[PATCH v4 0/3] virtiofs: export filesystem tags through sysfs

[Stefan Hajnoczi <stefanha@redhat.com>]

v4:
- Create kset before registering virtio driver because the kset needed in
  virtio_fs_probe(). Solves the empty /sys/fs/virtiofs bug. [Vivek]
v3:
- Use dev_dbg() to avoid spamming logs [Greg]
- Fix 644 mode on "tag" attr and use __ATTR_RO() [Greg]
- Use kset_uevent_ops and eliminate explicit KOBJ_REMOVE [Greg]
v2:
- Vivek mentioned that he didn't have time to work on this patch series
  recently so I gave it a shot.
- Information is now exposed in /sys/fs/virtiofs/ whereas before it was part of
  the generic virtio device kobject, which didn't really fit.

Userspace needs a way to enumerate available virtiofs filesystems and detect
when they are hotplugged or unplugged. This would allow systemd to wait for a
virtiofs filesystem during boot, for example.

This patch series adds the following in sysfs:

  /sys/fs/virtiofs/<n>/tag    - unique identifier for mount(8)
  /sys/fs/virtiofs/<n>/device - symlink to virtio device

A uevent is emitted when virtiofs devices are hotplugged or unplugged:

  KERNEL[111.113221] add      /fs/virtiofs/2 (virtiofs)
  ACTION=add
  DEVPATH=/fs/virtiofs/2
  SUBSYSTEM=virtiofs
  TAG=test

  KERNEL[165.527167] remove   /fs/virtiofs/2 (virtiofs)
  ACTION=remove
  DEVPATH=/fs/virtiofs/2
  SUBSYSTEM=virtiofs
  TAG=test

Stefan Hajnoczi (3):
  virtiofs: forbid newlines in tags
  virtiofs: export filesystem tags through sysfs
  virtiofs: emit uevents on filesystem events

 fs/fuse/virtio_fs.c                         | 137 ++++++++++++++++----
 Documentation/ABI/testing/sysfs-fs-virtiofs |  11 ++
 2 files changed, 126 insertions(+), 22 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-fs-virtiofs

-- 
2.43.0