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

[David Hildenbrand]

On 06.06.24 10:01, David Hildenbrand wrote:
> On 06.06.24 05:55, Lance Yang wrote:
> > On Wed, Jun 5, 2024 at 10:28 PM David Hildenbrand <david@xxxxxxxxxx> 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.
> >
> > Allow me to try explaining it again ;)
> >
> > > 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)".
> >
> > Yes, there is only a PTE-mapped THP when doing the "!folio_test_large(folio)"
> > check, as we will first conditionally split the PMD via
> > split_huge_pmd_address().
> >
> > > After this patch, we either
> >
> > Things will change. We'll first do the "!folio_test_large(folio)" check, then
> > conditionally split the PMD via split_huge_pmd_address().
> >
> > > 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)".
> >
> > Hmm...
> >
> > IIUC, we will skip it after this check, stop the page walk, and not
> > PTE-remap the THP.
> >
> > > 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?
> >
> > Before discarding a PMD-mapped THP as a whole, as patch #3 did,
> > we also perform the "!folio_test_large(folio)" check. If the THP coexists
> > with mlock, we will skip it, stop the page walk, and not discard it. IIUC.
>
> But "!folio_test_large(folio)" would *skip* the THP and not consider it
> regarding mlock.
>
> I'm probably missing something

I'm stupid, I missed that we still do the "goto walk_done_err;", only 
that we don't do the mlock_vma_folio(folio, vma);

Yes, let's drop it for now! :)

--
Cheers,

David / dhildenb