Re: [PATCH mm-new v3 3/3] mm/khugepaged: merge PTE scanning logic into a new helper

[Nico Pache]

On Tue, Oct 14, 2025 at 6:37 AM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> On Wed, Oct 08, 2025 at 12:37:48PM +0800, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > As David suggested, the PTE scanning logic in hpage_collapse_scan_pmd()
> > and __collapse_huge_page_isolate() was almost duplicated.
> >
> > This patch cleans things up by moving all the common PTE checking logic
> > into a new shared helper, thp_collapse_check_pte(). While at it, we use
> > vm_normal_folio() instead of vm_normal_page().
> >
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Suggested-by: Dev Jain <dev.jain@xxxxxxx>
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
>
> In general I like the idea, the implementation needs work though.
>
> Will check in more detail on respin
>
> > ---
> >  mm/khugepaged.c | 243 ++++++++++++++++++++++++++----------------------
> >  1 file changed, 130 insertions(+), 113 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index b5c0295c3414..7116caae1fa4 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -61,6 +61,12 @@ enum scan_result {
> >       SCAN_PAGE_FILLED,
> >  };
> >
> > +enum pte_check_result {
> > +     PTE_CHECK_SUCCEED,
> > +     PTE_CHECK_CONTINUE,
>
> Don't love this logic - this feels like we're essentially abstracting
> control flow, I mean what does 'continue' mean here? Other than you're in a
> loop and please continue, which is relying a little too much on what the
> caller does.
>
> if we retain this logic something like PTE_CHECK_SKIP would make more sense.
>
>
> > +     PTE_CHECK_FAIL,
> > +};
> > +
> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/huge_memory.h>
> >
> > @@ -533,62 +539,139 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> >       }
> >  }
> >
> > +/*
> > + * thp_collapse_check_pte - Check if a PTE is suitable for THP collapse
> > + * @pte:           The PTE to check
> > + * @vma:           The VMA the PTE belongs to
> > + * @addr:          The virtual address corresponding to this PTE
> > + * @foliop:        On success, used to return a pointer to the folio
> > + *                 Must be non-NULL
> > + * @none_or_zero:  Counter for none/zero PTEs. Must be non-NULL
> > + * @unmapped:      Counter for swap PTEs. Can be NULL if not scanning swaps
> > + * @shared:        Counter for shared pages. Must be non-NULL
> > + * @scan_result:   Used to return the failure reason (SCAN_*) on a
> > + *                 PTE_CHECK_FAIL return. Must be non-NULL
> > + * @cc:            Collapse control settings
>
> Do we really need this many parameters? THis is hard to follow.
>
> Of course it being me, I'd immediately prefer a helper struct :)

Hey!

Ive been trying to reimplement this patch, and started converting this
to a helper struct... At first glance it seems like the right
approach, but because the scan/isolate share two slightly different
implementations, we need to leave some logic in the parent. This
results in polluting the rest of the code significantly just to save
one ugly function call with many parameters.

not sure what to do about that tbh.

e.g., (just one of many examples)


-                               NR_ISOLATED_ANON + folio_is_file_lru(folio),
-                               folio_nr_pages(folio));
-               VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
-               VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
+                               NR_ISOLATED_ANON + folio_is_file_lru(ctx.folio),
+                               folio_nr_pages(ctx.folio));
+               VM_BUG_ON_FOLIO(!folio_test_locked(ctx.folio), ctx.folio);
+               VM_BUG_ON_FOLIO(folio_test_lru(ctx.folio), ctx.folio);

-               if (folio_test_large(folio))
-                       list_add_tail(&folio->lru, compound_pagelist);
+               if (folio_test_large(ctx.folio))
+                       list_add_tail(ctx.folio->lru, compound_pagelist);

Are we sure this helper structure is the right approach?

