On Mon, Feb 24, 2020 at 3:55 PM James Bottomley
[off-list ref] wrote:

Once it's table driven, certainly a sysfs directory becomes possible.
The problem with ST_DEV is filesystems like btrfs and xfs that may have
multiple devices.

For XFS there's always  a single sb->s_dev though, that's what st_dev
will be set to on all files.

Btrfs subvolume is sort of a lightweight superblock, so basically all
such st_dev's are aliases of the same master superblock.  So lookup of
all subvolume st_dev's could result in referencing the same underlying
struct super_block (just like /proc/$PID will reference the same
underlying task group regardless of which of the task group member's
PID is used).

Having this info in sysfs would spare us a number of issues that a set
of new syscalls would bring.  The question is, would that be enough,
or is there a reason that sysfs can't be used to present the various
filesystem related information that fsinfo is supposed to present?

Thanks,
Miklos