Re: [PATCH mm-new v7 2/5] mm: khugepaged: refine scan progress number

[Vernon Yang <vernon2gm@gmail.com>]

On Sun, Feb 8, 2026 at 5:17 PM Dev Jain <dev.jain@arm.com> wrote:
>
> On 07/02/26 1:46 pm, Vernon Yang wrote:
> > From: Vernon Yang <yanglincheng@kylinos.cn>
> >
> > Currently, each scan always increases "progress" by HPAGE_PMD_NR,
> > even if only scanning a single PTE/PMD entry.
> >
> > - When only scanning a sigle PTE entry, let me provide a detailed
> >   example:
> >
> > static int hpage_collapse_scan_pmd()
> > {
> >       for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> >            _pte++, addr += PAGE_SIZE) {
> >               pte_t pteval = ptep_get(_pte);
> >               ...
> >               if (pte_uffd_wp(pteval)) { <-- first scan hit
> >                       result = SCAN_PTE_UFFD_WP;
> >                       goto out_unmap;
> >               }
> >       }
> > }
> >
> > During the first scan, if pte_uffd_wp(pteval) is true, the loop exits
> > directly. In practice, only one PTE is scanned before termination.
> > Here, "progress += 1" reflects the actual number of PTEs scanned, but
> > previously "progress += HPAGE_PMD_NR" always.
> >
> > - When the memory has been collapsed to PMD, let me provide a detailed
> >   example:
> >
> > The following data is traced by bpftrace on a desktop system. After
> > the system has been left idle for 10 minutes upon booting, a lot of
> > SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan
> > by khugepaged.
> >
> > From trace_mm_khugepaged_scan_pmd and trace_mm_khugepaged_scan_file, the
> > following statuses were observed, with frequency mentioned next to them:
> >
> > SCAN_SUCCEED          : 1
> > SCAN_EXCEED_SHARED_PTE: 2
> > SCAN_PMD_MAPPED       : 142
> > SCAN_NO_PTE_TABLE     : 178
> > total progress size   : 674 MB
> > Total time            : 419 seconds, include khugepaged_scan_sleep_millisecs
> >
> > The khugepaged_scan list save all task that support collapse into hugepage,
> > as long as the task is not destroyed, khugepaged will not remove it from
> > the khugepaged_scan list. This exist a phenomenon where task has already
> > collapsed all memory regions into hugepage, but khugepaged continues to
> > scan it, which wastes CPU time and invalid, and due to
> > khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for
> > scanning a large number of invalid task, so scanning really valid task
> > is later.
> >
> > After applying this patch, when the memory is either SCAN_PMD_MAPPED or
> > SCAN_NO_PTE_TABLE, just skip it, as follow:
> >
> > SCAN_EXCEED_SHARED_PTE: 2
> > SCAN_PMD_MAPPED       : 147
> > SCAN_NO_PTE_TABLE     : 173
> > total progress size   : 45 MB
> > Total time            : 20 seconds
> >
> > Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> > ---
> >  mm/khugepaged.c | 38 ++++++++++++++++++++++++++++----------
> >  1 file changed, 28 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 4049234e1c8b..8b68ae3bc2c5 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -68,7 +68,10 @@ enum scan_result {
> >  static struct task_struct *khugepaged_thread __read_mostly;
> >  static DEFINE_MUTEX(khugepaged_mutex);
> >
> > -/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */
> > +/*
> > + * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas
> > + * every 10 second.
> > + */
> >  static unsigned int khugepaged_pages_to_scan __read_mostly;
> >  static unsigned int khugepaged_pages_collapsed;
> >  static unsigned int khugepaged_full_scans;
> > @@ -1240,7 +1243,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> >  }
> >
> >  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 vm_area_struct *vma, unsigned long start_addr,
> > +             bool *mmap_locked, unsigned int *cur_progress,
> >               struct collapse_control *cc)
> >  {
> >       pmd_t *pmd;
> > @@ -1256,19 +1260,27 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> >       VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
> >
> >       result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
> > -     if (result != SCAN_SUCCEED)
> > +     if (result != SCAN_SUCCEED) {
> > +             if (cur_progress)
> > +                     *cur_progress = 1;
> >               goto out;
> > +     }
> >
> >       memset(cc->node_load, 0, sizeof(cc->node_load));
> >       nodes_clear(cc->alloc_nmask);
> >       pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
> >       if (!pte) {
> > +             if (cur_progress)
> > +                     *cur_progress = 1;
> >               result = SCAN_NO_PTE_TABLE;
> >               goto out;
> >       }
> >
> >       for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> >            _pte++, addr += PAGE_SIZE) {
> > +             if (cur_progress)
> > +                     *cur_progress += 1;
> > +
> >               pte_t pteval = ptep_get(_pte);
> >               if (pte_none_or_zero(pteval)) {
> >                       ++none_or_zero;
> > @@ -2288,8 +2300,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> >       return result;
> >  }
> >
> > -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)
> > +static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm,
> > +             unsigned long addr, struct file *file, pgoff_t start,
> > +             unsigned int *cur_progress, struct collapse_control *cc)
> >  {
> >       struct folio *folio = NULL;
> >       struct address_space *mapping = file->f_mapping;
> > @@ -2378,6 +2391,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
> >                       cond_resched_rcu();
> >               }
> >       }
> > +     if (cur_progress)
> > +             *cur_progress = HPAGE_PMD_NR;
> >       rcu_read_unlock();
> >
> >
>
> Nit: Could move this at the end of the function. Looks weird before the
> rcu_read_unlock.

I placed it on the next line of rcu_read_unlock() because it follows
the loop more clearly for readability.

> Reviewed-by: Dev Jain <dev.jain@arm.com>

Thank you for review.