-- Nico

>
> > + *
> > + * Returns:
> > + *   PTE_CHECK_SUCCEED  - PTE is suitable, proceed with further checks
> > + *   PTE_CHECK_CONTINUE - Skip this PTE and continue scanning
> > + *   PTE_CHECK_FAIL     - Abort collapse scan
> > + */
> > +static inline int thp_collapse_check_pte(pte_t pte, struct vm_area_struct *vma,
>
> There's no need for inline in an internal static function in a file.
>
> > +             unsigned long addr, struct folio **foliop, int *none_or_zero,
> > +             int *unmapped, int *shared, int *scan_result,
> > +             struct collapse_control *cc)
> > +{
> > +     struct folio *folio = NULL;
> > +
> > +     if (pte_none(pte) || is_zero_pfn(pte_pfn(pte))) {
> > +             (*none_or_zero)++;
> > +             if (!userfaultfd_armed(vma) &&
> > +                 (!cc->is_khugepaged ||
> > +                  *none_or_zero <= khugepaged_max_ptes_none)) {
> > +                     return PTE_CHECK_CONTINUE;
> > +             } else {
> > +                     *scan_result = SCAN_EXCEED_NONE_PTE;
> > +                     count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> > +                     return PTE_CHECK_FAIL;
> > +             }
> > +     } else if (!pte_present(pte)) {
>
> You're now making the if-else issues with previous patches worse by
> returning which gets even checkpatch to warn you.
>
>         if (pte_none(pte) || is_zero_pfn(pte_pfn(pte))) {
>
> (of course note that I am not convinced you can drop the pte_present()
> check here)
>
>                 (*none_or_zero)++;
>                 if (!userfaultfd_armed(vma) &&
>                     (!cc->is_khugepaged ||
>                      *none_or_zero <= khugepaged_max_ptes_none))
>                         return PTE_CHECK_CONTINUE;
>
>                 *scan_result = SCAN_EXCEED_NONE_PTE;
>                 count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>                 return PTE_CHECK_FAIL;
>         }
>
>         if (!pte_present(pte)) {
>                 ...
>         }
>
> But even that really needs seriously more refactoring - that condition
> above is horrible for instance so, e.g.:
>
>         if (pte_none(pte) || is_zero_pfn(pte_pfn(pte))) {
>                 (*none_or_zero)++;
>
>                 /* Cases where this is acceptable. */
>                 if (!userfaultfd_armed(vma))
>                         return PTE_CHECK_SKIP;
>                 if (!cc->is_khugepaged)
>                         return PTE_CHECK_SKIP;
>                 if (*none_or_zero <= khugepaged_max_ptes_none)
>                         return PTE_CHECK_SKIP;
>
>                 /* Otherwise, we must abort the scan. */
>                 *scan_result = SCAN_EXCEED_NONE_PTE;
>                 count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>                 return PTE_CHECK_FAIL;
>         }
>
>         if (!pte_present(pte)) {
>                 ...
>         }
>
> Improves things a lot.
>
> I do however _hate_ this (*blah)++; thing. A helper struct would help :)
>
>
>
> > +             if (!unmapped) {
> > +                     *scan_result = SCAN_PTE_NON_PRESENT;
> > +                     return PTE_CHECK_FAIL;
>
> Can't we have a helper that sets the result, e.g.:
>
>         return pte_check_fail(scan_result, SCAN_PTE_NON_PRESENT);
>
> static enum pte_check_result pte_check_fail(int *scan_result,
>                 enum pte_check_result result)
> {
>         *scan_result = result;
>         return PTE_CHECK_FAIL;
> }
>
> And same for success/skip
>
> Then all of these horrible if (blah) { *foo = bar; return baz; } can be
> made into
>
>         if (blah)
>                 return pte_check_xxx(..., SCAN_PTE_...);
>
> > +             }
> > +
> > +             if (non_swap_entry(pte_to_swp_entry(pte))) {
> > +                     *scan_result = SCAN_PTE_NON_PRESENT;
> > +                     return PTE_CHECK_FAIL;
> > +             }
> > +
> > +             (*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(pte)) {
> > +                             *scan_result = SCAN_PTE_UFFD_WP;
> > +                             return PTE_CHECK_FAIL;
> > +                     }
> > +                     return PTE_CHECK_CONTINUE;
> > +             } else {
> > +                     *scan_result = SCAN_EXCEED_SWAP_PTE;
> > +                     count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> > +                     return PTE_CHECK_FAIL;
> > +             }
> > +     } else if (pte_uffd_wp(pte)) {
> > +             /*
