linux-kernel - Re: [PATCH v5 05/11] thp: change_huge_pmd(): keep huge zero page write-protected

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Date:	Fri, 16 Nov 2012 20:13:21 +0200
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v5 05/11] thp: change_huge_pmd(): keep huge zero page
 write-protected

On Thu, Nov 15, 2012 at 01:47:33PM -0800, David Rientjes wrote:
> On Thu, 15 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.

> > What's wrong with the check?
> > 
> 
> Anybody using pmd_modify() to set new protections in the future perhaps 
> without knowledge of huge zero page can incorrectly make the huge zero 
> page writable, which can never be allowed to happen.  It's better to make 
> sure it can never happen with the usual interface to modify protections.

I haven't found where we check if the page is a 4k zero page, but it's
definitely not pte_modify().

-- 
 Kirill A. Shutemov

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