linux-kernel - Re: [PATCH 3/4] s390/mm: prevent and break zero page mappings in case of storage keys

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Date:	Wed, 22 Oct 2014 12:32:08 +0200
From:	Dominik Dingel <dingel@...ux.vnet.ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	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>,
	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>
Subject: Re: [PATCH 3/4] s390/mm: prevent and break zero page mappings in
 case of storage keys

On Wed, 22 Oct 2014 12:09:31 +0200
Paolo Bonzini <pbonzini@...hat.com> wrote:

> On 10/22/2014 10:30 AM, Dominik Dingel wrote:
> > As use_skey is already the condition on which we call s390_enable_skey
> > we need to introduce a new flag for the mm->context on which we decide
> > if zero page mapping is allowed.
> 
> Can you explain better why "mm->context.use_skey = 1" cannot be done
> before the walk_page_range?  Where does the walk or __s390_enable_skey
> or (after the next patch) ksm_madvise rely on
> "mm->context.forbids_zeropage && !mm->context.use_skey"?

I can't, my reasoning there is wrong.
I remembered incorrectly that we use mm_use_skey in arch/s390/kvm/priv.c to
check if we need to call s390_enable_skey, but that does happen
with the interception bits.

So every vCPU which get the a interception for a storage key instruction
will call s390_enable_skey and wait there for the mmap_sem.

> The only reason I can think of, is that the next patch does not reset
> "mm->context.forbids_zeropage" to 0 if the ksm_madvise fails.  Why
> doesn't it do that---or is it a bug?

You are right, this is a bug, where we will drop to userspace with -ENOMEM.

I will fix this as well. 


> Thanks, and sorry for the flurry of questions! :)

I really appreciate your questions and remarks. Thank you!

> Paolo
> 

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