Re: [PATCH v12 mm-new 05/15] khugepaged: generalize __collapse_huge_page_* for mTHP support

[Nico Pache <npache@redhat.com>]

On Mon, Oct 27, 2025 at 10:02 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Wed, Oct 22, 2025 at 12:37:07PM -0600, Nico Pache wrote:
> > generalize the order of the __collapse_huge_page_* functions
> > to support future mTHP collapse.
> >
> > mTHP collapse will not honor the khugepaged_max_ptes_shared or
> > khugepaged_max_ptes_swap parameters, and will fail if it encounters a
> > shared or swapped entry.
> >
> > No functional changes in this patch.
> >
> > Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Co-developed-by: Dev Jain <dev.jain@arm.com>
> > Signed-off-by: Dev Jain <dev.jain@arm.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
>
> Thanks for addressing the v10 stuff (didn't check at v11).
>
> Overall LGTM, so:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks!
>
> Few minor nits below.
>
> > ---
> >  mm/khugepaged.c | 78 ++++++++++++++++++++++++++++++-------------------
> >  1 file changed, 48 insertions(+), 30 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 36ee659acfbb..4ccebf5dda97 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -537,25 +537,25 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> >  }
> >
> >  static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > -                                     unsigned long start_addr,
> > -                                     pte_t *pte,
> > -                                     struct collapse_control *cc,
> > -                                     struct list_head *compound_pagelist)
> > +             unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
> > +             unsigned int order, struct list_head *compound_pagelist)
>
> This series isn't the right place for it, but god do we need helper structs in
> this code... :)

Well we have collapse_control! I can spend some time in a follow up
series to better leverage this struct.

>
> >  {
> >       struct page *page = NULL;
> >       struct folio *folio = NULL;
> >       unsigned long addr = start_addr;
> >       pte_t *_pte;
> >       int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
> > +     const unsigned long nr_pages = 1UL << order;
> > +     int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
>
> Nit, but we should const-ify this too.

This gets converted to collapse_max_ptes_none in the future.