> > +              * Don't collapse the page if any of the small PTEs are
> > +              * armed with uffd write protection. Here we can also mark
> > +              * the new huge pmd as write protected if any of the small
> > +              * ones is marked but that could bring unknown userfault
> > +              * messages that falls outside of the registered range.
> > +              * So, just be simple.
> > +              */
> > +             *scan_result = SCAN_PTE_UFFD_WP;
> > +             return PTE_CHECK_FAIL;
> > +     }
> > +
>
> Again as all of the comments for previous series still apply here so
> obviously I don't like this as-is :)
>
> And as Andrew has pointed out, now checkpatch is finding the 'pointless
> else' issue (as mentioned above also).
>
> > +     folio = vm_normal_folio(vma, addr, pte);
> > +     if (unlikely(!folio) || unlikely(folio_is_zone_device(folio))) {
> > +             *scan_result = SCAN_PAGE_NULL;
> > +             return PTE_CHECK_FAIL;
> > +     }
> > +
> > +     if (!folio_test_anon(folio)) {
> > +             VM_WARN_ON_FOLIO(true, folio);
> > +             *scan_result = SCAN_PAGE_ANON;
> > +             return PTE_CHECK_FAIL;
> > +     }
> > +
> > +     /*
> > +      * We treat a single page as shared if any part of the THP
> > +      * is shared.
> > +      */
> > +     if (folio_maybe_mapped_shared(folio)) {
> > +             (*shared)++;
> > +             if (cc->is_khugepaged && *shared > khugepaged_max_ptes_shared) {
> > +                     *scan_result = SCAN_EXCEED_SHARED_PTE;
> > +                     count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > +                     return PTE_CHECK_FAIL;
> > +             }
> > +     }
> > +
> > +     *foliop = folio;
> > +
> > +     return PTE_CHECK_SUCCEED;
> > +}
> > +
> >  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)
> >  {
> > -     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;
> > +     int pte_check_res;
> >
> >       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> >            _pte++, addr += PAGE_SIZE) {
> >               pte_t pteval = ptep_get(_pte);
> > -             if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > -                     ++none_or_zero;
> > -                     if (!userfaultfd_armed(vma) &&
> > -                         (!cc->is_khugepaged ||
> > -                          none_or_zero <= khugepaged_max_ptes_none)) {
> > -                             continue;
> > -                     } else {
> > -                             result = SCAN_EXCEED_NONE_PTE;
> > -                             count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> > -                             goto out;
> > -                     }
> > -             } else if (!pte_present(pteval)) {
> > -                     result = SCAN_PTE_NON_PRESENT;
> > -                     goto out;
> > -             } else if (pte_uffd_wp(pteval)) {
> > -                     result = SCAN_PTE_UFFD_WP;
> > -                     goto out;
> > -             }
> > -             page = vm_normal_page(vma, addr, pteval);
> > -             if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
> > -                     result = SCAN_PAGE_NULL;
> > -                     goto out;
> > -             }
> >
> > -             folio = page_folio(page);
> > -             if (!folio_test_anon(folio)) {
> > -                     VM_WARN_ON_FOLIO(true, folio);
> > -                     result = SCAN_PAGE_ANON;
> > -                     goto out;
> > -             }
> > +             pte_check_res = thp_collapse_check_pte(pteval, vma, addr,
> > +                                     &folio, &none_or_zero, NULL, &shared,
> > +                                     &result, cc);
> >
> > -             /* See hpage_collapse_scan_pmd(). */
> > -             if (folio_maybe_mapped_shared(folio)) {
> > -                     ++shared;
> > -                     if (cc->is_khugepaged &&
> > -                         shared > khugepaged_max_ptes_shared) {
> > -                             result = SCAN_EXCEED_SHARED_PTE;
> > -                             count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > -                             goto out;
> > -                     }
> > -             }
> > +             if (pte_check_res == PTE_CHECK_CONTINUE)
> > +                     continue;
> > +             else if (pte_check_res == PTE_CHECK_FAIL)
> > +                     goto out;
> >
> >               if (folio_test_large(folio)) {
> >                       struct folio *f;
