Re: [RFC v2 7/9] khugepaged: add mTHP support

[Nico Pache <npache@redhat.com>]

On Wed, Feb 12, 2025 at 10:04 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 11/02/2025 00:30, Nico Pache wrote:
> > Introduce the ability for khugepaged to collapse to different mTHP sizes.
> > While scanning a PMD range for potential collapse candidates, keep track
> > of pages in MIN_MTHP_ORDER chunks via a bitmap. Each bit represents a
> > utilized region of order MIN_MTHP_ORDER ptes. We remove the restriction
> > of max_ptes_none during the scan phase so we dont bailout early and miss
> > potential mTHP candidates.
> >
> > After the scan is complete we will perform binary recursion on the
> > bitmap to determine which mTHP size would be most efficient to collapse
> > to. max_ptes_none will be scaled by the attempted collapse order to
> > determine how full a THP must be to be eligible.
> >
> > If a mTHP collapse is attempted, but contains swapped out, or shared
> > pages, we dont perform the collapse.
> >
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >  mm/khugepaged.c | 122 ++++++++++++++++++++++++++++++++----------------
> >  1 file changed, 83 insertions(+), 39 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index c8048d9ec7fb..cd310989725b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1127,13 +1127,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >  {
> >       LIST_HEAD(compound_pagelist);
> >       pmd_t *pmd, _pmd;
> > -     pte_t *pte;
> > +     pte_t *pte, mthp_pte;
> >       pgtable_t pgtable;
> >       struct folio *folio;
> >       spinlock_t *pmd_ptl, *pte_ptl;
> >       int result = SCAN_FAIL;
> >       struct vm_area_struct *vma;
> >       struct mmu_notifier_range range;
> > +     unsigned long _address = address + offset * PAGE_SIZE;
> >       VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> >
> >       /*
> > @@ -1148,12 +1149,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >               *mmap_locked = false;
> >       }
> >
> > -     result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
> > +     result = alloc_charge_folio(&folio, mm, cc, order);
> >       if (result != SCAN_SUCCEED)
> >               goto out_nolock;
> >
> >       mmap_read_lock(mm);
> > -     result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> > +     *mmap_locked = true;
> > +     result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
> >       if (result != SCAN_SUCCEED) {
> >               mmap_read_unlock(mm);
> >               goto out_nolock;
> > @@ -1171,13 +1173,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >                * released when it fails. So we jump out_nolock directly in
> >                * that case.  Continuing to collapse causes inconsistency.
> >                */
> > -             result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> > -                             referenced, HPAGE_PMD_ORDER);
> > +             result = __collapse_huge_page_swapin(mm, vma, _address, pmd,
> > +                             referenced, order);
> >               if (result != SCAN_SUCCEED)
> >                       goto out_nolock;
> >       }
> >
> >       mmap_read_unlock(mm);
> > +     *mmap_locked = false;
> >       /*
> >        * Prevent all access to pagetables with the exception of
> >        * gup_fast later handled by the ptep_clear_flush and the VM
> > @@ -1187,7 +1190,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >        * mmap_lock.
> >        */
> >       mmap_write_lock(mm);
> > -     result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> > +     result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
> >       if (result != SCAN_SUCCEED)
> >               goto out_up_write;
> >       /* check if the pmd is still valid */
> > @@ -1198,11 +1201,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >       vma_start_write(vma);
> >       anon_vma_lock_write(vma->anon_vma);
> >
> > -     mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
> > -                             address + HPAGE_PMD_SIZE);
> > +     mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, _address,
> > +                             _address + (PAGE_SIZE << order));
> >       mmu_notifier_invalidate_range_start(&range);
> >
> >       pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> > +
> >       /*
> >        * This removes any huge TLB entry from the CPU so we won't allow
> >        * huge and small TLB entries for the same virtual address to
> > @@ -1216,10 +1220,10 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >       mmu_notifier_invalidate_range_end(&range);
> >       tlb_remove_table_sync_one();
> >
> > -     pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> > +     pte = pte_offset_map_lock(mm, &_pmd, _address, &pte_ptl);
> >       if (pte) {
> > -             result = __collapse_huge_page_isolate(vma, address, pte, cc,
> > -                                     &compound_pagelist, HPAGE_PMD_ORDER);
> > +             result = __collapse_huge_page_isolate(vma, _address, pte, cc,
> > +                                     &compound_pagelist, order);
> >               spin_unlock(pte_ptl);
> >       } else {
> >               result = SCAN_PMD_NULL;
> > @@ -1248,8 +1252,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >       anon_vma_unlock_write(vma->anon_vma);
> >
> >       result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> > -                                        vma, address, pte_ptl,
> > -                                        &compound_pagelist, HPAGE_PMD_ORDER);
> > +                                        vma, _address, pte_ptl,
> > +                                        &compound_pagelist, order);
> >       pte_unmap(pte);
> >       if (unlikely(result != SCAN_SUCCEED))
> >               goto out_up_write;
> > @@ -1260,20 +1264,37 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >        * write.
> >        */
> >       __folio_mark_uptodate(folio);
> > -     pgtable = pmd_pgtable(_pmd);
> > -
> > -     _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> > -     _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> > -
> > -     spin_lock(pmd_ptl);
> > -     BUG_ON(!pmd_none(*pmd));
> > -     folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
> > -     folio_add_lru_vma(folio, vma);
> > -     pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > -     set_pmd_at(mm, address, pmd, _pmd);
> > -     update_mmu_cache_pmd(vma, address, pmd);
> > -     deferred_split_folio(folio, false);
> > -     spin_unlock(pmd_ptl);
> > +     if (order == HPAGE_PMD_ORDER) {
> > +             pgtable = pmd_pgtable(_pmd);
> > +             _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> > +             _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> > +
> > +             spin_lock(pmd_ptl);
> > +             BUG_ON(!pmd_none(*pmd));
> > +             folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> > +             folio_add_lru_vma(folio, vma);
> > +             pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > +             set_pmd_at(mm, address, pmd, _pmd);
> > +             update_mmu_cache_pmd(vma, address, pmd);
> > +             deferred_split_folio(folio, false);
> > +             spin_unlock(pmd_ptl);
> > +     } else { //mTHP
> > +             mthp_pte = mk_pte(&folio->page, vma->vm_page_prot);
> > +             mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
> > +
> > +             spin_lock(pmd_ptl);
> > +             folio_ref_add(folio, (1 << order) - 1);
> > +             folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> > +             folio_add_lru_vma(folio, vma);
> > +             spin_lock(pte_ptl);
> > +             set_ptes(vma->vm_mm, _address, pte, mthp_pte, (1 << order));
> > +             update_mmu_cache_range(NULL, vma, _address, pte, (1 << order));
> > +             spin_unlock(pte_ptl);
> > +             smp_wmb(); /* make pte visible before pmd */
> > +             pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> > +             deferred_split_folio(folio, false);
>
>
> Hi Nico,
>
> This patch will have the same issue as the one I pointed out in
> https://lore.kernel.org/all/82b9efd1-f2a6-4452-b2ea-6c163e17cdf7@gmail.com/ ?

Hi Usama,

Yes thanks for pointing that out! I'll make sure to remove the
deferred_split_folio call.

>