Re: [PATCH mm-unstable v1 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Nico Pache]

On Thu, Feb 12, 2026 at 1:04 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 2/12/26 03:25, Nico Pache wrote:
> > The khugepaged daemon and madvise_collapse have two different
> > implementations that do almost the same thing.
> >
> > Create collapse_single_pmd to increase code reuse and create an entry
> > point to these two users.
> >
> > Refactor madvise_collapse and collapse_scan_mm_slot to use the new
> > collapse_single_pmd function. This introduces a minor behavioral change
> > that is most likely an undiscovered bug. The current implementation of
> > khugepaged tests collapse_test_exit_or_disable before calling
> > collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
> > case. By unifying these two callers madvise_collapse now also performs
> > this check. We also modify the return value to be SCAN_ANY_PROCESS which
> > properly indicates that this process is no longer valid to operate on.
> >
> > We also guard the khugepaged_pages_collapsed variable to ensure its only
> > incremented for khugepaged.
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> > ---
> >   mm/khugepaged.c | 121 ++++++++++++++++++++++++++----------------------
> >   1 file changed, 66 insertions(+), 55 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index fa41480f6948..0839a781bedd 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2395,6 +2395,62 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, unsigned long a
> >       return result;
> >   }
> >
> > +/*
> > + * Try to collapse a single PMD starting at a PMD aligned addr, and return
> > + * the results.
> > + */
> > +static enum scan_result collapse_single_pmd(unsigned long addr,
> > +             struct vm_area_struct *vma, bool *mmap_locked,
> > +             struct collapse_control *cc)
> > +{
> > +     struct mm_struct *mm = vma->vm_mm;
> > +     enum scan_result result;
> > +     struct file *file;
> > +     pgoff_t pgoff;
> > +
> > +     if (vma_is_anonymous(vma)) {
> > +             result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
> > +             goto end;
> > +     }
> > +
> > +     file = get_file(vma->vm_file);
> > +     pgoff = linear_page_index(vma, addr);
> > +
> > +     mmap_read_unlock(mm);
> > +     *mmap_locked = false;
> > +     result = collapse_scan_file(mm, addr, file, pgoff, cc);
> > +
> > +     if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
> > +         mapping_can_writeback(file->f_mapping)) {
> > +             const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> > +             const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> > +
> > +             filemap_write_and_wait_range(file->f_mapping, lstart, lend);
> > +     }
> > +     fput(file);
> > +
> > +     if (result != SCAN_PTE_MAPPED_HUGEPAGE)
> > +             goto end;
> > +
> > +     mmap_read_lock(mm);
> > +     *mmap_locked = true;
> > +     if (collapse_test_exit_or_disable(mm)) {
> > +             mmap_read_unlock(mm);
> > +             *mmap_locked = false;
> > +             return SCAN_ANY_PROCESS;
> > +     }
> > +     result = try_collapse_pte_mapped_thp(mm, addr, !cc->is_khugepaged);
> > +     if (result == SCAN_PMD_MAPPED)
> > +             result = SCAN_SUCCEED;
> > +     mmap_read_unlock(mm);
> > +     *mmap_locked = false;
> > +
> > +end:
> > +     if (cc->is_khugepaged && result == SCAN_SUCCEED)
> > +             ++khugepaged_pages_collapsed;
> > +     return result;
> > +}
> > +
> >   static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *result,
> >                                           struct collapse_control *cc)
> >       __releases(&khugepaged_mm_lock)
> > @@ -2466,34 +2522,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *
> >                       VM_BUG_ON(khugepaged_scan.address < hstart ||
> >                                 khugepaged_scan.address + HPAGE_PMD_SIZE >
> >                                 hend);
> > -                     if (!vma_is_anonymous(vma)) {
> > -                             struct file *file = get_file(vma->vm_file);
> > -                             pgoff_t pgoff = linear_page_index(vma,
> > -                                             khugepaged_scan.address);
> > -
> > -                             mmap_read_unlock(mm);
> > -                             mmap_locked = false;
> > -                             *result = collapse_scan_file(mm,
> > -                                     khugepaged_scan.address, file, pgoff, cc);
> > -                             fput(file);
> > -                             if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > -                                     mmap_read_lock(mm);
