Getting Call Traces on VM console related to virtio_balloon for s390x RHEL container disk images

JIRA: https://issues.redhat.com/browse/RHEL-87556

commit 2ccd42b959aaf490333dbd3b9b102eaf295c036a
Author: David Hildenbrand <david@redhat.com>
Date:   Wed Apr 2 22:36:21 2025 +0200

    s390/virtio_ccw: Don't allocate/assign airqs for non-existing queues
    
    If we finds a vq without a name in our input array in
    virtio_ccw_find_vqs(), we treat it as "non-existing" and set the vq pointer
    to NULL; we will not call virtio_ccw_setup_vq() to allocate/setup a vq.
    
    Consequently, we create only a queue if it actually exists (name != NULL)
    and assign an incremental queue index to each such existing queue.
    
    However, in virtio_ccw_register_adapter_ind()->get_airq_indicator() we
    will not ignore these "non-existing queues", but instead assign an airq
    indicator to them.
    
    Besides never releasing them in virtio_ccw_drop_indicators() (because
    there is no virtqueue), the bigger issue seems to be that there will be a
    disagreement between the device and the Linux guest about the airq
    indicator to be used for notifying a queue, because the indicator bit
    for adapter I/O interrupt is derived from the queue index.
    
    The virtio spec states under "Setting Up Two-Stage Queue Indicators":
    
            ... indicator contains the guest address of an area wherein the
            indicators for the devices are contained, starting at bit_nr, one
            bit per virtqueue of the device.
    
    And further in "Notification via Adapter I/O Interrupts":
    
            For notifying the driver of virtqueue buffers, the device sets the
            bit in the guest-provided indicator area at the corresponding
            offset.
    
    For example, QEMU uses in virtio_ccw_notify() the queue index (passed as
    "vector") to select the relevant indicator bit. If a queue does not exist,
    it does not have a corresponding indicator bit assigned, because it
    effectively doesn't have a queue index.
    
    Using a virtio-balloon-ccw device under QEMU with free-page-hinting
    disabled ("free-page-hint=off") but free-page-reporting enabled
    ("free-page-reporting=on") will result in free page reporting
    not working as expected: in the virtio_balloon driver, we'll be stuck
    forever in virtballoon_free_page_report()->wait_event(), because the
    waitqueue will not be woken up as the notification from the device is
    lost: it would use the wrong indicator bit.
    
    Free page reporting stops working and we get splats (when configured to
    detect hung wqs) like:
    
     INFO: task kworker/1:3:463 blocked for more than 61 seconds.
           Not tainted 6.14.0 #4
     "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
     task:kworker/1:3 [...]
     Workqueue: events page_reporting_process
     Call Trace:
      [<000002f404e6dfb2>] __schedule+0x402/0x1640
      [<000002f404e6f22e>] schedule+0x3e/0xe0
      [<000002f3846a88fa>] virtballoon_free_page_report+0xaa/0x110 [virtio_balloon]
      [<000002f40435c8a4>] page_reporting_process+0x2e4/0x740
      [<000002f403fd3ee2>] process_one_work+0x1c2/0x400
      [<000002f403fd4b96>] worker_thread+0x296/0x420
      [<000002f403fe10b4>] kthread+0x124/0x290
      [<000002f403f4e0dc>] __ret_from_fork+0x3c/0x60
      [<000002f404e77272>] ret_from_fork+0xa/0x38
    
    There was recently a discussion [1] whether the "holes" should be
    treated differently again, effectively assigning also non-existing
    queues a queue index: that should also fix the issue, but requires other
    workarounds to not break existing setups.
    
    Let's fix it without affecting existing setups for now by properly ignoring
    the non-existing queues, so the indicator bits will match the queue
    indexes.
    
    [1] https://lore.kernel.org/all/cover.1720611677.git.mst@redhat.com/
    
    Fixes: a229989d975e ("virtio: don't allocate vqs when names[i] = NULL")
    Reported-by: Chandra Merla <cmerla@redhat.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: David Hildenbrand <david@redhat.com>
    Tested-by: Thomas Huth <thuth@redhat.com>
    Reviewed-by: Thomas Huth <thuth@redhat.com>
    Reviewed-by: Cornelia Huck <cohuck@redhat.com>
    Acked-by: Michael S. Tsirkin <mst@redhat.com>
    Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
    Link: https://lore.kernel.org/r/20250402203621.940090-1-david@redhat.com
    Signed-off-by: Heiko Carstens <hca@linux.ibm.com>```

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

---

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