# Re: \[RFC PATCH v3 12/24\] x86/mm: Modify ptep\_set\_wrprotect and pmdp\_set\_wrprotect for \_PAGE\_DIRTY\_SW

- From: Yu-cheng Yu \<hidden\>
- Date: 2018-08-31 14:38:11 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2018/08/5368679
- Thread: https://ratatoskr.run/linux-doc/2018/08/5367691/t.md

```text
On Fri, 2018-08-31 at 11:53 +0200, Peter Zijlstra wrote:
> On Thu, Aug 30, 2018 at 11:47:16PM +0200, Jann Horn wrote:
> > 
> >         do {
> >                 pte = pte_wrprotect(pte);
> >                 /* note: relies on _PAGE_DIRTY_HW < _PAGE_DIRTY_SW
> > */
> >                 /* dirty direct bit-twiddling; you can probably
> > write
> > this in a nicer way */
> >                 pte.pte |= (pte.pte & _PAGE_DIRTY_HW) >>
> > _PAGE_BIT_DIRTY_HW << _PAGE_BIT_DIRTY_SW;
> >                 pte.pte &= ~_PAGE_DIRTY_HW;
> >                 pte = cmpxchg(ptep, pte, new_pte);
> >         } while (pte != new_pte);
> Please use the form:
> 
> 	pte_t new_pte, pte = READ_ONCE(*ptep);
> 	do {
> 		new_pte = /* ... */;
> 	} while (!try_cmpxchg(ptep, &pte, new_pte);
> 
> Also, this will fail to build on i386-PAE, but I suspect this code
> will
> be under some CONFIG option specific to x86_64 anyway.

Thanks!  I will work on it.

Yu-cheng
```