Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy

[Christian Borntraeger]

On 18.05.21 17:36, Claudio Imbrenda wrote:
> On Tue, 18 May 2021 17:05:37 +0200
> Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
>
> > On Mon, 17 May 2021 22:07:47 +0200
> > Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx> wrote:
> >
> > > Previously, when a protected VM was rebooted or when it was shut
> > > down, its memory was made unprotected, and then the protected VM
> > > itself was destroyed. Looping over the whole address space can take
> > > some time, considering the overhead of the various Ultravisor Calls
> > > (UVCs).  This means that a reboot or a shutdown would take a
> > > potentially long amount of time, depending on the amount of used
> > > memory.
> > >
> > > This patchseries implements a deferred destroy mechanism for
> > > protected guests. When a protected guest is destroyed, its memory
> > > is cleared in background, allowing the guest to restart or
> > > terminate significantly faster than before.
> > >
> > > There are 2 possibilities when a protected VM is torn down:
> > > * it still has an address space associated (reboot case)
> > > * it does not have an address space anymore (shutdown case)
> > >
> > > For the reboot case, the reference count of the mm is increased, and
> > > then a background thread is started to clean up. Once the thread
> > > went through the whole address space, the protected VM is actually
> > > destroyed.
> > >
> > > For the shutdown case, a list of pages to be destroyed is formed
> > > when the mm is torn down. Instead of just unmapping the pages when
> > > the address space is being torn down, they are also set aside.
> > > Later when KVM cleans up the VM, a thread is started to clean up
> > > the pages from the list.
> >
> > Just to make sure, 'clean up' includes doing uv calls?
>
> yes
>
> > > This means that the same address space can have memory belonging to
> > > more than one protected guest, although only one will be running,
> > > the others will in fact not even have any CPUs.
> >
> > Are those set-aside-but-not-yet-cleaned-up pages still possibly
> > accessible in any way? I would assume that they only belong to the
>
> in case of reboot: yes, they are still in the address space of the
> guest, and can be swapped if needed
>
> > 'zombie' guests, and any new or rebooted guest is a new entity that
> > needs to get new pages?
>
> the rebooted guest (normal or secure) will re-use the same pages of the
> old guest (before or after cleanup, which is the reason of patches 3
> and 4)
>
> the KVM guest is not affected in case of reboot, so the userspace
> address space is not touched.
>
> > Can too many not-yet-cleaned-up pages lead to a (temporary) memory
> > exhaustion?
>
> in case of reboot, not much; the pages were in use are still in use
> after the reboot, and they can be swapped.
>
> in case of a shutdown, yes, because the pages are really taken aside
> and cleared/destroyed in background. they cannot be swapped. they are
> freed immediately as they are processed, to try to mitigate memory
> exhaustion scenarios.
>
> in the end, this patchseries is a tradeoff between speed and memory
> consumption. the memory needs to be cleared up at some point, and that
> requires time.
>
> in cases where this might be an issue, I introduced a new KVM flag to
> disable lazy destroy (patch 10)

Maybe we could piggy-back on the OOM-kill notifier and then fall back to
synchronous freeing for some pages?