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[Bug 1584597] Re: Hyper-V Memory Ballooning re-broken in 16.04


@l-ububtu-3 What was the procedure on the reinstall?

Based on my results, I will add some more checks to our long-term
testing efforts to see if ballooning messages are being dropped.

An explanation of Hyper-V dynamic memory in general is here:

For the Linux driver the process is relatively straightforward. When
there's less memory pressure than what is allocated, the guest balloon
driver allocates buffers to itself and releases that memory to Hyper-V.
As a result, the Linux guest will think the memory is there but in use,
but Hyper-V will report a lower assigned memory. When there is more
pressure than what is available, free, first the balloon driver will
request memory and release the buffers it had allocated when they are
assigned. If there are no more buffers available, Hyper-V will assign
more memory and the driver will "hot add" that memory to the guest. If
there is no pressure, nothing will change.

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  Hyper-V Memory Ballooning re-broken in 16.04

Status in linux package in Ubuntu:
Status in linux source package in Xenial:

Bug description:
  Regression: #1294283 fixed memory ballooning in Hyper-V in 14.04, but
  the bug has returned in 16.04.

  Steps to reproduce:

  1. Create Gen2 Hyper-V VM
  2. Install Ubuntu 16.04
  3. modprobe hv_balloon
  Memory usage according to Hyper-V Manager will not change

  This has been reproduced on 2 fresh Ubuntu Server 16.04 instances.

  I installed linux-generic-xenial on a 14.04 instance, rebooted,
  modprobe'd hv_balloon and memory usage in Hyper-V Manager reduced to
  the correct level.

  Host: Windows NanoServer 2016

  Ubuntu Server 16.04 version_signature: `Ubuntu 4.4.0-21.37-generic 4.4.6`
  Ubuntu Server 14.04 version_signature: `Ubuntu 4.4.0-22.40~14.04.1-generic 4.4.8`

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