blkfront: add uevent for size change

[Mark Thomas]

A Xen blkfront(xen-blkfront:) patch has been submitted upstream, regarding the resizing of a blkfront device from dom0. This patch would emit a KOBJ_CHANGE uevent, to notify a guest of the change. This allows for custom udev rules, such
as automatically resizing a filesystem, when an event occurs.

We are requesting that this patch be cherry-picked/backported to the supported Ubuntu kernels.

Reference: https://patchwork.kernel.org/patch/9676017/
Reference: https://lkml.org/lkml/2017/4/11/736

[Kamal Mostafa (kamalmostafa)]

linux-aws test kernel with the patch applied: http://kernel.ubuntu.com/~kamal/lp1683878/

[Mark Thomas (markthomas)]

I performed this test in EC2:

- Launched an instance of latest Xenial AMI in us-west-1 (ami-2afbde4a, kernel 4.4.0-1013-aws) with 8GB root EBS volume
- Created a second EBS volume, 100GB, and attached as /dev/sdf.
- Ran sudo udevadm monitor --kernel --udev and resized both volumes. No udev events were registered.

I repeated the test with the test kernel in comment #1 (4.4.0-1016-aws). Upon resizing both volumes, this output was captured:
KERNEL[100.355617] change /devices/vbd-768/block/xvda (block)
UDEV [100.373979] change /devices/vbd-768/block/xvda (block)
KERNEL[127.255821] change /devices/vbd-51792/block/xvdf (block)
UDEV [127.269313] change /devices/vbd-51792/block/xvdf (block)

The volume modifications were causing udev events to be emitted.

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux-aws - 4.4.0-1016.25

---------------
linux-aws (4.4.0-1016.25) xenial; urgency=low

  * linux-aws: 4.4.0-1016.25 -proposed tracker (LP: #1684458)

  * blkfront: add uevent for size change (LP: #1683878)
    - SAUCE: blkfront: add uevent for size change

  [ Ubuntu: 4.4.0-75.96 ]

  * linux: 4.4.0-75.96 -proposed tracker (LP: #1684441)
  * [Hyper-V] hv: util: move waiting for release to hv_utils_transport itself
    (LP: #1682561)
    - Drivers: hv: util: move waiting for release to hv_utils_transport itself

linux-aws (4.4.0-1015.24) xenial; urgency=low

  * linux-aws: 4.4.0-1015.24 -proposed tracker (LP: #1682047)

  [ Ubuntu: 4.4.0-74.95 ]

  * linux: 4.4.0-74.95 -proposed tracker (LP: #1682041)
  * [Hyper-V] hv: vmbus: Raise retry/wait limits in vmbus_post_msg()
    (LP: #1681893)
    - Drivers: hv: vmbus: Raise retry/wait limits in vmbus_post_msg()

linux-aws (4.4.0-1014.23) xenial; urgency=low

  * linux-aws: 4.4.0-1014.23 -proposed tracker (LP: #1680424)

  [ Ubuntu: 4.4.0-73.94 ]

  * linux: 4.4.0-73.94 -proposed tracker (LP: #1680416)
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