Re: [RFC PATCH 0/4] KVM: x86/mmu: Rework marking folios dirty/accessed

Sean Christopherson
2024-04-04T22:02:52Z

On Thu, Apr 04, 2024, David Hildenbrand wrote:
> On 04.04.24 19:31, Sean Christopherson wrote:
> > On Thu, Apr 04, 2024, David Hildenbrand wrote:
> > > On 04.04.24 00:19, Sean Christopherson wrote:
> > > > Hmm, we essentially already have an mmu_notifier today, since secondary MMUs need
> > > > to be invalidated before consuming dirty status. Isn't the end result essentially
> > > > a sane FOLL_TOUCH?
> > >
> > > Likely. As stated in my first mail, FOLL_TOUCH is a bit of a mess right now.
> > >
> > > Having something that makes sure the writable PTE/PMD is dirty (or
> > > alternatively sets it dirty), paired with MMU notifiers notifying on any
> > > mkclean would be one option that would leave handling how to handle dirtying
> > > of folios completely to the core. It would behave just like a CPU writing to
> > > the page table, which would set the pte dirty.
> > >
> > > Of course, if frequent clearing of the dirty PTE/PMD bit would be a problem
> > > (like we discussed for the accessed bit), that would not be an option. But
> > > from what I recall, only clearing the PTE/PMD dirty bit is rather rare.
> >
> > And AFAICT, all cases already invalidate secondary MMUs anyways, so if anything
> > it would probably be a net positive, e.g. the notification could more precisely
> > say that SPTEs need to be read-only, not blasted away completely.
>
> As discussed, I think at least madvise_free_pte_range() wouldn't do that.

I'm getting a bit turned around. Are you talking about what madvise_free_pte_range()
would do in this future world, or what madvise_free_pte_range() does today? Because
today, unless I'm really misreading the code, secondary MMUs are invalidated before
the dirty bit is cleared.

mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
range.start, range.end);

lru_add_drain();
tlb_gather_mmu(&tlb, mm);
update_hiwater_rss(mm);

mmu_notifier_invalidate_range_start(&range);
tlb_start_vma(&tlb, vma);
walk_page_range(vma->vm_mm, range.start, range.end,
&madvise_free_walk_ops, &tlb);
tlb_end_vma(&tlb, vma);
mmu_notifier_invalidate_range_end(&range);

KVM (or any other secondary MMU) can re-establish mapping with W=1,D=0 in the
PTE, but the costly invalidation (zap+flush+fault) still happens.

> Notifiers would only get called later when actually zapping the folio.

And in case we're talking about a hypothetical future, I was thinking the above
could do MMU_NOTIFY_WRITE_PROTECT instead of MMU_NOTIFY_CLEAR.

> So at least for some time, you would have the PTE not dirty, but the SPTE
> writable or even dirty. So you'd have to set the page dirty when zapping the
> SPTE ... and IMHO that is what we should maybe try to avoid :)