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Date: Tue, 21 Aug 2012 21:12:09 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
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To: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
cc: Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, Avi Kivity <avi@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, KVM <kvm@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [PATCH] mm: mmu_notifier: fix inconsistent memory between
 secondary MMU and host
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On Wed, 22 Aug 2012, Xiao Guangrong wrote:
> On 08/21/2012 11:06 PM, Andrea Arcangeli wrote:
> > 
> > The KSM usage of it looks safe because it will only establish readonly
> > ptes with it.
> 
> Hmm, in KSM code, i found this code in replace_page:
> 
> set_pte_at_notify(mm, addr, ptep, mk_pte(kpage, vma->vm_page_prot));
> 
> It is possible to establish a writable pte, no?

No: we only do KSM in private vmas (!VM_SHARED), and because of the
need to CopyOnWrite in those, vm_page_prot excludes write permission:
write permission has to be added on COW fault.

Hugh