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

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:

quoted

+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