On Wed, Jul 28, 2021 at 1:44 AM NeilBrown <neilb@xxxxxxx> wrote: > > When a filesystem has internal mounts, it controls the filehandles > across all those mounts (subvols) in the filesystem. So it is useful to > be able to look up a filehandle again one mount, and get a result which > is in a different mount (part of the same overall file system). > > This patch makes that possible by changing export_decode_fh() and > export_decode_fh_raw() to take a vfsmount pointer by reference, and > possibly change the vfsmount pointed to before returning. > > The core of the change is in reconnect_path() which now not only checks > that the dentry is fully connected, but also that the vfsmnt reported > has the same 'dev' (reported by vfs_getattr) as the dentry. > If it doesn't, we walk up the dparent() chain to find the highest place > where the dev changes without there being a mount point, and trigger an > automount there. > > As no filesystems yet provide local-mounts, this does not yet change any > behaviour. > > In exportfs_decode_fh_raw() we previously tested for DCACHE_DISCONNECT > before calling reconnect_path(). That test is dropped. It was only a > minor optimisation and is now inconvenient. > > The change in overlayfs needs more careful thought than I have yet given > it. Just note that overlayfs does not support following auto mounts in layers. See ovl_dentry_weird(). ovl_lookup() fails if it finds such a dentry. So I think you need to make sure that the vfsmount was not crossed when decoding an overlayfs real fh. Apart from that, I think that your new feature should be opt-in w.r.t the exportfs_decode_fh() vfs api and that overlayfs should not opt-in for the cross mount decode. Thanks, Amir.