> > -                                     if (collapse_test_exit_or_disable(mm))
> > -                                             goto breakouterloop;
> > -                                     *result = try_collapse_pte_mapped_thp(mm,
> > -                                             khugepaged_scan.address, false);
> > -                                     if (*result == SCAN_PMD_MAPPED)
> > -                                             *result = SCAN_SUCCEED;
> > -                                     mmap_read_unlock(mm);
> > -                             }
> > -                     } else {
> > -                             *result = collapse_scan_pmd(mm, vma,
> > -                                     khugepaged_scan.address, &mmap_locked, cc);
> > -                     }
> > -
> > -                     if (*result == SCAN_SUCCEED)
> > -                             ++khugepaged_pages_collapsed;
> >
> > +                     *result = collapse_single_pmd(khugepaged_scan.address,
> > +                                                   vma, &mmap_locked, cc);
> >                       /* move to next address */
> >                       khugepaged_scan.address += HPAGE_PMD_SIZE;
> >                       progress += HPAGE_PMD_NR;
> > @@ -2799,6 +2830,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >                       cond_resched();
> >                       mmap_read_lock(mm);
> >                       mmap_locked = true;
> > +                     *lock_dropped = true;
> >                       result = hugepage_vma_revalidate(mm, addr, false, &vma,
> >                                                        cc);
> >                       if (result  != SCAN_SUCCEED) {
> > @@ -2809,46 +2841,25 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >                       hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
> >               }
> >               mmap_assert_locked(mm);
> > -             if (!vma_is_anonymous(vma)) {
> > -                     struct file *file = get_file(vma->vm_file);
> > -                     pgoff_t pgoff = linear_page_index(vma, addr);
> >
> > -                     mmap_read_unlock(mm);
> > -                     mmap_locked = false;
> > -                     *lock_dropped = true;
> > -                     result = collapse_scan_file(mm, addr, file, pgoff, cc);
> > -
> > -                     if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
> > -                         mapping_can_writeback(file->f_mapping)) {
> > -                             loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> > -                             loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> > +             result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
> >
> > -                             filemap_write_and_wait_range(file->f_mapping, lstart, lend);
> > -                             triggered_wb = true;
> > -                             fput(file);
> > -                             goto retry;
> > -                     }
> > -                     fput(file);
> > -             } else {
> > -                     result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, cc);
> > -             }
> >               if (!mmap_locked)
> >                       *lock_dropped = true;
> >
> > -handle_result:
> > +             if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) {
> > +                     triggered_wb = true;
> > +                     goto retry;
> > +             }
>
> Having triggered_wb set where writeback is not actually triggered is
> suboptimal.

It took me a second to figure out what you were referring to, but I
see it now. if we return SCAN_PAGE_D_OR_WB but the can_writeback fails
it still retries.

Would be an appropriate solution if can_writeback fails to modify the
return value.
ie)

if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK) {
    if (mapping_can_writeback(file->f_mapping)) {
       const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
       const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;

       filemap_write_and_wait_range(file->f_mapping, lstart, lend);
    } else {
       result = SCAN_(SOMETHING?)
    }
}
fput(file);

we dont have a enum that fits this description but we want one that
will continue.

Cheers!
-- Nico

>
> And you can tell that by realizing that you would now retry once even
> thought the mapping does not support writeback (
> mapping_can_writeback(file->f_mapping)) and no writeback actually happened.
>
> Further, we would also try to call filemap_write_and_wait_range() now
> twice instead of only during the first round.
>
> --
> Cheers,
>
> David
>