Messages in this thread |  | | Date | Fri, 5 Apr 2024 08:53:49 +0200 | | Subject | Re: [RFC PATCH 0/4] KVM: x86/mmu: Rework marking folios dirty/accessed | | From | David Hildenbrand <> |
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On 05.04.24 00:02, Sean Christopherson wrote: > 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.
Likely I missed it, sorry! I was talking about the possible future. :)
> > 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); >
Indeed, we do setup the MMU notifier invalidation. We do the start/end .. I was looking for PTE notifications.
I spotted the set_pte_at(), not a set_pte_at_notify() like we do in other code. Maybe that's not required here (digging through documentation I'm still left clueless).
" set_pte_at_notify() sets the pte _after_ running the notifier. This is safe to start by updating the secondary MMUs, because the primary MMU pte invalidate must have already happened with a ptep_clear_flush() before set_pte_at_notify() has been invoked. ... "
Absolutely unclear to me when we *must* to use it, or if it is. Likely its a pure optimization when we're *changing* a PTE.
KSM and wp_page_copy() uses it with MMU_NOTIFY_CLEAR, when replacing the mapped page by another page (or write-protecting an existing one). __replace_page() uses it with __replace_page() when replacing the mapped page. And migrate_vma_insert_page() uses it with MMU_NOTIFY_MIGRATE.
Other code (e.g., khugepaged) doesn't seem to use it as well.
.. so I guess it's fine? Confusing.
-- Cheers,
David / dhildenb
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