Re: [PATCH v13 mm-new 02/16] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

Lorenzo Stoakes
2026-01-08T17:02:55Z

Sorry I realise it's due to delay in review but, unsurprisingly got a knock-on
merge conflict here:

Looks like it's commit 823953d831d8 ("mm/khugepaged: use enum scan_result for
result variables and return types").

<<<<<<< HEAD

static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *result,
struct collapse_control *cc)
=======
/*
* Try to collapse a single PMD starting at a PMD aligned addr, and return
* the results.
*/
static int 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;
int 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);
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 = 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, int *result,
struct collapse_control *cc)
>>>>>>> introduce collapse_single_pmd to unify khugepaged and madvise_collapse

On Mon, Dec 01, 2025 at 10:46:13AM -0700, 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: 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>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> mm/khugepaged.c | 97 ++++++++++++++++++++++++++-----------------------
> 1 file changed, 52 insertions(+), 45 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 959be77f2e65..433ea7283488 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2392,6 +2392,53 @@ static int collapse_scan_file(struct mm_struct *mm, unsigned long addr,
> return result;
> }
>
> +/*
> + * Try to collapse a single PMD starting at a PMD aligned addr, and return
> + * the results.
> + */
> +static int 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;
> + int 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);
> + 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 = 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, int *result,
> struct collapse_control *cc)
> __releases(&khugepaged_mm_lock)
> @@ -2462,34 +2509,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *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 = 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;
> @@ -2801,35 +2823,20 @@ 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;
> - result = collapse_scan_file(mm, addr, file, pgoff, cc);
> - fput(file);
> - } else {
> - result = collapse_scan_pmd(mm, vma, addr,
> - &mmap_locked, cc);
> - }
> + result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
> +
> if (!mmap_locked)
> *lock_dropped = true;
>
> -handle_result:
> switch (result) {
> case SCAN_SUCCEED:
> case SCAN_PMD_MAPPED:
> ++thps;
> break;
> - case SCAN_PTE_MAPPED_HUGEPAGE:
> - BUG_ON(mmap_locked);
> - mmap_read_lock(mm);
> - result = collapse_pte_mapped_thp(mm, addr, true);
> - mmap_read_unlock(mm);
> - goto handle_result;
> /* Whitelisted set of results where continuing OK */
> case SCAN_NO_PTE_TABLE:
> + case SCAN_PTE_MAPPED_HUGEPAGE:
> case SCAN_PTE_NON_PRESENT:
> case SCAN_PTE_UFFD_WP:
> case SCAN_LACK_REFERENCED_PAGE:
> --
> 2.51.1
>