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

[Peter Zijlstra]

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);