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

[Lance Yang]

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 ;)

[1] https://lore.kernel.org/all/20230918073318.1181104-3-fengwei.yin@xxxxxxxxx/T/#mdab40248cf3705581d8bfb64e1ebf2d9cd81c095

>
> >                       goto walk_done_err;
> >               }
> >
> > +             if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > +                     /*
> > +                      * We temporarily have to drop the PTL and start once
> > +                      * again from that now-PTE-mapped page table.
> > +                      */
> > +                     split_huge_pmd_locked(vma, range.start, pvmw.pmd, false,
> > +                                           folio);
>
> Using range.start here is a bit weird. Wouldn't this be pvmw.address?
> [did not check]

Hmm... we may adjust range.start before the page walk, but pvmw.address
does not.

At least for now, pvmw.address seems better. Will adjust as you suggested.

>
> > +                     pvmw.pmd = NULL;
> > +                     spin_unlock(pvmw.ptl);
> > +                     pvmw.ptl = NULL;
>
>
> Would we want a
>
> page_vma_mapped_walk_restart() that is exactly for that purpose?

Nice, let's add page_vma_mapped_walk_restart() for that purpose :)

Thanks again for your time!

Lance

>
> > +                     flags &= ~TTU_SPLIT_HUGE_PMD;
> > +                     continue;
> > +             }
> > +
> > +             /* Unexpected PMD-mapped THP? */
> > +             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>
> --
> Cheers,
>
> David / dhildenb
>