Re: [PATCH v9 05/14] khugepaged: generalize __collapse_huge_page_* for mTHP support

[Nico Pache <npache@redhat.com>]

On Wed, Jul 16, 2025 at 7:53 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 14.07.25 02:31, Nico Pache wrote:
> > generalize the order of the __collapse_huge_page_* functions
> > to support future mTHP collapse.
> >
> > mTHP collapse can suffer from incosistant behavior, and memory waste
> > "creep". disable swapin and shared support for mTHP collapse.
> >
> > No functional changes in this patch.
> >
> > Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.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>
> > ---
> >   mm/khugepaged.c | 49 +++++++++++++++++++++++++++++++------------------
> >   1 file changed, 31 insertions(+), 18 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index cc9a35185604..ee54e3c1db4e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -552,15 +552,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >                                       unsigned long address,
> >                                       pte_t *pte,
> >                                       struct collapse_control *cc,
> > -                                     struct list_head *compound_pagelist)
> > +                                     struct list_head *compound_pagelist,
> > +                                     u8 order)
>
> u8 ... (applies to all instances)
Fixed all instances of this (other than those that need to stay)
>
> >   {
> >       struct page *page = NULL;
> >       struct folio *folio = NULL;
> >       pte_t *_pte;
> >       int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
> >       bool writable = false;
> > +     int scaled_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
>
> "scaled_max_ptes_none" maybe?
done!
>
> >
> > -     for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > +     for (_pte = pte; _pte < pte + (1 << order);
> >            _pte++, address += PAGE_SIZE) {
> >               pte_t pteval = ptep_get(_pte);
> >               if (pte_none(pteval) || (pte_present(pteval) &&
> > @@ -568,7 +570,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >                       ++none_or_zero;
> >                       if (!userfaultfd_armed(vma) &&
> >                           (!cc->is_khugepaged ||
> > -                          none_or_zero <= khugepaged_max_ptes_none)) {
> > +                          none_or_zero <= scaled_none)) {
> >                               continue;
> >                       } else {
> >                               result = SCAN_EXCEED_NONE_PTE;
> > @@ -596,8 +598,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >               /* See hpage_collapse_scan_pmd(). */
> >               if (folio_maybe_mapped_shared(folio)) {
> >                       ++shared;
> > -                     if (cc->is_khugepaged &&
> > -                         shared > khugepaged_max_ptes_shared) {
> > +                     if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
> > +                         shared > khugepaged_max_ptes_shared)) {
>
> Please add a comment why we do something different with PMD. As
> commenting below, does this deserve a TODO?
>
> >                               result = SCAN_EXCEED_SHARED_PTE;
> >                               count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> >                               goto out;
> > @@ -698,13 +700,14 @@ 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 list_head *compound_pagelist,
> > +                                             u8 order)
> >   {
> >       struct folio *src, *tmp;
> >       pte_t *_pte;
> >       pte_t pteval;
> >
> > -     for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > +     for (_pte = pte; _pte < pte + (1 << order);
> >            _pte++, address += PAGE_SIZE) {
> >               pteval = ptep_get(_pte);
> >               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > @@ -751,7 +754,8 @@ 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)
> > +                                          struct list_head *compound_pagelist,
> > +                                          u8 order)
> >   {
> >       spinlock_t *pmd_ptl;
> >
> > @@ -768,7 +772,7 @@ 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 + (1 << order), compound_pagelist);
> >   }
> >
> >   /*
> > @@ -789,7 +793,7 @@ 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,
> > -             struct list_head *compound_pagelist)
> > +             struct list_head *compound_pagelist, u8 order)
> >   {
> >       unsigned int i;
> >       int result = SCAN_SUCCEED;
> > @@ -797,7 +801,7 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> >       /*
> >        * Copying pages' contents is subject to memory poison at any iteration.
> >        */
> > -     for (i = 0; i < HPAGE_PMD_NR; i++) {
> > +     for (i = 0; i < (1 << order); i++) {
> >               pte_t pteval = ptep_get(pte + i);
> >               struct page *page = folio_page(folio, i);
> >               unsigned long src_addr = address + i * PAGE_SIZE;
> > @@ -816,10 +820,10 @@ 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);
> > +                                                 compound_pagelist, order);
> >       else
> >               __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> > -                                              compound_pagelist);
> > +                                              compound_pagelist, order);
> >
> >       return result;
> >   }
> > @@ -994,11 +998,11 @@ static int check_pmd_still_valid(struct mm_struct *mm,
> >   static int __collapse_huge_page_swapin(struct mm_struct *mm,
> >                                      struct vm_area_struct *vma,
> >                                      unsigned long haddr, pmd_t *pmd,
> > -                                    int referenced)
> > +                                    int referenced, u8 order)
> >   {
> >       int swapped_in = 0;
> >       vm_fault_t ret = 0;
> > -     unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
> > +     unsigned long address, end = haddr + (PAGE_SIZE << order);
> >       int result;
> >       pte_t *pte = NULL;
> >       spinlock_t *ptl;
> > @@ -1029,6 +1033,15 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> >               if (!is_swap_pte(vmf.orig_pte))
> >                       continue;
> >
> > +             /* Dont swapin for mTHP collapse */
>
> Should we turn this into a TODO, because it's something to figure out
> regarding the scaling etc?
Good idea, I changed both of these into TODOs
>
> > +             if (order != HPAGE_PMD_ORDER) {
> > +                     count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
> > +                     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);
> > @@ -1149,7 +1162,7 @@ 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);
>
> Indent messed up. Feel free to exceed 80 chars if it aids readability.
Fixed!
>
> >               if (result != SCAN_SUCCEED)
> >                       goto out_nolock;
> >       }
> > @@ -1197,7 +1210,7 @@ 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,
> > -                                                   &compound_pagelist);
> > +                                     &compound_pagelist, HPAGE_PMD_ORDER);
>
> Dito.
Fixed!
>
>
> Apart from that, nothing jumped at me
>
> Acked-by: David Hildenbrand <david@redhat.com>
Thanks for the ack! I fixed the compile issue you noted too.
>
> --
> Cheers,
>
> David / dhildenb
>