Possible bug: detached mounts difficult to cleanup

Eric W. Biederman ebiederm at xmission.com
Wed Jan 11 02:27:05 UTC 2017

Krister Johansen <kjlx at templeofstupid.com> writes:

> Gents,
> I wondered if a naive solution could re-walk the list of mounts
> processed in umount_tree() and if all of the detached but locked mounts
> had a refcount that indicated they're unused, they could be unlocked and
> unmounted.  At least in the case of the containers I'm dealing with, the
> the container software should be ensuring that nothing in the container
> has a reference on anything that's under the detached portion of the
> tree.  However, there's probably a better way to do this.

So if the code is working correctly that should already happen.

The design is for the parent mount to hold a reference to the submounts.
And when the reference on the parent drops to 0.  The references on
all of the submounts will also be dropped.

I was hoping to read the code and point it out to you quickly, but I am
not seeing it now.  I am wondering if in all of the refactoring of that
code something was dropped/missed :(

Somewhere there is supposed to be the equivalent of:
	pin_insert_group(&p->mnt_umount, &p->mnt_parent->mnt, &unmounted);
when we unhash those mounts because the last count has gone away.
Either it is very sophisticated or I am missing it.  Grr....


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