Re: [PATCH mm-unstable v15 03/13] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Nico Pache <npache@redhat.com>]

On Thu, Mar 12, 2026 at 2:56 PM David Hildenbrand (Arm)
<david@kernel.org> wrote:
>
> On 3/12/26 21:36, David Hildenbrand (Arm) wrote:
> > On 3/12/26 21:32, David Hildenbrand (Arm) wrote:
> >> On 2/26/26 04:23, 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: Wei Yang <richard.weiyang@gmail.com>
> >>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> >>> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> >>> 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 | 73 +++++++++++++++++++++++++++++++------------------
> >>>  1 file changed, 47 insertions(+), 26 deletions(-)
> >>>
> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> >>> index a9b645402b7f..ecdbbf6a01a6 100644
> >>> --- a/mm/khugepaged.c
> >>> +++ b/mm/khugepaged.c
> >>> @@ -535,7 +535,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> >>>
> >>>  static enum scan_result __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 int order, struct list_head *compound_pagelist)
> >>>  {
> >>>     struct page *page = NULL;
> >>>     struct folio *folio = NULL;
> >>> @@ -543,15 +543,17 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >>>     pte_t *_pte;
> >>>     int none_or_zero = 0, shared = 0, referenced = 0;
> >>>     enum scan_result result = SCAN_FAIL;
> >>> +   const unsigned long nr_pages = 1UL << order;
> >>> +   int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
> >>
> >> It might be a bit more readable to move "const unsigned long
> >> nr_pages = 1UL << order;" all the way to the top.
> >>
> >> Then, have here
> >>
> >>      int max_ptes_none = 0;
> >>
> >> and do at the beginning of the function:
> >>
> >>      /* For MADV_COLLAPSE, we always collapse ... */
> >>      if (!cc->is_khugepaged)
> >>              max_ptes_none = HPAGE_PMD_NR;
> >>      /*  ... except if userfaultf relies on MISSING faults. */
> >>      if (!userfaultfd_armed(vma))
> >>              max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
> >>
> >> (but see below regarding helper function)
> >>
> >> then the code below becomes ...
> >>
> >>>
> >>> -   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)) {
> >>
> >> ...
> >>
> >>      if (none_or_zero <= max_ptes_none) {
> >>
> >>
> >> I see that you do something like that (but slightly different) in the next
> >> patch. You could easily extend the above by it.
> >>
> >> Or go one step further and move all of that conditional into collapse_max_ptes_none(), whereby
> >> you simply also pass the cc and the vma.
> >>
> >> Then this all gets cleaned up and you'd end up above with
> >>
> >> max_ptes_none = collapse_max_ptes_none(cc, vma, order);
> >> if (max_ptes_none < 0)
> >>      return result;
> >>
> >> I'd do all that in this patch here, getting rid of #4.
> >>
> >>
> >>>                             continue;
> >>>                     } else {
> >>>                             result = SCAN_EXCEED_NONE_PTE;
> >>> @@ -585,8 +587,14 @@ static enum scan_result __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.
> >>> +                    */
> >>> +                   if (!is_pmd_order(order) || (cc->is_khugepaged &&
> >>> +                       shared > khugepaged_max_ptes_shared)) {
> >>
> >> That's not how we indent within a nested ().
> >>
> >> To make this easier to read, what about similarly having at the beginning
> >> of the function:
> >>
> >> int max_ptes_shared = 0;
> >>
> >> /* For MADV_COLLAPSE, we always collapse. */
> >> if (cc->is_khugepaged)
> >>      max_ptes_none = HPAGE_PMD_NR;
> >> /* TODO ... */
> >> if (is_pmd_order(order))
> >>      max_ptes_none = khugepaged_max_ptes_shared;
> >>
> >> to turn this code into a
> >>
> >>      if (shared > khugepaged_max_ptes_shared)
> >>
> >> Also, here, might make sense to have a collapse_max_ptes_swap(cc, order)
> >> to do that and clean it up.
> >>
> >>
> >>>                             result = SCAN_EXCEED_SHARED_PTE;
> >>>                             count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> >>>                             goto out;
> >>> @@ -679,18 +687,18 @@ static enum scan_result __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;
> >>
> >> Move it further to the top.
> >>
> >>>
> >>> -   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);
> >>> @@ -743,13 +751,11 @@ 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
> >>> @@ -763,7 +769,7 @@ 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);
> >>>  }
> >>>
> >>>  /*
> >>> @@ -783,16 +789,16 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> >>>   */
> >>>  static enum scan_result __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;
> >>>     enum scan_result result = SCAN_SUCCEED;
> >>> -
> >>> +   const unsigned long nr_pages = 1UL << order;
> >>
> >> Same here, all the way to the top.
> >>
> >>>     /*
> >>>      * 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;
> >>> @@ -811,10 +817,10 @@ static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *foli
> >>>
> >>>     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;
> >>>  }
> >>> @@ -985,12 +991,12 @@ static enum scan_result check_pmd_still_valid(struct mm_struct *mm,
> >>>   * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
> >>>   */
> >>>  static enum scan_result __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)
> >>>  {
> >>>     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);
> >>>     enum scan_result result;
> >>>     pte_t *pte = NULL;
> >>>     spinlock_t *ptl;
> >>> @@ -1022,6 +1028,19 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
> >>>                 pte_present(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.
> >>> +            */
> >>> +           if (!is_pmd_order(order)) {
> >>> +                   pte_unmap(pte);
> >>> +                   mmap_read_unlock(mm);
> >>> +                   result = SCAN_EXCEED_SWAP_PTE;
> >>> +                   goto out;
> >>> +           }
> >>> +
> >>
> >> Interesting, we just swapin everything we find :)
> >>
> >> But do we really need this check here? I mean, we just found it to be present.
> >>
> >> In the rare event that there was a race, do we really care? It was just
> >> present, now it's swapped. Bad luck. Just swap it in.
> >>
> >
> > Okay, now I am confused. Why are you not taking care of
> > collapse_scan_pmd() in the same context?
> >
> > Because if you make sure that we properly check against a max_ptes_swap
> > similar as in the style above, we'd rule out swapin right from the start?
> >
> > Also, I would expect that all other parameters in there are similarly
> > handled?
> >
>
> Okay, I think you should add the following:

