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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

-- 
MST