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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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
>