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Subject: Re: [PATCH 1/6] mm: tracking shared dirty pages
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Andrew Morton <akpm@osdl.org>
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Date: Thu, 22 Jun 2006 13:33:51 +0200
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On Wed, 2006-06-21 at 22:56 -0700, Andrew Morton wrote:
> On Mon, 19 Jun 2006 19:52:53 +0200

> > +		vma->vm_page_prot =
> > +			__pgprot(pte_val
> > +				(pte_wrprotect
> > +				 (__pte(pgprot_val(vma->vm_page_prot)))));
> > +
> 
> Is there really no simpler way?

	pgprot_t prot_shared = protection_map[vm_flags & 
		(VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];
	pgprot_t prot_priv = protection_map[vm_flags & 
		(VM_READ|VM_WRITE|VM_EXEC)];

	typeof(pgprot_val(prot_shared)) mask = 
		~(pgprot_val(prot_shared) ^ pgprot_val(prot_priv));

	pgprot_val(vma->vm_page_prot) &= mask;
	pgprot_val(vma->vm_page_prot) |= 
		(pgprot_val(prot_priv) & mask);

its more readable, but barely so.

BTW, is there a difference between:
  (VM_READ|VM_WRITE|VM_EXEC)
and
  (VM_READ|VM_EXEC|VM_SHARED)
in this context?

Or I can make it a generic arch specific function and override for i386
and x86-64. That way I can also cleanup drivers/char/drm/drm_vm.c where
I found this thing.

include/asm-generic/pgtable.h

#ifndef __HAVE_ARCH_PGPROT_WRPROTECT
#define pgprot_wrprotect(prot) \
({ (prot) = __pgprot(pte_val \
		(pte_wrprotect	\
		(__pte(pgprot_val(prot))))) \
})
#endif

include/asm-{i386,x86-64}/pgtable.h

#define pgprot_wrprotect(prot) ({ pgprot_val(prot) &= ~_PAGE_RW; })
#define __HAVE_ARCH_PGPROT_WRPROTECT

I can go through some other archs and see what I can do.

Hmm, now that I look at this, might give a include-dependency problem.
Awell, thoughts, comments?