>
> >
> > -     for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > +     for (_pte = pte; _pte < pte + nr_pages;
> >            _pte++, addr += PAGE_SIZE) {
> >               pte_t pteval = ptep_get(_pte);
> >               if (pte_none_or_zero(pteval)) {
> >                       ++none_or_zero;
> >                       if (!userfaultfd_armed(vma) &&
> >                           (!cc->is_khugepaged ||
> > -                          none_or_zero <= khugepaged_max_ptes_none)) {
> > +                          none_or_zero <= max_ptes_none)) {
> >                               continue;
> >                       } else {
> >                               result = SCAN_EXCEED_NONE_PTE;
> > @@ -583,8 +583,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >               /* See collapse_scan_pmd(). */
> >               if (folio_maybe_mapped_shared(folio)) {
> >                       ++shared;
> > -                     if (cc->is_khugepaged &&
> > -                         shared > khugepaged_max_ptes_shared) {
> > +                     /*
> > +                      * TODO: Support shared pages without leading to further
> > +                      * mTHP collapses. Currently bringing in new pages via
> > +                      * shared may cause a future higher order collapse on a
> > +                      * rescan of the same range.
> > +                      */
>
> Yeah, I wish we could find a way to address this in some other way but given the
> mire of THP code putting this comment here for now is probably the only sensible
> way.
>
> > +                     if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
> > +                         shared > khugepaged_max_ptes_shared)) {
> >                               result = SCAN_EXCEED_SHARED_PTE;
> >                               count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> >                               goto out;
> > @@ -677,18 +683,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >  }
> >
> >  static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> > -                                             struct vm_area_struct *vma,
> > -                                             unsigned long address,
> > -                                             spinlock_t *ptl,
> > -                                             struct list_head *compound_pagelist)
> > +             struct vm_area_struct *vma, unsigned long address,
> > +             spinlock_t *ptl, unsigned int order,
> > +             struct list_head *compound_pagelist)
> >  {
> > -     unsigned long end = address + HPAGE_PMD_SIZE;
> > +     unsigned long end = address + (PAGE_SIZE << order);
> >       struct folio *src, *tmp;
> >       pte_t pteval;
> >       pte_t *_pte;
> >       unsigned int nr_ptes;
> > +     const unsigned long nr_pages = 1UL << order;
> >
> > -     for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
> > +     for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes,
> >            address += nr_ptes * PAGE_SIZE) {
> >               nr_ptes = 1;
> >               pteval = ptep_get(_pte);
> > @@ -741,13 +747,11 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> >  }
> >
> >  static void __collapse_huge_page_copy_failed(pte_t *pte,
> > -                                          pmd_t *pmd,
> > -                                          pmd_t orig_pmd,
> > -                                          struct vm_area_struct *vma,
> > -                                          struct list_head *compound_pagelist)
> > +             pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
> > +             unsigned int order, struct list_head *compound_pagelist)
> >  {
> >       spinlock_t *pmd_ptl;
> > -
> > +     const unsigned long nr_pages = 1UL << order;
> >       /*
> >        * Re-establish the PMD to point to the original page table
> >        * entry. Restoring PMD needs to be done prior to releasing
> > @@ -761,7 +765,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> >        * Release both raw and compound pages isolated
> >        * in __collapse_huge_page_isolate.
> >        */
> > -     release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
> > +     release_pte_pages(pte, pte + nr_pages, compound_pagelist);
> >  }
> >
> >  /*
> > @@ -781,16 +785,16 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> >   */
> >  static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> >               pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
> > -             unsigned long address, spinlock_t *ptl,
> > +             unsigned long address, spinlock_t *ptl, unsigned int order,
> >               struct list_head *compound_pagelist)
> >  {
> >       unsigned int i;
> >       int result = SCAN_SUCCEED;
> > -
> > +     const unsigned long nr_pages = 1UL << order;
> >       /*
> >        * Copying pages' contents is subject to memory poison at any iteration.
> >        */
> > -     for (i = 0; i < HPAGE_PMD_NR; i++) {
> > +     for (i = 0; i < nr_pages; i++) {
> >               pte_t pteval = ptep_get(pte + i);
> >               struct page *page = folio_page(folio, i);
> >               unsigned long src_addr = address + i * PAGE_SIZE;
> > @@ -809,10 +813,10 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> >
> >       if (likely(result == SCAN_SUCCEED))
> >               __collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
> > -                                                 compound_pagelist);
> > +                                                 order, compound_pagelist);
> >       else
> >               __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> > -                                              compound_pagelist);
> > +                                              order, compound_pagelist);
> >
> >       return result;
> >  }
> > @@ -985,13 +989,12 @@ static int check_pmd_still_valid(struct mm_struct *mm,
> >   * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
> >   */
> >  static int __collapse_huge_page_swapin(struct mm_struct *mm,
> > -                                    struct vm_area_struct *vma,
> > -                                    unsigned long start_addr, pmd_t *pmd,
> > -                                    int referenced)
> > +             struct vm_area_struct *vma, unsigned long start_addr,
> > +             pmd_t *pmd, int referenced, unsigned int order)
>
> Nit, super nit really, but since other __collapse_huge_page_*() functions have
> ..., order, param) as their last parameters, perhaps worth flipping referenced +
> order here?
>
> Not a big deal though.
>
> >  {
> >       int swapped_in = 0;
> >       vm_fault_t ret = 0;
> > -     unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE);
> > +     unsigned long addr, end = start_addr + (PAGE_SIZE << order);
> >       int result;
> >       pte_t *pte = NULL;
> >       spinlock_t *ptl;
> > @@ -1022,6 +1025,19 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> >               if (!is_swap_pte(vmf.orig_pte))
> >                       continue;
> >
> > +             /*
> > +              * TODO: Support swapin without leading to further mTHP
> > +              * collapses. Currently bringing in new pages via swapin may
> > +              * cause a future higher order collapse on a rescan of the same
> > +              * range.
> > +              */
>
> Same comment as above re: this, i.e. that it's a pity but probably unavoidable
> for now.
>
> > +             if (order != HPAGE_PMD_ORDER) {
> > +                     pte_unmap(pte);
> > +                     mmap_read_unlock(mm);
> > +                     result = SCAN_EXCEED_SWAP_PTE;
> > +                     goto out;
> > +             }
> > +
> >               vmf.pte = pte;
> >               vmf.ptl = ptl;
> >               ret = do_swap_page(&vmf);
> > @@ -1142,7 +1158,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >                * that case.  Continuing to collapse causes inconsistency.
> >                */
> >               result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> > -                                                  referenced);
> > +                                                  referenced, HPAGE_PMD_ORDER);
> >               if (result != SCAN_SUCCEED)
> >                       goto out_nolock;
> >       }
> > @@ -1190,6 +1206,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >       pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> >       if (pte) {
> >               result = __collapse_huge_page_isolate(vma, address, pte, cc,
> > +                                                   HPAGE_PMD_ORDER,
> >                                                     &compound_pagelist);
> >               spin_unlock(pte_ptl);
> >       } else {
> > @@ -1220,6 +1237,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >
> >       result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> >                                          vma, address, pte_ptl,
> > +                                        HPAGE_PMD_ORDER,
> >                                          &compound_pagelist);
> >       pte_unmap(pte);
> >       if (unlikely(result != SCAN_SUCCEED))
> > --
> > 2.51.0
> >
>