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Date: Mon, 20 Oct 2014 20:14:53 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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To: Dave Hansen <dave.hansen@intel.com>,
        Dominik Dingel <dingel@linux.vnet.ibm.com>
CC: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@suse.cz>,
        Rik van Riel <riel@redhat.com>, Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Bob Liu <lliubbo@gmail.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Cornelia Huck <cornelia.huck@de.ibm.com>,
        Gleb Natapov <gleb@kernel.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        Hugh Dickins <hughd@google.com>, Ingo Molnar <mingo@kernel.org>,
        Jianyu Zhan <nasa4836@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Konstantin Weitz <konstantin.weitz@gmail.com>, kvm@vger.kernel.org,
        linux390@de.ibm.com, linux-kernel@vger.kernel.org,
        linux-s390@vger.kernel.org,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sasha Levin <sasha.levin@oracle.com>
Subject: Re: [PATCH 2/4] mm: introduce new VM_NOZEROPAGE flag
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On 10/18/2014 06:28 PM, Dave Hansen wrote:
> > Currently it is an all or nothing thing, but for a future change we might want to just
> > tag the guest memory instead of the complete user address space.
>
> I think it's a bad idea to reserve a flag for potential future use.  If
> you_need_  it in the future, let's have the discussion then.  For now, I
> think it should probably just be stored in the mm somewhere.

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.

Paolo (who was just taught what storage keys really are)