Re: [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY_SW

Yu-cheng Yu
2019-08-27T22:46:32Z

On Fri, 2019-08-23 at 16:02 +0200, Peter Zijlstra wrote:
> On Tue, Aug 13, 2019 at 01:52:09PM -0700, Yu-cheng Yu wrote:
>
> > +static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
> > +{
> > + if (pte_flags(pte) & from)
> > + pte = pte_set_flags(pte_clear_flags(pte, from), to);
> > + return pte;
> > +}
>
> Aside of the whole conditional thing (I agree it would be better to have
> this unconditionally); the function doesn't really do as advertised.
>
> That is, if @from is clear, it doesn't endeavour to make sure @to is
> also clear.
>
> Now it might be sufficient, but in that case it really needs a comment
> and or different name.
>
> An implementation that actually moves the bit is something like:
>
> pteval_t a,b;
>
> a = native_pte_value(pte);
> b = (a >> from_bit) & 1;
> a &= ~((1ULL << from_bit) | (1ULL << to_bit));
> a |= b << to_bit;
> return make_native_pte(a);

There can be places calling pte_wrprotect() on a PTE that is already RO +
DIRTY_SW. Then in pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW) we do not
want to clear _PAGE_DIRTY_SW. But, I will look into this and make it more
obvious.

Thanks,
Yu-cheng