Re: [Qemu-devel] [PATCH 1/1] balloon: add a feature bit to let Guest OS deflate balloon on oom

["Denis V. Lunev" <den@openvz.org>]

On 10/06/15 16:13, Michael S. Tsirkin wrote:
> On Wed, Jun 10, 2015 at 03:02:21PM +0300, Denis V. Lunev wrote:
>> On 09/06/15 13:37, Christian Borntraeger wrote:
>>> Am 09.06.2015 um 12:19 schrieb Denis V. Lunev:
>>>> Excessive virtio_balloon inflation can cause invocation of OOM-killer,
>>>> when Linux is under severe memory pressure. Various mechanisms are
>>>> responsible for correct virtio_balloon memory management. Nevertheless it
>>>> is often the case that these control tools does not have enough time to
>>>> react on fast changing memory load. As a result OS runs out of memory and
>>>> invokes OOM-killer. The balancing of memory by use of the virtio balloon
>>>> should not cause the termination of processes while there are pages in the
>>>> balloon. Now there is no way for virtio balloon driver to free memory at
>>>> the last moment before some process get killed by OOM-killer.
>>>>
>>>> This does not provide a security breach as balloon itself is running
>>>> inside Guest OS and is working in the cooperation with the host. Thus
>>>> some improvements from Guest side should be considered as normal.
>>>>
>>>> To solve the problem, introduce a virtio_balloon callback which is
>>>> expected to be called from the oom notifier call chain in out_of_memory()
>>>> function. If virtio balloon could release some memory, it will make the
>>>> system return and retry the allocation that forced the out of memory
>>>> killer to run.
>>>>
>>>> This behavior should be enabled if and only if appropriate feature bit
>>>> is set on the device. It is off by default.
>>> The balloon frees pages in this way
>>>
>>> static void balloon_page(void *addr, int deflate)
>>> {
>>> #if defined(__linux__)
>>>      if (!kvm_enabled() || kvm_has_sync_mmu())
>>>          qemu_madvise(addr, TARGET_PAGE_SIZE,
>>>                  deflate ? QEMU_MADV_WILLNEED : QEMU_MADV_DONTNEED);
>>> #endif
>>> }
>>>
>>> The guest can re-touch that page and get a empty zero or the old page back without
>>> tampering the host integrity. This should work for all cases I am aware of (without sync_mmu its a nop anyway) so why not enable that by default? Anything that I missed?
>>>
>>> Christian
>> I'd like to do that :) Actually original version of kernel patch
>> has enabled this unconditionally. But Michael asked to make
>> it configurable and off by default.
>>
>> Den
> That's not the question here.  The question is why is it limited by kvm_has_sync_mmu.
>
original comment about this is quite simple

"    Until 2.6.27, KVM forced memory pinning so we must disable 
ballooning unless the
     kernel actually supports it when using KVM.  It's always safe when 
using TCG."

Thus this check is a rudiment of a very-very old kernels.
Actually I do not know whether current QEMU will start
on such kernels :)

Den