Re: [PATCH mm-unstable v2 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2/26/26 9:29 AM, 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 properly indicates that this process is no longer valid to operate on.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>
> ---

[snip]

>   	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);
>   			mmap_locked = true;
> +			*lock_dropped = true;
IIUC, this should be '*lock_dropped = false', right?

>   			result = hugepage_vma_revalidate(mm, addr, false, &vma,
>   							 cc);
>   			if (result  != SCAN_SUCCEED) {
> @@ -2836,46 +2872,20 @@ 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;
> -			result = collapse_scan_file(mm, addr, file, pgoff, NULL, 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, NULL, 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, NULL, 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 = try_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: