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Date: Sat, 18 Oct 2014 09:28:17 -0700
From: Dave Hansen <dave.hansen@intel.com>
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To: 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>,
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        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
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        Christian Borntraeger <borntraeger@de.ibm.com>,
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        Paolo Bonzini <pbonzini@redhat.com>,
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Subject: Re: [PATCH 2/4] mm: introduce new VM_NOZEROPAGE flag
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On 10/18/2014 07:49 AM, Dominik Dingel wrote:
> On Fri, 17 Oct 2014 15:04:21 -0700
> Dave Hansen <dave.hansen@intel.com> wrote:
>> Is there ever a time where the VMAs under an mm have mixed VM_NOZEROPAGE
>> status?  Reading the patches, it _looks_ like it might be an all or
>> nothing thing.
> 
> 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.

>> Full disclosure: I've got an x86-specific feature I want to steal a flag
>> for.  Maybe we should just define another VM_ARCH bit.
>>
> 
> So you think of something like:
> 
> #if defined(CONFIG_S390)
> # define VM_NOZEROPAGE	VM_ARCH_1
> #endif
> 
> #ifndef VM_NOZEROPAGE
> # define VM_NOZEROPAGE	VM_NONE
> #endif
> 
> right?

Yeah, something like that.