Re: [PATCH v5 05/11] thp: change_huge_pmd(): keep huge zero pagewrite-protected

[Kirill A. Shutemov]

On Tue, Nov 20, 2012 at 06:00:40PM +0200, Kirill A. Shutemov wrote:
> On Fri, Nov 16, 2012 at 12:10:39PM -0800, David Rientjes wrote:
> > 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??
> 
> Looks like we don't need the patch at all.
> 
> IIUC, if you ask for PROT_WRITE vm_get_page_prot() will translate it to
> _PAGE_COPY or similar and you'll only get the page writable on pagefault.
> 
> Could anybody confirm that it's correct?
> 
> -- 
>  Kirill A. Shutemov

Andrew, please drop the patch or replace it with the patch below, if you
wish.

From 048e3d4c97202cfecab55ead2a816421dce4b382 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Date: Tue, 7 Aug 2012 06:09:56 -0700
Subject: [PATCH] thp: change_huge_pmd(): make sure we don't try to make a
 page writable

mprotect core never tries to make page writable using change_huge_pmd().
Let's add an assert that the assumption is true. It's important to be
sure we will not make huge zero page writable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/huge_memory.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f5589c0..5fba83b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1245,6 +1245,7 @@ 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);
+		BUG_ON(pmd_write(entry));
 		set_pmd_at(mm, addr, pmd, entry);
 		spin_unlock(&vma->vm_mm->page_table_lock);
 		ret = 1;
-- 
 Kirill A. Shutemov

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