linux-kernel - Re: [PATCH v3 0/4] mm: new function to forbid zeropage mappings for a process

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Message-ID: <5447B84C.4000909@redhat.com>
Date:	Wed, 22 Oct 2014 15:59:40 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	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>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>
CC:	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>,
	kvm@...r.kernel.org, linux390@...ibm.com,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH v3 0/4] mm: new function to forbid zeropage mappings for
 a process



On 10/22/2014 01:09 PM, Dominik Dingel wrote:
> s390 has the special notion of storage keys which are some sort of page flags
> associated with physical pages and live outside of direct addressable memory.
> These storage keys can be queried and changed with a special set of instructions.
> The mentioned instructions behave quite nicely under virtualization, if there is: 
> - an invalid pte, then the instructions will work on memory in the host page table
> - a valid pte, then the instructions will work with the real storage key
> 
> Thanks to Martin with his software reference and dirty bit tracking,
> the kernel does not issue any storage key instructions as now a 
> software based approach will be taken, on the other hand distributions 
> in the wild are currently using them.
> 
> However, for virtualized guests we still have a problem with guest pages 
> mapped to zero pages and the kernel same page merging.  
> With each one multiple guest pages will point to the same physical page
> and share the same storage key.
> 
> Let's fix this by introducing a new function which s390 will define to
> forbid new zero page mappings.  If the guest issues a storage key related 
> instruction we flag the mm_struct, drop existing zero page mappings
> and unmerge the guest memory.
> 
> v2 -> v3:
>  - Clearing up patch description Patch 3/4
>  - removing unnecessary flag in mmu_context (Paolo)

... and zero the mm_use_skey flag correctly, too. :)

> v1 -> v2: 
>  - Following Dave and Paolo suggestion removing the vma flag

Thanks, the patches look good.  I expect that they will either go in
through the s390 tree, or come in via Christian.

If the latter, Martin, please reply with your Acked-by.

Paolo
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