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From: David Hildenbrand <>
To: Claudio Imbrenda <>,
	"" <>,
	Michal Hocko <>
Subject: Re: [PATCH v3 00/14] KVM: s390: pv: implement lazy destroy
Date: Fri, 6 Aug 2021 09:10:28 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 04.08.21 17:40, Claudio Imbrenda 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.

That doesn't sound too hacky to me, and actually sounds like a good 
idea, doing what the guest would do either way but speeding it up 
asynchronously, but ...

> 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.

... this ...

> 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.

... this ...
> When a guest is destroyed, its memory still counts towards its memory
> control group until it's actually freed (I tested this experimentally)
> When the system runs out of memory, if a guest has terminated and its
> memory is being cleaned asynchronously, the OOM killer will wait a
> little and then see if memory has been freed. This has the practical
> effect of slowing down memory allocations when the system is out of
> memory to give the cleanup thread time to cleanup and free memory, and
> avoid an actual OOM situation.

... and this sound like the kind of arch MM hacks that will bite us in 
the long run. Of course, I might be wrong, but already doing excessive 
GFP_ATOMIC allocations or messing with the OOM killer that way for a 
pure (shutdown) optimization is an alarm signal. Of course, I might be 

You should at least CC linux-mm. I'll do that right now and also CC 
Michal. He might have time to have a quick glimpse at patch #11 and #13.

IMHO, we should proceed with patch 1-10, as they solve a really 
important problem ("slow reboots") in a nice way, whereby patch 11 
handles a case that can be worked around comparatively easily by 
management tools -- my 2 cents.


David / dhildenb

  parent reply	other threads:[~2021-08-06  7:10 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-04 15:40 Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 01/14] KVM: s390: pv: add macros for UVC CC values Claudio Imbrenda
2021-08-06  7:26   ` David Hildenbrand
2021-08-06  9:34     ` Claudio Imbrenda
2021-08-06 15:15     ` Janosch Frank
2021-08-04 15:40 ` [PATCH v3 02/14] KVM: s390: pv: avoid stall notifications for some UVCs Claudio Imbrenda
2021-08-06  7:30   ` David Hildenbrand
2021-08-06  9:33     ` Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 03/14] KVM: s390: pv: leak the ASCE page when destroy fails Claudio Imbrenda
2021-08-06  7:31   ` David Hildenbrand
2021-08-06  9:32     ` Claudio Imbrenda
2021-08-06 11:39       ` David Hildenbrand
2021-08-04 15:40 ` [PATCH v3 04/14] KVM: s390: pv: properly handle page flags for protected guests Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 05/14] KVM: s390: pv: handle secure storage violations " Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 06/14] KVM: s390: pv: handle secure storage exceptions for normal guests Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 07/14] KVM: s390: pv: refactor s390_reset_acc Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 08/14] KVM: s390: pv: usage counter instead of flag Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 09/14] KVM: s390: pv: add export before import Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 10/14] KVM: s390: pv: lazy destroy for reboot Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 11/14] KVM: s390: pv: extend lazy destroy to handle shutdown Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 12/14] KVM: s390: pv: module parameter to fence lazy destroy Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 13/14] KVM: s390: pv: add OOM notifier for " Claudio Imbrenda
2021-08-04 15:40 ` [PATCH v3 14/14] KVM: s390: pv: avoid export before import if possible Claudio Imbrenda
2021-08-06  7:10 ` David Hildenbrand [this message]
2021-08-06  9:30   ` [PATCH v3 00/14] KVM: s390: pv: implement lazy destroy Claudio Imbrenda
2021-08-06 11:30     ` David Hildenbrand
2021-08-06 13:44       ` Claudio Imbrenda
2021-08-09  8:50         ` David Hildenbrand

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