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

[Dev Jain <dev.jain@xxxxxxx>]

On 04/03/25 12:48 am, Nico Pache wrote:
> On Mon, Feb 17, 2025 at 9:23 PM Dev Jain <dev.jain@xxxxxxx> wrote:
> > On 11/02/25 6:00 am, 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@xxxxxxxxxx>
> > > ---
> > >    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));
> >
> > As I have mentioned before, since you are isolating the PTE table in
> > both cases, you need to do the mmu_notifier_* stuff for HPAGE_PMD_SIZE
> > in any case. Check out patch 8 from my patchset.
>
> Why do we need to invalidate the whole PMD if we are only changing a
> section of it and no one can touch this memory?

I confess I do not understand mmu_notifiers properly, but on a proper 
look I guess you are correct; they are used to indicate to secondary 
TLBs that the TLB entry is now stale for the corresponding pte. w.r.t 
that you make sense, I just assumed that we are in a sense 
"invalidating" the whole PMD region since we are isolating the PTE table.

> > >        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);
> >
> > My personal opinion is that this if-else nesting looks really weird.
> > This is why I separated out mTHP collapse into a separate function.
> >
> >
> > > +     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);
> > > +             spin_unlock(pmd_ptl);
> > > +     }
> > >
> > >        folio = NULL;
> > >
> > > @@ -1353,21 +1374,27 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > >    {
> > >        pmd_t *pmd;
> > >        pte_t *pte, *_pte;
> > > +     int i;
> > >        int result = SCAN_FAIL, referenced = 0;
> > >        int none_or_zero = 0, shared = 0;
> > >        struct page *page = NULL;
> > >        struct folio *folio = NULL;
> > >        unsigned long _address;
> > > +     unsigned long enabled_orders;
> > >        spinlock_t *ptl;
> > >        int node = NUMA_NO_NODE, unmapped = 0;
> > >        bool writable = false;
> > > -
> > > +     int chunk_none_count = 0;
> > > +     int scaled_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - MIN_MTHP_ORDER);
> > > +     unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0;
> > >        VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> > >
> > >        result = find_pmd_or_thp_or_none(mm, address, &pmd);
> > >        if (result != SCAN_SUCCEED)
> > >                goto out;
> > >
> > > +     bitmap_zero(cc->mthp_bitmap, MAX_MTHP_BITMAP_SIZE);
> > > +     bitmap_zero(cc->mthp_bitmap_temp, MAX_MTHP_BITMAP_SIZE);
> > >        memset(cc->node_load, 0, sizeof(cc->node_load));
> > >        nodes_clear(cc->alloc_nmask);
> > >        pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> > > @@ -1376,8 +1403,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > >                goto out;
> > >        }
> > >
> > > -     for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> > > -          _pte++, _address += PAGE_SIZE) {
> > > +     for (i = 0; i < HPAGE_PMD_NR; i++) {
> > > +             if (i % MIN_MTHP_NR == 0)
> > > +                     chunk_none_count = 0;
> > > +
> > > +             _pte = pte + i;
> > > +             _address = address + i * PAGE_SIZE;
> > >                pte_t pteval = ptep_get(_pte);
> > >                if (is_swap_pte(pteval)) {
> > >                        ++unmapped;
> > > @@ -1400,16 +1431,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > >                        }
> > >                }
> > >                if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > > +                     ++chunk_none_count;
> > >                        ++none_or_zero;
> > > -                     if (!userfaultfd_armed(vma) &&
> > > -                         (!cc->is_khugepaged ||
> > > -                          none_or_zero <= khugepaged_max_ptes_none)) {
> > > -                             continue;
> > > -                     } else {
> > > +                     if (userfaultfd_armed(vma)) {
> > >                                result = SCAN_EXCEED_NONE_PTE;
> > >                                count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> > >                                goto out_unmap;
> > >                        }
> > > +                     continue;
> > >                }
> > >                if (pte_uffd_wp(pteval)) {
> > >                        /*
> > > @@ -1500,7 +1529,16 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > >                     folio_test_referenced(folio) || mmu_notifier_test_young(vma->vm_mm,
> > >                                                                     address)))
> > >                        referenced++;
> > > +
> > > +             /*
> > > +              * we are reading in MIN_MTHP_NR page chunks. if there are no empty
> > > +              * pages keep track of it in the bitmap for mTHP collapsing.
> > > +              */
> > > +             if (chunk_none_count < scaled_none &&
> > > +                     (i + 1) % MIN_MTHP_NR == 0)
> > > +                     bitmap_set(cc->mthp_bitmap, i / MIN_MTHP_NR, 1);
> > >        }
> > > +
> > >        if (!writable) {
> > >                result = SCAN_PAGE_RO;
> > >        } else if (cc->is_khugepaged &&
> > > @@ -1513,10 +1551,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> > >    out_unmap:
> > >        pte_unmap_unlock(pte, ptl);
> > >        if (result == SCAN_SUCCEED) {
> > > -             result = collapse_huge_page(mm, address, referenced,
> > > -                                         unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0);
> > > -             /* collapse_huge_page will return with the mmap_lock released */
> > > -             *mmap_locked = false;
> > > +             enabled_orders = thp_vma_allowable_orders(vma, vma->vm_flags,
> > > +                     tva_flags, THP_ORDERS_ALL_ANON);
> > > +             result = khugepaged_scan_bitmap(mm, address, referenced, unmapped, cc,
> > > +                            mmap_locked, enabled_orders);
> > > +             if (result > 0)
> > > +                     result = SCAN_SUCCEED;
> > > +             else
> > > +                     result = SCAN_FAIL;
> > >        }
> > >    out:
> > >        trace_mm_khugepaged_scan_pmd(mm, &folio->page, writable, referenced,
> > > @@ -2476,11 +2518,13 @@ static int khugepaged_collapse_single_pmd(unsigned long addr, struct mm_struct *
> > >                        fput(file);
> > >                        if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > >                                mmap_read_lock(mm);
> > > +                             *mmap_locked = true;
> > >                                if (khugepaged_test_exit_or_disable(mm))
> > >                                        goto end;
> > >                                result = collapse_pte_mapped_thp(mm, addr,
> > >                                                                 !cc->is_khugepaged);
> > >                                mmap_read_unlock(mm);
> > > +                             *mmap_locked = false;
> > >                        }
> > >                } else {
> > >                        result = khugepaged_scan_pmd(mm, vma, addr,