[PATCH v2 0/1] statmount: mountinfo for "unmounted" mounts

[Bhavik Sachdev <b.sachdev1904@gmail.com>]

By "unmounted" mounts we mean mounts that have been unmounted using
umount2(mnt, MNT_DETACH) but we still have file descriptors to files on
that mount. We want to add the ability to handle such mounts in CRIU
(Checkpoint/Restore in Userspace).

Currently, we have no way to get mount info for these mounts as they do
not appear in /proc/<pid>/mountinfo and statmount does not work on them.

We solve this problem by introducing a file descriptor parameter to
statmount, along with a STATMOUNT_FD flag. Even if this file descriptor
is on a "unmounted" mount we are still able to get mountinfo for the
mount. We report the mountpoint of the mount to be "[detached]" and
mnt_ns_id to be 0.

v1 of this patchset, took a different approach and introduced a new
umount_mnt_ns, to which "unmounted" mounts would be moved to (instead of
their namespace being NULL) thus allowing them to be still available via
statmount:
https://lore.kernel.org/linux-fsdevel/20251002125422.203598-1-b.sachdev1904@gmail.com/

That approach complicated namespace locking and modified performance
sensitive code.
See: https://lore.kernel.org/linux-fsdevel/7e4d9eb5-6dde-4c59-8ee3-358233f082d0@virtuozzo.com/

Christian also talked about a separate approach of tying the _lifetime_
of the mount namespace to the lifetime of the unmounted mounts through
the passive reference count by moving them to a separate rb_root
`unmounted` in the namespace instance.

This approach has a few problems, some of them are:
1. It further extends the scope of the namespace semaphore.
2. Weird to be able to statmount() via mount id if the mount namespace
is dead.

For a more complete description, see:
https://lore.kernel.org/linux-fsdevel/20251006-erlesen-anlagen-9af59899a969@brauner/

Aleska Sarai also pointed out that this fd based approach is similiar to
the fstatfs(2) which returns information about a mounted filesystem when
given a fd open on that filesystem.
https://lore.kernel.org/linux-fsdevel/2025-10-07-lavish-refried-navy-journey-EqHk9K@cyphar.com/

We use this patchset with CRIU to support checkpoint/restore of
"unmounted" mounts in this pull request:
https://github.com/checkpoint-restore/criu/pull/2754.

All these patches are also available in this branch on github:
https://github.com/bsach64/linux/tree/statmount-fd-v2

Bhavik Sachdev (1):
  statmount: accept fd as a parameter

 fs/namespace.c             | 80 ++++++++++++++++++++++++++++----------
 include/uapi/linux/mount.h |  8 ++++
 2 files changed, 67 insertions(+), 21 deletions(-)

-- 
2.51.0