Subvolumes and /proc/self/mountinfo

["H. Peter Anvin" <hpa@zytor.com>]

I'm trying to figure out an algorithm from taking an arbitrary mounted
btrfs directory and break it down into:

<device(s), subvolume, subpath>

where, keep in mind, <subpath> may not actually be part of the mount.

/proc/self/mountinfo seems to have some of that information, however, it
does not appear to distinguish between non-default subvolumes and
directories.  At the same time, once I have mounted a subvolume I see
its name in the root btrfs directory even if I didn't access it.

Questions, thus:

a. Are subvolumes always part of the "root" namespace?  If so, is it the
mounted root, the default subvolume, or subvolume 0 which always exposes
these other subvolumes?  Are there disambiguation rules so that if I
have /btrfs/root/blah and "blah" is both a subvolume and a directory (I
presume that can happen?)

b. Are there better ways (walking the tree using BTRFS_IOC_TREE_SEARCH?)
to accomplish this than using /proc/self/mountinfo?

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.