linux-kernel - Re: [PATCH 2/4] mm: introduce new VM

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Date:	Tue, 21 Oct 2014 10:11:43 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
CC:	Dave Hansen <dave.hansen@...el.com>,
	Dominik Dingel <dingel@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Bob Liu <lliubbo@...il.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Gleb Natapov <gleb@...nel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Jianyu Zhan <nasa4836@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Konstantin Weitz <konstantin.weitz@...il.com>,
	kvm@...r.kernel.org, linux390@...ibm.com,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH 2/4] mm: introduce new VM_NOZEROPAGE flag



On 10/21/2014 08:11 AM, Martin Schwidefsky wrote:
>> I agree with Dave (I thought I disagreed, but I changed my mind while
>> writing down my thoughts).  Just define mm_forbids_zeropage in
>> arch/s390/include/asm, and make it return mm->context.use_skey---with a
>> comment explaining how this is only for processes that use KVM, and then
>> only for guests that use storage keys.
>
> The mm_forbids_zeropage() sure will work for now, but I think a vma flag
> is the better solution. This is analog to VM_MERGEABLE or VM_NOHUGEPAGE,
> the best solution would be to only mark those vmas that are mapped to
> the guest. That we have not found a way to do that yet in a sensible way
> does not change the fact that "no-zero-page" is a per-vma property, no?

I agree it should be per-VMA.  However, right now the code is 
complicated unnecessarily by making it a per-VMA flag.  Also, setting 
the flag per VMA should probably be done in 
kvm_arch_prepare_memory_region together with some kind of storage key 
notifier.  This is not very much like Dominik's patch.  All in all, 
mm_forbids_zeropage() provides a non-intrusive and non-controversial way 
to fix the bug.  Later on, switching to vma_forbids_zeropage() will be 
trivial as far as mm/ code is concerned.

> But if you insist we go with the mm_forbids_zeropage() until we find a
> clever way to distinguish the guest vmas from the qemu ones.

Yeah, I think it is simpler for now.

Paolo
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