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

[Lance Yang]

On Thu, Jun 6, 2024 at 5:41 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> 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. :)

Agreed. Let's drop it for now :)

Thanks a lot for your thoughts!
Lance

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