[PATCH v2 0/4] virtio_balloon: quiesce balloon work on device shutdown

["Denis V. Lunev" <den@openvz.org>]

Since commit 8bd2fa086a04 ("virtio: break and reset virtio devices on
device_shutdown()") the virtio bus breaks and resets every virtio device
during device_shutdown(), i.e. on reboot and kexec. virtio_balloon has no
.shutdown of its own, so that generic path runs while the balloon's
asynchronous work is still armed: the free page reporting worker, the
inflate/deflate and stats workers, the OOM notifier and the free page
shrinker.

Once the device has been broken, virtqueue_add_inbuf() in
virtballoon_free_page_report() returns -EIO and trips its WARN_ON_ONCE().
On a kernel booted with panic_on_warn that turns an ordinary reboot into a
fatal panic in the middle of device_shutdown(), so the machine never
reaches the new kernel. The inflate/deflate and OOM paths do not warn but
are no better off: they call wait_event(vb->acked, ...) and would block
forever on a queue that can no longer complete.

This was hit in the field as an intermittent failure of a virtualization
cluster upgrade: guest storage nodes were rebooted via kexec into the new
kernel, and the ones whose free page reporting happened to run during
device_shutdown() panicked (the guests run with panic_on_warn) and never
came back, stalling the rolling upgrade. The crash dump showed the WARN at
virtio_balloon.c:216 in a page_reporting kworker, with all the balloon
virtqueues already broken.

Validated by churning balloon inflate/deflate from the host while
kexec-rebooting the guest in a loop under panic_on_warn: the unpatched
kernel reproduces the WARN within a couple of cycles, while the patched
kernel survives many consecutive kexec cycles cleanly (12/12 in the final
run, 0 WARNs). checkpatch is clean across the series.

Changes in v2:
- Add a virtio_device_shutdown() core helper and call it from the balloon
  .shutdown handler instead of open-coding break + synchronize_cbs + reset
  (David Hildenbrand).
- New patch: make tell_host() warn and bail instead of hanging if a buffer
  add ever fails (David Hildenbrand); kept as a separate patch
  (Michael S. Tsirkin).

v1: https://lore.kernel.org/all/20260622133715.3707707-1-den@openvz.org

Denis V. Lunev (4):
  virtio: add virtio_device_shutdown() helper
  virtio_balloon: factor out virtballoon_quiesce()
  virtio_balloon: quiesce balloon work before device shutdown
  virtio_balloon: warn on failed buffer add in tell_host()

 drivers/virtio/virtio.c         | 41 ++++++++++++++++++++++-----------
 drivers/virtio/virtio_balloon.c | 40 ++++++++++++++++++++++++--------
 include/linux/virtio.h          |  1 +
 3 files changed, 59 insertions(+), 23 deletions(-)

-- 
2.53.0