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

Yu-cheng Yu
2019-08-28T14:57:41Z

On Wed, 2019-08-28 at 09:03 +0200, Peter Zijlstra wrote:
> On Tue, Aug 27, 2019 at 03:37:12PM -0700, Yu-cheng Yu wrote:
> > 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.
>
> Well, then the name 'move' is just wrong, because that is not the
> semantics you're looking for.
>
> So the thing is; if you provide a generic function that 'munges' two
> bits, then it's name had better be accurate. But AFAICT you only ever
> used this for the DIRTY bits, so it might be better to have a function
> specifically for that and with a comment that spells out the exact
> semantics and reasons for them.

Yes, I will work on that.

Yu-cheng