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

[David Hildenbrand]

On 06.06.24 11:38, Lance Yang wrote:
> On Thu, Jun 6, 2024 at 4:06 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > 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);
>
> Should we still keep the '!pvmw.pte' here? Something like:
>
> if (!folio_test_large(folio) || !pvmw.pte)
>      mlock_vma_folio(folio, vma);

I was wondering the same the whole time ...

> We can mlock the THP to prevent it from being picked up during page reclaim.
>
> David, I’d like to hear your thoughts on this ;)

but I think there is no need to for now, in the context of your patchset. :)

--
Cheers,

David / dhildenb