file metadata via fs API

[Ian Kent]

On Wed, 2020-08-12 at 14:06 +0100, David Howells wrote:
> Miklos Szeredi <miklos at szeredi.hu> wrote:
> 
> > That presumably means the mount ID <-> mount path mapping already
> > exists, which means it's just possible to use the open(mount_path,
> > O_PATH) to obtain the base fd.
> 
> No, you can't.  A path more correspond to multiple mounts stacked on
> top of
> each other, e.g.:
> 
> 	mount -t tmpfs none /mnt
> 	mount -t tmpfs none /mnt
> 	mount -t tmpfs none /mnt
> 
> Now you have three co-located mounts and you can't use the path to
> differentiate them.  I think this might be an issue in autofs, but
> Ian would
> need to comment on that.

It is a problem for autofs, direct mounts in particular, but also
for mount ordering at times when umounting a tree of mounts where
mounts are covered or at shutdown.

Ian