On Wed, 28 Jul 2021, Amir Goldstein wrote: > 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. Sounds sensible - thanks. Does this mean that my change would cause problems for people using overlayfs with a btrfs lower layer? > > 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. I did consider making it opt-in, but it is easy enough for the caller to ignore the changed vfsmount, and only one (of 4) callers that it really makes a difference for. I will review the overlayfs in light of these comments. Thanks, NeilBrown