Hey! Thanks for all your reviews here.

For multiple reasons, here is the solution I developed:

Add a patch before the generalize __collapse.. patch that reworks the
max_ptes* handling and introduces the helpers (no functional changes).

I later updated these functions to follow the specific mthp rules in
the generalization patch. Honestly, refactoring much of this has been
very hard without one large patch, which is why we split it up
initially.

How does that sound?

-- Nico

>
> From 17bce81ab93f3b16e044ac2f4f62be19aac38180 Mon Sep 17 00:00:00 2001
> From: "David Hildenbrand (Arm)" <david@kernel.org>
> Date: Thu, 12 Mar 2026 21:54:22 +0100
> Subject: [PATCH] tmp
>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
>  mm/khugepaged.c | 89 +++++++++++++++++++++++++++++--------------------
>  1 file changed, 53 insertions(+), 36 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b7b4680d27ab..6a3773bfa0a2 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -318,6 +318,34 @@ static ssize_t max_ptes_shared_store(struct kobject *kobj,
>         return count;
>  }
>
> +static int collapse_max_ptes_none(struct collapse_control *cc,
> +               struct vm_area_struct *vma)
> +{
> +       /* We don't mess with MISSING faults. */
> +       if (vma && userfaultfd_armed(vma))
> +               return 0;
> +       /* MADV_COLLAPSE always collapses. */
> +       if (!cc->is_khugepaged)
> +               return HPAGE_PMD_NR;
> +       return khugepaged_max_ptes_none;
> +}
> +
> +static int collapse_max_ptes_shared(struct collapse_control *cc)
> +{
> +       /* MADV_COLLAPSE always collapses. */
> +       if (!cc->is_khugepaged)
> +               return HPAGE_PMD_NR;
> +       return khugepaged_max_ptes_shared;
> +}
> +
> +static int collapse_max_ptes_swap(struct collapse_control *cc)
> +{
> +       /* MADV_COLLAPSE always collapses. */
> +       if (!cc->is_khugepaged)
> +               return HPAGE_PMD_NR;
> +       return khugepaged_max_ptes_swap;
> +}
> +
>  static struct kobj_attribute khugepaged_max_ptes_shared_attr =
>         __ATTR_RW(max_ptes_shared);
>
> @@ -539,6 +567,8 @@ static enum scan_result __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)
>  {
> +       const int max_ptes_none = collapse_max_ptes_none(cc, vma);
> +       const int max_ptes_shared = collapse_max_ptes_shared(cc);
>         struct page *page = NULL;
>         struct folio *folio = NULL;
>         unsigned long addr = start_addr;
> @@ -550,16 +580,12 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>              _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)) {
> -                               continue;
> -                       } else {
> +                       if (++none_or_zero > max_ptes_none) {
>                                 result = SCAN_EXCEED_NONE_PTE;
>                                 count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>                                 goto out;
>                         }
> +                       continue;
>                 }
>                 if (!pte_present(pteval)) {
>                         result = SCAN_PTE_NON_PRESENT;
> @@ -586,9 +612,7 @@ static enum scan_result __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 (++shared > max_ptes_shared) {
>                                 result = SCAN_EXCEED_SHARED_PTE;
>                                 count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
>                                 goto out;
> @@ -1247,6 +1271,9 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
>                 struct vm_area_struct *vma, unsigned long start_addr,
>                 bool *mmap_locked, struct collapse_control *cc)
>  {
> +       const int max_ptes_none = collapse_max_ptes_none(cc, vma);
> +       const int max_ptes_swap = collapse_max_ptes_swap(cc);
> +       const int max_ptes_shared = collapse_max_ptes_shared(cc);
>         pmd_t *pmd;
>         pte_t *pte, *_pte;
>         int none_or_zero = 0, shared = 0, referenced = 0;
> @@ -1280,36 +1307,28 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
>
>                 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)) {
> -                               continue;
> -                       } else {
> +                       if (++none_or_zero > max_ptes_none) {
