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

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Date:   Tue, 18 May 2021 17:52:25 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org,
        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 Tue, 18 May 2021 17:45:18 +0200
Christian Borntraeger <borntraeger@...ibm.com> wrote:

> On 18.05.21 17:36, Claudio Imbrenda wrote:
> > On Tue, 18 May 2021 17:05:37 +0200
> > Cornelia Huck <cohuck@...hat.com> wrote:

> >> 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?

Sounds like a good idea. If delayed cleanup is safe, you probably want
to have the fast shutdown behaviour.