Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW

[Peter Zijlstra]

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.