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

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Date:   Tue, 18 May 2021 17:05:37 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, borntraeger@...ibm.com, frankja@...ux.ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, david@...hat.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy

On Mon, 17 May 2021 22:07:47 +0200
Claudio Imbrenda <imbrenda@...ux.ibm.com> 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?

> 
> 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
'zombie' guests, and any new or rebooted guest is a new entity that
needs to get new pages?

Can too many not-yet-cleaned-up pages lead to a (temporary) memory
exhaustion?