[CRIU] Sync TODO-s for mount.c work

[Oleg Nesterov]

On 04/21, Pavel Emelyanov wrote:
>
> This is just to make sure I properly track what's going on with mount.c :)

And let me report another problem, just for record. Sorry if this
was already discussed or documented somewhere. And please correct
me if I am wrong, I still do not really understand this "mount"
magic.

I do not see how CRIU can dump/restore a mount_nodev() mount, say,
tmpfs. Yes, tmpfs_dump() and tmpfs_restore() are clever and I am
not saying there are always wrong, but this depends on use-case.

Once again, my lovely trivial example:

	# unshare -m

	# grep run /proc/self/mountinfo
	52 26 0:18 / /run rw,nosuid,nodev shared:22 - tmpfs tmpfs rw,seclabel,mode=755

	# perl -e 'close STDIN; close STDOUT; close STDERR; sleep'

and dump/restore works. In particular it dumps/restores /run. But
it actually restores the "copy" of this mount and this is pointless
in this particular case.

CRIU doesn't have an option to change this behaviour but this is
minor, to simplify the discussion lets suppose we want to change
the current behaviour so /run is still "shared" after restore.

How can we do this?

"restore" does clean_mnt_ns(), this umounts "/run", and after that
I don't see how we can re-mount it "correctly" without nontrivial
setns-like hacks.

Oleg.