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

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Date:	Sat, 18 Oct 2014 09:28:17 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Dominik Dingel <dingel@...ux.vnet.ibm.com>
CC:	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,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH 2/4] mm: introduce new VM_NOZEROPAGE flag

On 10/18/2014 07:49 AM, Dominik Dingel wrote:
> On Fri, 17 Oct 2014 15:04:21 -0700
> Dave Hansen <dave.hansen@...el.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.

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