Linux does not use more than the startup RAM under Hyper-V with dynamic memory enabled

[Brian Wong <draconicpenguin1@yahoo.com>]

I'm new to LKML, so please don't be too hard on me :)

I'm running Gentoo Linux under Microsoft Client Hyper-V on Windows 8.1 
Pro, and I've noticed some odd behavior with respect to dynamic memory 
(aka memory ballooning). The system will never use more than the startup 
memory defined in the vitual machine's settings.

For example, if I set the startup memory to 512 MB, and enable dynamic 
memory with a minimum of 512 MB and a maximum of 8192 MB, the system 
will never allocate than 512 MB of physical memory, despite Hyper-V 
assigning more memory to the VM and the added memory being visible in 
the output of "free" and "htop". Attempting to use more memory causes 
the system to start paging to swap, rather than actually allocating the 
memory above the startup memory assigned to the VM.

The kernel is built with the full set of Hyper-V drivers, including the 
key "Microsoft Hyper-V Balloon Driver" as well as memory hot-add and 
hot-remove functionality. This is happening with both the Gentoo-patched 
3.10.32 kernel and the vanilla 3.12.5 kernel. The host machine has a 
total of 24 GB of memory.

For now, I am working around the issue by starting the VM with the 
startup memory set to the maximum and letting Hyper-V take the usused 
memory back when it is not in use. The VM will then get the extra memory 
when it needs it.

Have I encountered a bug in the Hyper-V balloon driver?

-- 
Brian Wong
http://www.fierydragonlord.com