> > @@ -1264,11 +1347,11 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> >       pte_t *pte, *_pte;
> >       int result = SCAN_FAIL, referenced = 0;
> >       int none_or_zero = 0, shared = 0;
> > -     struct page *page = NULL;
> >       struct folio *folio = NULL;
> >       unsigned long addr;
> >       spinlock_t *ptl;
> >       int node = NUMA_NO_NODE, unmapped = 0;
> > +     int pte_check_res;
> >
> >       VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
> >
> > @@ -1287,81 +1370,15 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> >       for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> >            _pte++, addr += PAGE_SIZE) {
> >               pte_t pteval = ptep_get(_pte);
> > -             if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > -                     ++none_or_zero;
> > -                     if (!userfaultfd_armed(vma) &&
> > -                         (!cc->is_khugepaged ||
> > -                          none_or_zero <= khugepaged_max_ptes_none)) {
> > -                             continue;
> > -                     } else {
> > -                             result = SCAN_EXCEED_NONE_PTE;
> > -                             count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> > -                             goto out_unmap;
> > -                     }
> > -             } else if (!pte_present(pteval)) {
> > -                     if (non_swap_entry(pte_to_swp_entry(pteval))) {
> > -                             result = SCAN_PTE_NON_PRESENT;
> > -                             goto out_unmap;
> > -                     }
> >
> > -                     ++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(pteval)) {
> > -                                     result = SCAN_PTE_UFFD_WP;
> > -                                     goto out_unmap;
> > -                             }
> > -                             continue;
> > -                     } else {
> > -                             result = SCAN_EXCEED_SWAP_PTE;
> > -                             count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> > -                             goto out_unmap;
> > -                     }
> > -             } else if (pte_uffd_wp(pteval)) {
> > -                     /*
> > -                      * Don't collapse the page if any of the small
> > -                      * PTEs are armed with uffd write protection.
> > -                      * Here we can also mark the new huge pmd as
> > -                      * write protected if any of the small ones is
> > -                      * marked but that could bring unknown
> > -                      * userfault messages that falls outside of
> > -                      * the registered range.  So, just be simple.
> > -                      */
> > -                     result = SCAN_PTE_UFFD_WP;
> > -                     goto out_unmap;
> > -             }
> > +             pte_check_res = thp_collapse_check_pte(pteval, vma, addr,
> > +                                     &folio, &none_or_zero, &unmapped,
> > +                                     &shared, &result, cc);
> >
> > -             page = vm_normal_page(vma, addr, pteval);
> > -             if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
> > -                     result = SCAN_PAGE_NULL;
> > -                     goto out_unmap;
> > -             }
> > -             folio = page_folio(page);
> > -
> > -             if (!folio_test_anon(folio)) {
> > -                     VM_WARN_ON_FOLIO(true, folio);
> > -                     result = SCAN_PAGE_ANON;
> > +             if (pte_check_res == PTE_CHECK_CONTINUE)
> > +                     continue;
> > +             else if (pte_check_res == PTE_CHECK_FAIL)
> >                       goto out_unmap;
> > -             }
> > -
> > -             /*
> > -              * We treat a single page as shared if any part of the THP
> > -              * is shared.
> > -              */
> > -             if (folio_maybe_mapped_shared(folio)) {
> > -                     ++shared;
> > -                     if (cc->is_khugepaged &&
> > -                         shared > khugepaged_max_ptes_shared) {
> > -                             result = SCAN_EXCEED_SHARED_PTE;
> > -                             count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > -                             goto out_unmap;
> > -                     }
> > -             }
> >
> >               /*
> >                * Record which node the original page is from and save this
> > --
> > 2.49.0
> >
>