Miklos Szeredi wrote:
> > That makes less sense if we allow other tasks to be using a namespace
> > through a passing a file descriptor, and then the last task which has
> > current->namespace equal to that namespace exits. It makes no sense
> > to me that the mount which is still accessible through the file
> > descriptor is suddenly detached from it's parent and children mounts.
>
> I see your point. I don't yet see a solution.
>
> Currently detach is an explicit action, not something automatic which
> happens when there are no more references to a vfsmount.
It's implicit, when the last task calls put_namespace:
void __put_namespace(struct namespace *namespace)
{
[...]
umount_tree(namespace->root);
-> calls detach_mnt for each vfsmnt in namespace.
[...]
}
> > Why is it not good enough to detach each vfsmnt when the last
> > reference to each vfsmnt is dropped? In other words, simply when the
> > vfsmnt becomes unreachable?
>
> Define unreachable.
Unreachable as in no file descriptors (or chroot/cwd) refer to the
vfsmnt, either directly or indirectly through a path traversal.
> Then define a mechanism, by which it can be detected.
There aren't any vfsmnt->vfsmnt cycles... They're a forest, vfsmnts
don't move from one tree to another (bind mounts don't link them, they
create new vfsmnts), and each tree can be referenced by a file
descriptor at any point on the tree.
It rather hinges on which of these behaviours you prefer:
1. A file descriptor/chroot/cwd reference to any point in a vfsmnt
tree means the whole tree is retained. This means ".." remains
always accessible: fchdir(fd); open("..") continues to access
that whole tree as you still have fd.
2. A file descriptor/chroot/cwd reference to any point in a vfsmnt
tree means the subtree from that point is retained, and parents
may disappear if there are no references (not counting ".." as a
reference). This behaviour is more sensible for chroots, where
the parents should be inaccessible anyway.
3. A mixture, where current->root references only maintain the
subtree rooted at that point, and other references, if outside
the current->root subtree, retain the whole tree accessible from
those references.
The appropriate data structure / algorithm depends on which behaviour
is preferred. So which is it? 1 Is best done with a mnt_namespace
structure, but references to it counted when vfsmnts are referenced by
file descriptors/root/cwd, _not_ references by tasks (no
current->namespace). 2 is best done by simply reference counting
vfsmnts.
-- Jamie
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