[v3,0/9] vhost-scsi: log write descriptors for live migration (and three bugfix)

Message

Dongli Zhang April 3, 2025, 6:29 a.m. UTC

The live migration with vhost-scsi has been enabled by QEMU commit
b3e89c941a85 ("vhost-scsi: Allow user to enable migration"), which
thoroughly explains the workflow that QEMU collaborates with vhost-scsi on
the live migration.

Although it logs dirty data for the used ring, it doesn't log any write
descriptor (VRING_DESC_F_WRITE).

In comparison, vhost-net logs write descriptors via vhost_log_write(). The
SPDK (vhost-user-scsi backend) also logs write descriptors via
vhost_log_req_desc().

As a result, there is likely data mismatch between memory and vhost-scsi
disk during the live migration.

1. Suppose there is high workload and high memory usage. Suppose some
systemd userspace pages are swapped out to the swap disk.

2. Upon request from systemd, the kernel reads some pages from the swap
disk to the memory via vhost-scsi.

3. Although those userspace pages' data are updated, they are not marked as
dirty by vhost-scsi (this is the bug). They are not going to migrate to the
target host during memory transfer iterations.

4. Suppose systemd doesn't write to those pages any longer. Those pages
never get the chance to be dirty or migrated any longer.

5. Once the guest VM is resumed on the target host, because of the lack of
those dirty pages' data, the systemd may run into abnormal status, i.e.,
there may be systemd segfault.

Log all write descriptors to fix the issue.

In addition, the patchset also fixes three bugs in vhost-scsi.

Changed since v1:
  - Rebase on top of most recent vhost changes.
  - Don't allocate log buffer during initialization. Allocate only once for
    each command. Don't free until not used any longer.
  - Add bugfix for vhost_scsi_send_status().
Changed since v2:
  - Document parameters of vhost_log_write().
  - Use (len == U64_MAX) to indicate whether log all pages, instead of
    introducing a new parameter.
  - Merge PATCH 6 and PATCH 7 from v2 as one patch, to Allocate for only
    once in submission path in runtime. Reclaim int
    VHOST_SET_FEATURES/VHOST_SCSI_SET_ENDPOINT.
  - Encapsulate the one-time on-demand per-cmd log buffer alloc/copy in a
    helper, as suggested by Mike.

Dongli Zhang (vhost-scsi bugfix):
  vhost-scsi: protect vq->log_used with vq->mutex
  vhost-scsi: Fix vhost_scsi_send_bad_target()
  vhost-scsi: Fix vhost_scsi_send_status()

Dongli Zhang (log descriptor, suggested by Joao Martins):
  vhost: modify vhost_log_write() for broader users
  vhost-scsi: adjust vhost_scsi_get_desc() to log vring descriptors
  vhost-scsi: log I/O queue write descriptors
  vhost-scsi: log control queue write descriptors
  vhost-scsi: log event queue write descriptors
  vhost: add WARNING if log_num is more than limit

 drivers/vhost/scsi.c  | 264 ++++++++++++++++++++++++++++++++++++++++-----
 drivers/vhost/vhost.c |  46 ++++++--
 2 files changed, 273 insertions(+), 37 deletions(-)


base-commit: ac34bd6a617c03cad0fbb61f189f7c4dafbbddfb
branch: remotes/origin/linux-next
tree: https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git

Thank you very much!

Dongli Zhang