On Tue, Oct 19, 2021 at 08:22:08AM +0800, Anand Jain wrote:

btrfs-progs tries to read the fsid from the super-block for a missing
device and, it fails. It needs to find out if the device is a seed
device. It does it by comparing the device's fsid with the fsid of the
mounted filesystem. To help this scenario introduce a new sysfs file to
read the fsid from the kernel.
     /sys/fs/btrfs/<fsid>/devinfo/<devid>/fsid

Patch 1 is a cleanup converts scnprtin()f and snprintf() to sysfs_emit()
Patch 2 introduces the new sysfs interface as above

The other implementation choice is to add another parameter to the
struct btrfs_ioctl_dev_info_args and use BTRFS_IOC_DEV_INFO ioctl. But
then backward kernel compatibility with the newer btrfs-progs is more
complicated. If needed, we can add that too.

Related btrfs-progs patches:
  btrfs-progs: prepare helper device_is_seed
  btrfs-progs: read fsid from the sysfs in device_is_seed

Anand Jain (2):
  btrfs: sysfs convert scnprintf and snprintf to use sysfs_emit
  btrfs: sysfs add devinfo/fsid to retrieve fsid from the device

 fs/btrfs/sysfs.c | 113 +++++++++++++++++++++++++----------------------
 1 file changed, 60 insertions(+), 53 deletions(-)

You can add

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

to the series.  Thanks,

Josef