Re: [PATCH v6 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

[Lance Yang]

On Wed, Jun 5, 2024 at 10:39 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 05.06.24 16:28, David Hildenbrand wrote:
> > On 05.06.24 16:20, Lance Yang wrote:
> >> Hi David,
> >>
> >> On Wed, Jun 5, 2024 at 8:46 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>>
> >>> On 21.05.24 06:02, Lance Yang wrote:
> >>>> In preparation for supporting try_to_unmap_one() to unmap PMD-mapped
> >>>> folios, start the pagewalk first, then call split_huge_pmd_address() to
> >>>> split the folio.
> >>>>
> >>>> Since TTU_SPLIT_HUGE_PMD will no longer perform immediately, we might
> >>>> encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range during
> >>>> the page walk. It’s probably necessary to mlock this THP to prevent it from
> >>>> being picked up during page reclaim.
> >>>>
> >>>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> >>>> Suggested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> >>>> Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> >>>> ---
> >>>
> >>> [...] again, sorry for the late review.
> >>
> >> No worries at all, thanks for taking time to review!
> >>
> >>>
> >>>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>>> index ddffa30c79fb..08a93347f283 100644
> >>>> --- a/mm/rmap.c
> >>>> +++ b/mm/rmap.c
> >>>> @@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >>>>         if (flags & TTU_SYNC)
> >>>>                 pvmw.flags = PVMW_SYNC;
> >>>>
> >>>> -     if (flags & TTU_SPLIT_HUGE_PMD)
> >>>> -             split_huge_pmd_address(vma, address, false, folio);
> >>>> -
> >>>>         /*
> >>>>          * For THP, we have to assume the worse case ie pmd for invalidation.
> >>>>          * For hugetlb, it could be much worse if we need to do pud
> >>>> @@ -1668,20 +1665,35 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >>>>         mmu_notifier_invalidate_range_start(&range);
> >>>>
> >>>>         while (page_vma_mapped_walk(&pvmw)) {
> >>>> -             /* Unexpected PMD-mapped THP? */
> >>>> -             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
> >>>> -
> >>>>                 /*
> >>>>                  * If the folio is in an mlock()d vma, we must not swap it out.
> >>>>                  */
> >>>>                 if (!(flags & TTU_IGNORE_MLOCK) &&
> >>>>                     (vma->vm_flags & VM_LOCKED)) {
> >>>>                         /* Restore the mlock which got missed */
> >>>> -                     if (!folio_test_large(folio))
> >>>> +                     if (!folio_test_large(folio) ||
> >>>> +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
> >>>>                                 mlock_vma_folio(folio, vma);
> >>>
> >>> Can you elaborate why you think this would be required? If we would have
> >>> performed the  split_huge_pmd_address() beforehand, we would still be
> >>> left with a large folio, no?
> >>
> >> Yep, there would still be a large folio, but it wouldn't be PMD-mapped.
> >>
> >> After Weifeng's series[1], the kernel supports mlock for PTE-mapped large
> >> folio, but there are a few scenarios where we don't mlock a large folio, such
> >> as when it crosses a VM_LOCKed VMA boundary.
> >>
> >>    -                     if (!folio_test_large(folio))
> >>    +                     if (!folio_test_large(folio) ||
> >>    +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
> >>
> >> And this check is just future-proofing and likely unnecessary. If encountering a
> >> PMD-mapped THP missing the mlock for some reason, we can mlock this
> >> THP to prevent it from being picked up during page reclaim, since it is fully
> >> mapped and doesn't cross the VMA boundary, IIUC.
> >>
> >> What do you think?
> >> I would appreciate any suggestions regarding this check ;)
> >
> > Reading this patch only, I wonder if this change makes sense in the
> > context here.
> >
> > Before this patch, we would have PTE-mapped the PMD-mapped THP before
> > reaching this call and skipped it due to "!folio_test_large(folio)".
> >
> > After this patch, we either
> >
> > a) PTE-remap the THP after this check, but retry and end-up here again,
> > whereby we would skip it due to "!folio_test_large(folio)".
> >
> > b) Discard the PMD-mapped THP due to lazyfree directly. Can that
> > co-exist with mlock and what would be the problem here with mlock?
> >
> >

Thanks a lot for clarifying!

> > So if the check is required in this patch, we really have to understand
> > why. If not, we should better drop it from this patch.
> >
> > At least my opinion, still struggling to understand why it would be
> > required (I have 0 knowledge about mlock interaction with large folios :) ).
> >
>
> Looking at that series, in folio_references_one(), we do
>
>                         if (!folio_test_large(folio) || !pvmw.pte) {
>                                 /* Restore the mlock which got missed */
>                                 mlock_vma_folio(folio, vma);
>                                 page_vma_mapped_walk_done(&pvmw);
>                                 pra->vm_flags |= VM_LOCKED;
>                                 return false; /* To break the loop */
>                         }
>
> I wonder if we want that here as well now: in case of lazyfree we
> would not back off, right?
>
> But I'm not sure if lazyfree in mlocked areas are even possible.
>
> Adding the "!pvmw.pte" would be much clearer to me than the flag check.

Hmm... How about we drop it from this patch for now, and add it back if needed
in the future?

Thanks,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>