Re: subvols, ro- and bind mounts - how?

[Christoph Anton Mitterer <calestyo@scientia.net>]

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Hey.

I'd have an additional question about subvols O:-)

Given the following setup:
5
|
+--root (subvol, /)
   +-- mnt (dir)

with the following done:
- init 1
- remount,ro / (i.e. the subvol root)
- mount /dev/btrfs-device /mnt (i.e. mount the top subvol at /mnt)

The following happened:
- / was ro-mounted (obviously, at least one thing that I had expected
  correctly)
- /mnt was ro-mounted either (and the /mnt/root/ nested subvol then as
  well).
  => why is /mnt (i.e. the top level subvol) mounted ro??
  => I would have expected that, since / (i.e. the subvol "root" is ro
     mounted), it's also ro mounted as the nested subvol below 5, i.e.
     my naive thinking was in terms of logic:
     "/ mounted ro" => "subvol root is mounted ro (everywhere)"
       => "thus /mnt/root/ is mounted ro as well"

However, the later doesn't seem to be true, cause then I did:
- remount,rw /mnt
=> now /mnt/*, including /mnt/root/* was rw moutned



So I guess my assumption of subvols behaving more or less as if they'd
be a fs (and thus mounted at one place ro => everywhere ro) is not
true, is it?

Do, ro,rw (and possibly others) instead only affect the respective
mountpoint?
And automatically any nested subvols of that mountpoint?

So I could have basically:
/mount-point1/subvol-a	; ro, noexec
/mount-point2/subvol-a	; rw, compress=yes
/root			; rw, compress=no
/root/here/it/is/nested/subvol-a ; (no mountpoint)

(with subvol-a being the same subvol)

And when I write via mount-point1 I'd get an error, but via mount-
point2 it works and in addition I get compression, while when writing
via the /root mountpoint, where it is nested, I'd get the rw and
compress=no from the "parent" mountpoint /root


Does that sounds correct?
It seems to make sense actually, though it's a bit unfamiliar... if I'm
not correctly wrong, than e.g. in terms of ext* I cannot have the same
fs mounted with different settings,... of course I cannot have it
mounted twice at all, but speaking of bind mounts.

So I guess, that when I'd do --bind mounts instead, I actually do get
the "old" behaviour, i.e. when the source is ro, then the --bind
mount's target is also forcibly ro.


Still, one unclear thing, why got /mnt mounted ro very above?



Thanks,
Chris.

btw: Not sure if I just missed it, but I guess the above should be more
or less documented, showing people that mounting subvols (especially
when mounting the same several times, either directly or as nested
subvol) has these implications.

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