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

[Rusty Russell <rusty@rustcorp.com.au>]

Heinz Graalfs <graalfs@linux.vnet.ibm.com> writes:
> 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:

OK, we still have the problem of in-flight requests.

I think the correct answer is to drop all requests if the virtqueue
is broken:

 -	blk_cleanup_queue(vblk->disk->queue);
 +	if (virtqueue_is_broken(vblk->vq))
 +              /* Don't wait for completion, just drop queue. */
 +              blk_abandon_queue(vblk->disk->queue);
 +      else
 +		blk_cleanup_queue(vblk->disk->queue);
 +

Unfortunately blk_abandon_queue() doesn't exist.  Your previous patch
did nothing in that path, which I suspect may leak memory.  That may be
acceptable given that this Shouldn't Happen (often).

At this point, I ask Jens :)

Cheers,
Rusty.

>
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>   #8 [163f7640] invalidate_partition at 407ba8
>   #9 [163f7668] del_gendisk at 408a4c
> #10 [163f76c0] virtblk_remove at 46f81e
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> #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
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> #35 [163f7be0] slow_eval_known_fn at 4d3cea
> #36 [163f7c10] bus_for_each_dev at 45ea56
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> #38 [163f7c98] css_slow_path_func at 4d3450
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> #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
>>