Re: [PATCH mm-new v5 4/5] mm: khugepaged: skip lazy-free folios

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

On 2026/1/23 23:08, Vernon Yang wrote:
> On Fri, Jan 23, 2026 at 5:09 PM Lance Yang <lance.yang@linux.dev> wrote:
>>
>> On 2026/1/23 16:22, Vernon Yang wrote:
>>> From: Vernon Yang <yanglincheng@kylinos.cn>
>>>

[...]

>>> @@ -583,6 +584,11 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>>>                folio = page_folio(page);
>>>                VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
>>>
>>> +             if (!pte_dirty(pteval) && folio_test_lazyfree(folio)) {
>>
>> I'm wondering if we need "cc->is_khugepaged &&" as well here?
>>
>> We should allow users to enforce collapse via the madvise_collapse()
>> path even if pages are marked lazyfree, IMHO.
> 
> $ man madvise
> MADV_COLLAPSE
>          Perform a best-effort synchronous collapse of the native pages
>          mapped by the memory range into Transparent Huge Pages (THPs).
> 
> The semantics of MADV_COLLAPSE are best-effort and do not imply to enforce
> collapsing, so we don't need "cc->is_khugepaged" here.
> 
> We can imagine that if a user simultaneously uses MADV_FREE and
> MADV_COLLAPSE, it indicates a misunderstanding of their semantics.
> As the kernel, we need to safeguard the baseline.

No. Afraid I don't think so.

To be clear, what I meant by "enforce":

Yep, MADV_COLLAPSE is best-effort - it can fail. But when users
call MADV_COLLAPSE, they're explicitly asking for collapse.

Compared to khugepaged just scanning around, that's already "enforce"
- users are actively requesting it, not passively waiting for.

Note that you're *breaking* userspace. Users would not be able
to collapse the range where there are any lazyfree pages anymore,
even when they explicitly call MADV_COLLAPSE.

For khugepaged, skipping lazyfree makes sense.

> 
>>> +                     result = SCAN_PAGE_LAZYFREE;
>>> +                     goto out;
>>> +             }
>>> +
>>>                /* See hpage_collapse_scan_pmd(). */
>>>                if (folio_maybe_mapped_shared(folio)) {
>>>                        ++shared;
>>> @@ -1330,6 +1336,11 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
>>>                }
>>>                folio = page_folio(page);
>>>
>>> +             if (!pte_dirty(pteval) && folio_test_lazyfree(folio)) {
>>
>> Ditto.
>>
>>> +                     result = SCAN_PAGE_LAZYFREE;
>>> +                     goto out_unmap;
>>> +             }
>>> +
>>>                if (!folio_test_anon(folio)) {
>>>                        result = SCAN_PAGE_ANON;
>>>                        goto out_unmap;
>>