svn commit: r267858 - in head/sys/dev: virtio/balloon xen/balloon

[Roger Pau Monné]

On 25/06/14 18:47, Alan Cox wrote:
> On 06/25/2014 08:29, Roger Pau Monné wrote:
>> On 25/06/14 13:58, Attilio Rao wrote:
>>> On Wed, Jun 25, 2014 at 11:51 AM, Roger Pau Monné <royger at freebsd.org> wrote:
>>>> Author: royger
>>>> Date: Wed Jun 25 09:51:08 2014
>>>> New Revision: 267858
>>>> URL: http://svnweb.freebsd.org/changeset/base/267858
>>>>
>>>> Log:
>>>>   xen/virtio: fix balloon drivers to not mark pages as WIRED
>>>>
>>>>   Prevent the Xen and VirtIO balloon drivers from marking pages as
>>>>   wired. This prevents them from increasing the system wired page count,
>>>>   which can lead to mlock failing because of hitting the limit in
>>>>   vm.max_wired.
>>> This change is conceptually wrong.
>>> The pages balloon is allocating are unmanaged and they should be wired
>>> by definition. Alan and I are considering enforcing this (mandatory
>>> wired pages for unmanaged pages allocation) directly in the KPI.
>>> This in practice just seem an artifact to deal with scarce  wired
>>> memory limit. I suggest that for the XEN case this limit gets bumped
>>> rather relying on similar type of hacks.
>> IMHO, marking them as wired seems wrong too, those pages are not wired,
>> they are simply not there any more. This was discussed in:
>>
>> http://lists.freebsd.org/pipermail/freebsd-virtualization/2014-June/002643.html
>>
>> If there's consensus I will revert the change, but I would say that
>> increasing vm.max_wired for VMs is also a gross hack.
> 
> We wouldn't be changing it just for VMs.  Our current definition of
> vm_max_wired dates back to r5455, or in human years 1995.  If the ZFS
> ARC has taught us anything, we, or really the page daemon, can cope with
> having more than 1/3 of a machine's pages wired.  :-)
> 
> I have a couple questions for you.  Exactly how much memory will Xen try
> to steal from a domain via ballooning?  Clearly the domain can fail to
> respond to the balloon request and Xen has to cope with that.  Can the
> domain also explicitly say, "No, I'm not giving you any pages."

Xen itself won't try to steal any memory from a VM, this is AFAIK
initiated by the host admin (or maybe some high-level orchestration
toolstack), which requests the domain to balloon down. The domain is
never forced to balloon down, is just a petition that the VM usually
tries to satisfy as much as it can (or completely ignore it).

As said before, I'm not sure what high-level toolstacks will do, maybe
some of them will just kill the domain if it doesn't balloon down as
expected, but I would be quite surprised if that's the case.

For example I've tried ballooning down a FreeBSD VM from 2GB to 256MB,
and only got it to ~300MB, but that's perfectly fine IMHO.

Roger.