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

[Lance Yang <lance.yang@linux.dev>]

On 2026/2/13 04:26, Nico Pache wrote:
> On Thu, Feb 12, 2026 at 1:04 PM David Hildenbrand (Arm)
> <david@kernel.org> 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 properly indicates that this process is no longer valid to operate on.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@oracle.com>
>>> Signed-off-by: Nico Pache <npache@redhat.com>
>>> ---
>>>    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.

Good catch!

> 
> 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)

Yep, we're on the right track. IIRC, David's concern has two parts:

1) Avoid retry when writeback wasn't actually triggered
    (mapping_can_writeback() fails)
2) Avoid calling filemap_write_and_wait_range() twice on retry

The proposed approach below addresses #1, but we still need to tackle #2.

The issue is that on the retry, collapse_single_pmd() doesn't know that
writeback was already performed in the previous round, so it could call
filemap_write_and_wait_range() again if the page is stil dirty.

> 
> 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.

Right. Let's avoid calling it twice.


Cheers,
Lance