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

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Message-ID: <20141021081131.641c6104@mschwide>
Date:	Tue, 21 Oct 2014 08:11:31 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.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 Mon, 20 Oct 2014 20:14:53 +0200
Paolo Bonzini <pbonzini@...hat.com> wrote:

> 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.

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?

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.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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