linux-kernel - Re: [PATCH v3 0/2] s390/mm: shared zeropage + KVM fixes

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Message-ID: <3dbf706f-b6ca-45d7-a293-12aa5807fcdf@redhat.com>
Date: Thu, 11 Apr 2024 23:56:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>, Heiko Carstens
 <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Peter Xu <peterx@...hat.com>, Alexander Gordeev <agordeev@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
 linux-s390@...r.kernel.org
Subject: Re: [PATCH v3 0/2] s390/mm: shared zeropage + KVM fixes

On 11.04.24 23:28, Andrew Morton wrote:
> On Thu, 11 Apr 2024 18:14:39 +0200 David Hildenbrand <david@...hat.com> wrote:
> 
>> This series fixes one issue with uffd + shared zeropages on s390x and
>> fixes that "ordinary" KVM guests can make use of shared zeropages again.
>>
>> ...
>>
>> Without the shared zeropage, during (2), the VM would suddenly consume
>> 100 GiB on the migration source and destination. On the migration source,
>> where we don't excpect memory overcommit, we could easilt end up crashing
>> the VM during migration.
>>
>> Independent of that, memory handed back to the hypervisor using "free page
>> reporting" would end up consuming actual memory after the migration on the
>> destination, not getting freed up until reused+freed again.
>>
> 
> Is a backport desirable?
> 
> If so, the [1/2] Fixes dates back to 2015 and the [2/2] Fixes is from
> 2017.  Is it appropriate that the patches be backported so far back,
> and into different kernel versions?
> 

[2/2] won't be easy to backport to kernels without FAULT_FLAG_UNSHARE, 
so I wouldn't really suggest backports to kernels before that. [1/2] 
might be reasonable to backport, but might require some tweaking (page 
vs. folio).

-- 
Cheers,

David / dhildenb