, "bdi");
+	if (error)
+		goto failure;
+
 	fs_info->space_info_kobj = kobject_create_and_add("allocation",
 						  fsid_kobj);
 	if (!fs_info->space_info_kobj) {