Re: [PATCH v4 RFC 0/3] virtio: add 'device_lost' to virtio_device

[Heinz Graalfs <graalfs@linux.vnet.ibm.com>]

On 23/01/14 05:51, Rusty Russell wrote:
> Heinz Graalfs <graalfs@linux.vnet.ibm.com> writes:
>> Hi, here is my v4 patch-set update to the v3 RFC submitted on Nov 27th.
>
> Hi Heinz,
>
>          I didn't get a response on my 'break all the virtqueues' patch
> series.  Could your System Z code work with this?
>
> Rusty.
>
>

Sorry Rusty, I'm back as of today.

I applied your patch series and did some testing...

Removing a disk while reading from it mostly still ends up
in hangs as of below:

PID: 13     TASK: 163f8000          CPU: 0   COMMAND: "kworker/u128:1"
  #0 [163f72e0] __schedule at 6aa22c
  #1 [163f7428] io_schedule at 6aab6c
  #2 [163f7448] sleep_on_page at 22cbb2
  #3 [163f7460] __wait_on_bit at 6ab394
  #4 [163f74b0] wait_on_page_bit at 22cef4
  #5 [163f7508] filemap_fdatawait_range at 22d0a6
  #6 [163f75e8] filemap_write_and_wait at 22de62
  #7 [163f7618] fsync_bdev at 2dc5d8
  #8 [163f7640] invalidate_partition at 407ba8
  #9 [163f7668] del_gendisk at 408a4c
#10 [163f76c0] virtblk_remove at 46f81e
#11 [163f76f8] virtio_dev_remove at 43d302
#12 [163f7730] __device_release_driver at 4604c4
#13 [163f7758] device_release_driver at 46057c
#14 [163f7780] bus_remove_device at 45ff74
#15 [163f77b0] device_del at 45cf54
#16 [163f77e8] device_unregister at 45d00e
#17 [163f7808] unregister_virtio_device at 43d5ba
#18 [163f7828] virtio_ccw_remove at 55156c
#19 [163f7850] ccw_device_remove at 4d7e22
#20 [163f78d8] __device_release_driver at 4604c4
#21 [163f7900] device_release_driver at 46057c
#22 [163f7928] bus_remove_device at 45ff74
#23 [163f7958] device_del at 45cf54
#24 [163f7990] ccw_device_unregister at 4d86a0
#25 [163f79b0] io_subchannel_remove at 4d8d1a
#26 [163f79e8] css_remove at 4d2856
#27 [163f7a08] __device_release_driver at 4604c4
#28 [163f7a30] device_release_driver at 46057c
#29 [163f7a58] bus_remove_device at 45ff74
#30 [163f7a88] device_del at 45cf54
#31 [163f7ac0] device_unregister at 45d00e
#32 [163f7ae0] css_sch_device_unregister at 4d29d4
#33 [163f7b08] io_subchannel_sch_event at 4daad6
#34 [163f7b80] css_evaluate_known_subchannel at 4d2cc0
#35 [163f7be0] slow_eval_known_fn at 4d3cea
#36 [163f7c10] bus_for_each_dev at 45ea56
#37 [163f7c50] for_each_subchannel_staged at 4d337e
#38 [163f7c98] css_slow_path_func at 4d3450
#39 [163f7cc0] process_one_work at 164ff4
#40 [163f7d60] worker_thread at 166500
#41 [163f7da8] kthread at 16e67c
#42 [163f7eb0] kernel_thread_starter at 6b0a5e

Removing a disk while writing to it now ends up mostly
with errors (which is new behavior and good).
However, the detached device is still listed under /dev, and a 
subsequent umount ends up in a hang. Latter also occurred with
my approach, sometimes.

Sometimes everything ends up in QEMU crashes, which is, however, not 
reproducible. I will investigate on this.

Heinz

>> When an active virtio block device is hot-unplugged from a KVM guest,
>> affected guest user applications are not aware of any errors that occur
>> due to the lost device. This patch-set adds code to avoid further request
>> queueing when a lost block device is detected, resulting in appropriate
>> error info. Additionally a potential hang situation can be avoided by not
>> waiting for requests (e.g. in-flight requests) in blk_cleanup_queue() that
>> will never complete.
>>
>> On System z there exists no handshake mechanism between host and guest
>> when a device is hot-unplugged. The device is removed and no further I/O
>> is possible.
>>
>> When an online channel device disappears on System z the kernel's CIO layer
>> informs the driver (virtio_ccw) about the lost device.
>>
>> Here are some more error details:
>>
>> For a particular block device virtio's request function virtblk_request()
>> is called by the block layer to queue requests to be handled by the host.
>> In case of a lost device requests can still be queued, but an appropriate
>> subsequent host kick usually fails. This leads to situations where no error
>> feedback is shown.
>>
>> In order to prevent request queueing for lost devices appropriate settings
>> in the block layer should be made. Exploiting System z's CIO notify handler
>> callback, and passing on device loss information via the surprize_removal
>> flag to the remove callback of the backend driver, can solve this task.
>>
>> v3->v4 changes:
>>   - patch 1: solves some vcdev pointer handling issues in the virtio_ccw driver
>>     (e.g. locked vcdev pointer reset/query; serialize remove()/set_offline()
>>     callback processing).
>>   - patch 2: introduces 'device_lost' atomic in virtio_device and use in
>>     backend driver virtio_blk accordingly (original 3 patches merged).
>>   - patch 3: the notify() callback is now serialized with remove()/set_offline()
>>     callbacks. The notification is ignored if the vcdev pointer has been cleared
>>     already (by remove() or set_offline()).
>>
>> v2->v3 changes:
>>   - remove virtio_driver's notify callback (and appropriate code) introduced
>>     in my v1 RFC
>>   - introduce 'surprize_removal' in struct virtio_device
>>   - change virtio_blk's remove callback to perform special actions when the
>>     surprize_removal flag is set
>>     - avoid final I/O by preventing further request queueing
>>     - avoid hangs in blk_cleanup_queue() due to waits on 'in-flight' requests
>>   - set surprize_removal in virtio_ccw's notify callback when a device is lost
>>
>> v1->v2 changes:
>>   - add include of linux/notifier.h (I also added it to the 3rd patch)
>>   - get queue lock in order to be able to use safe queue_flag_set() functions
>>     in virtblk_notify() handler
>>
>>
>> Heinz Graalfs (3):
>>    virtio_ccw: fix vcdev pointer handling issues
>>    virtio: introduce 'device_lost' flag in virtio_device
>>    virtio_ccw: set 'device_lost' on CIO_GONE notification
>>
>>   drivers/block/virtio_blk.c    | 14 ++++++++++-
>>   drivers/s390/kvm/virtio_ccw.c | 58 ++++++++++++++++++++++++++++++++++++-------
>>   include/linux/virtio.h        |  2 ++
>>   3 files changed, 64 insertions(+), 10 deletions(-)
>>
>> --
>> 1.8.3.1
>