>                                 result = SCAN_EXCEED_NONE_PTE;
>                                 count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>                                 goto out_unmap;
>                         }
> +                       continue;
>                 }
>                 if (!pte_present(pteval)) {
> -                       ++unmapped;
> -                       if (!cc->is_khugepaged ||
> -                           unmapped <= khugepaged_max_ptes_swap) {
> -                               /*
> -                                * Always be strict with uffd-wp
> -                                * enabled swap entries.  Please see
> -                                * comment below for pte_uffd_wp().
> -                                */
> -                               if (pte_swp_uffd_wp_any(pteval)) {
> -                                       result = SCAN_PTE_UFFD_WP;
> -                                       goto out_unmap;
> -                               }
> -                               continue;
> -                       } else {
> +                       if (++unmapped > max_ptes_swap) {
>                                 result = SCAN_EXCEED_SWAP_PTE;
>                                 count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
>                                 goto out_unmap;
>                         }
> +                       /*
> +                        * Always be strict with uffd-wp enabled swap entries.
> +                        * See the comment below for pte_uffd_wp().
> +                        */
> +                       if (pte_swp_uffd_wp_any(pteval)) {
> +                               result = SCAN_PTE_UFFD_WP;
> +                               goto out_unmap;
> +                       }
> +                       continue;
>                 }
>                 if (pte_uffd_wp(pteval)) {
>                         /*
> @@ -1348,9 +1367,7 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
>                  * is shared.
>                  */
>                 if (folio_maybe_mapped_shared(folio)) {
> -                       ++shared;
> -                       if (cc->is_khugepaged &&
> -                           shared > khugepaged_max_ptes_shared) {
> +                       if (++shared > max_ptes_shared) {
>                                 result = SCAN_EXCEED_SHARED_PTE;
>                                 count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
>                                 goto out_unmap;
> @@ -2305,6 +2322,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm,
>                 unsigned long addr, struct file *file, pgoff_t start,
>                 struct collapse_control *cc)
>  {
> +       const int max_ptes_none = collapse_max_ptes_none(cc, NULL);
> +       const int max_ptes_swap = collapse_max_ptes_swap(cc);
>         struct folio *folio = NULL;
>         struct address_space *mapping = file->f_mapping;
>         XA_STATE(xas, &mapping->i_pages, start);
> @@ -2323,8 +2342,7 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm,
>
>                 if (xa_is_value(folio)) {
>                         swap += 1 << xas_get_order(&xas);
> -                       if (cc->is_khugepaged &&
> -                           swap > khugepaged_max_ptes_swap) {
> +                       if (swap > max_ptes_swap) {
>                                 result = SCAN_EXCEED_SWAP_PTE;
>                                 count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
>                                 break;
> @@ -2395,8 +2413,7 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm,
>                 cc->progress += HPAGE_PMD_NR;
>
>         if (result == SCAN_SUCCEED) {
> -               if (cc->is_khugepaged &&
> -                   present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
> +               if (present < HPAGE_PMD_NR - max_ptes_none) {
>                         result = SCAN_EXCEED_NONE_PTE;
>                         count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>                 } else {
> --
> 2.43.0
>
>
> Then extend it by passing an order + return value check in this patch here. You can
> directly squash changes from patch #4 in here then.
>
> --
> Cheers,
>
> David
>