Re: [PATCH mm-unstable v3 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/11/26 22:13, 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 properly indicates that this process is no longer valid to operate on.
> 
> By moving the madvise_collapse writeback-retry logic into the helper
> function we can also avoid having to revalidate the VMA.
> 
> We also guard the khugepaged_pages_collapsed variable to ensure its only
> incremented for khugepaged.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 120 +++++++++++++++++++++++++-----------------------
>  1 file changed, 63 insertions(+), 57 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 33ae56e313ed..733c4a42c2ce 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2409,6 +2409,65 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  	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;
> +	bool triggered_wb = false;
> +	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;
> +retry:
> +	result = collapse_scan_file(mm, addr, file, pgoff, cc);
> +
> +	/*
> +	 * For MADV_COLLAPSE, when encountering dirty pages, try to writeback,
> +	 * then retry the collapse one time.
> +	 */
> +	if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
> +	    !triggered_wb && 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);
> +		triggered_wb = true;
> +		goto retry;
> +	}
> +	fput(file);
> +
> +	if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> +		mmap_read_lock(mm);
> +		if (collapse_test_exit_or_disable(mm))
> +			result = SCAN_ANY_PROCESS;
> +		else
> +			result = try_collapse_pte_mapped_thp(mm, addr,
> +							     !cc->is_khugepaged);
> +		if (result == SCAN_PMD_MAPPED)
> +			result = SCAN_SUCCEED;
> +		mmap_read_unlock(mm);
> +	}
> +end:
> +	if (cc->is_khugepaged && result == SCAN_SUCCEED)
> +		++khugepaged_pages_collapsed;
> +	return result;
> +}
> +
>  static void collapse_scan_mm_slot(unsigned int progress_max,
>  		enum scan_result *result, struct collapse_control *cc)
>  	__releases(&khugepaged_mm_lock)
> @@ -2479,34 +2538,9 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
>  			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;
>  			if (!mmap_locked)
> @@ -2806,9 +2840,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>  
>  	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
>  		enum scan_result result = SCAN_FAIL;
> -		bool triggered_wb = false;
>  
> -retry:
>  		if (!mmap_locked) {
>  			cond_resched();
>  			mmap_read_lock(mm);
> @@ -2823,46 +2855,20 @@ 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;
> -			*lock_dropped = true;

Okay, we can get rid of this because collapse_single_pmd() will never
drop the lock, to retake it, returning with "mmap_locked == true".

If it dropped the lock, even if it relocked, it will always return with
the lock dropped and "mmap_locked == false".

The "if (!mmap_locked)" will properly set "*lock_dropped = true;".

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David