Date: Fri, 16 Nov 2012 12:10:39 -0800 (PST)
From: David Rientjes <rientjes@google.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
cc: Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>, linux-mm@kvack.org,
        Andi Kleen <ak@linux.intel.com>,
        "H. Peter Anvin" <hpa@linux.intel.com>, linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH v5 05/11] thp: change_huge_pmd(): keep huge zero page
 write-protected
In-Reply-To: <20121116181321.GA18313@otc-wbsnb-06>
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References: <1352300463-12627-1-git-send-email-kirill.shutemov@linux.intel.com> <1352300463-12627-6-git-send-email-kirill.shutemov@linux.intel.com> <alpine.DEB.2.00.1211141512400.22537@chino.kir.corp.google.com> <20121115084635.GC9676@otc-wbsnb-06>
 <alpine.DEB.2.00.1211151344100.27188@chino.kir.corp.google.com> <20121116181321.GA18313@otc-wbsnb-06>
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On Fri, 16 Nov 2012, Kirill A. Shutemov wrote:

> > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > > index d767a7c..05490b3 100644
> > > > > --- a/mm/huge_memory.c
> > > > > +++ b/mm/huge_memory.c
> > > > > @@ -1259,6 +1259,8 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> > > > >  		pmd_t entry;
> > > > >  		entry = pmdp_get_and_clear(mm, addr, pmd);
> > > > >  		entry = pmd_modify(entry, newprot);
> > > > > +		if (is_huge_zero_pmd(entry))
> > > > > +			entry = pmd_wrprotect(entry);
> > > > >  		set_pmd_at(mm, addr, pmd, entry);
> > > > >  		spin_unlock(&vma->vm_mm->page_table_lock);
> > > > >  		ret = 1;
> > > > 
> > > > Nack, this should be handled in pmd_modify().
> > > 
> > > I disagree. It means we will have to enable hzp per arch. Bad idea.
> > > 
> > 
> > pmd_modify() only exists for those architectures with thp support already, 
> > so you've already implicitly enabled for all the necessary architectures 
> > with your patchset.
> 
> Now we have huge zero page fully implemented inside mm/huge_memory.c. Push
> this logic to pmd_modify() means we expose hzp implementation details to
> arch code. Looks ugly for me.
> 

So you are suggesting that anybody who ever does pmd_modify() in the 
future is responsible for knowing about the zero page and to protect 
against giving it write permission in the calling code??
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