[Virtual-pkg-base-maintainers] Bug#825212: Ballooned memory is visible via proc

[Anna Melekhova (Vorobyova)]

Package: base
Version: Debian 7, Debian 8

There is QEMU/KVM and a Debian Linux running inside the guest. The amount
of memory available for guest could be adjusted by balloon for better
host scalability. The problem that this change is visible for end-user
actually using the guest. This could (potentially) result in lawsuite
from the end-user to hosting provides. 

Though there is a problem in this setup. The end-user and hosting provider
have signed SLA agreement in which some amount of memory is guaranted for
the guest. The good thing is that this memory will be given to the guest
when the guest will really need it (f.e. with OOM in guest and with
VIRTIO_BALLOON_F_DEFLATE_ON_OOM configuration flag set). The bad thing
is that end-user does not know this.

Balloon by default reduce the amount of memory exposed to the end-user
each time when the page is stolen from guest or returned back by using
adjust_managed_page_count and thus /proc/meminfo shows reduced amount
of memory.

Steps to Reproduce:
1. inflate guest balloon
2. amount of memory in /proc/meminfo in reduced in guest

Expected results:
no changes in /proc/meminfo in guest if properly configured in host

The problem is addressed in mainstream Linux with the following patchset:

commit 997e120843e82609c8d99a9d5714e6cf91e14cbe
Author: Denis V. Lunev <den at openvz.org>
Date:   Thu Aug 20 00:49:49 2015 +0300
virtio_balloon: do not change memory amount visible via /proc/meminfo

   Balloon device is frequently used as a mean of cooperative memory control
   in between guest and host to manage memory overcommitment. This is the
   typical case for any hosting workload when KVM guest is provided for
   end-user.

   Though there is a problem in this setup. The end-user and hosting provider
   have signed SLA agreement in which some amount of memory is guaranted for
   the guest. The good thing is that this memory will be given to the guest
   when the guest will really need it (f.e. with OOM in guest and with
   VIRTIO_BALLOON_F_DEFLATE_ON_OOM configuration flag set). The bad thing
   is that end-user does not know this.

   Balloon by default reduce the amount of memory exposed to the end-user
   each time when the page is stolen from guest or returned back by using
   adjust_managed_page_count and thus /proc/meminfo shows reduced amount
   of memory.

   Fortunately the solution is simple, we should just avoid to call
   adjust_managed_page_count with VIRTIO_BALLOON_F_DEFLATE_ON_OOM set.

   Signed-off-by: Denis V. Lunev <den at openvz.org>
   CC: Michael S. Tsirkin <mst at redhat.com>
   Signed-off-by: Michael S. Tsirkin <mst at redhat.com>


commit b4d34037329f46ed818d3b0a6e1e23b9c8721f79
Author: Denis V. Lunev <den at openvz.org>
Date:   Thu Aug 20 00:49:48 2015 +0300
virtio_ballon: change stub of release_pages_by_pfn

   and rename it to release_pages_balloon. The function originally takes
   arrays of pfns and now it takes pointer to struct virtio_ballon.
   This change is necessary to conditionally call adjust_managed_page_count
   in the next patch.

   Signed-off-by: Denis V. Lunev <den at openvz.org>
   CC: Michael S. Tsirkin <mst at redhat.com>
   Signed-off-by: Michael S. Tsirkin <mst at redhat.com>