On Thu, Jun 11, 2026 at 08:42:28AM +0800, Baolin Wang wrote:
>
>
>On 6/10/26 8:44 PM, Lance Yang wrote:
>>
>> On Wed, Jun 10, 2026 at 06:29:12PM +0800, Baolin Wang wrote:
>> [...]
>>> @@ -1512,8 +1517,12 @@ static enum scan_result mthp_collapse(struct
>>> mm_struct *mm,
>>> enum scan_result ret;
>>>
>>> collapse_address = address + offset * PAGE_SIZE;
>>> - ret = collapse_huge_page(mm, collapse_address,
>>> referenced,
>>> - unmapped, cc, order);
>>> + if (file)
>>> + ret = collapse_file(mm, collapse_address, file,
>>> + start + offset, cc, order);
>>> + else
>>> + ret = collapse_huge_page(mm, collapse_address,
>>> + referenced, unmapped, cc,
>>> order);
>>>
>>> switch (ret) {
>>> /* Cases where we continue to next collapse candidate */
>>> @@ -1521,6 +1530,7 @@ static enum scan_result mthp_collapse(struct
>>> mm_struct *mm,
>>> collapsed += nr_ptes;
>>> fallthrough;
>>> case SCAN_PTE_MAPPED_HUGEPAGE:
>>
>> Looks like SCAN_PTE_MAPPED_HUGEPAGE from collapse_file() get lost for
>> the PMD-order case.
>>
>> Previously, collapse_file() returned it straight back to
>> collapse_single_pmd(), so we would run try_collapse_pte_mapped_thp().
>>
>> Now it hits mthp_collapse() fitst, and that case just goes to
>> next_offset ...
>
>This is the expected behavior. SCAN_PTE_MAPPED_HUGEPAGE is only returned
>when the MADV_COLLAPSE collapse succeeds. In this case, if
>collapse_file() still returns SCAN_PTE_MAPPED_HUGEPAGE, then
>mthp_collapse() will ultimately return SCAN_FAIL (because collapsed ==
>0), even though the MADV_COLLAPSE collapse has already succeeded.
>
>Additionally, the SCAN_PTE_MAPPED_HUGEPAGE logic is already handled in
>collapse_scan_file(), see:
>
>result = mthp_collapse(mm, file, start, addr, 0, 0, cc, enabled_orders);
>if (result == SCAN_SUCCEED && !cc->is_khugepaged) {
> /* If MADV_COLLAPSE, adjust result to call
>collapse_pte_mapped_thp(). */
> result = SCAN_PTE_MAPPED_HUGEPAGE;
>}
Right, I agree for that case. The one I meant is a bit different.
I was looking at this earlier return from collapse_file():
if (is_pmd_order(folio_order(folio))) {
result = SCAN_PTE_MAPPED_HUGEPAGE;
goto out_unlock;
}
Old code let it bubble back up:
collapse_single_pmd()
-> collapse_scan_file()
-> collapse_file()
-> SCAN_PTE_MAPPED_HUGEPAGE
-> SCAN_PTE_MAPPED_HUGEPAGE
-> try_collapse_pte_mapped_thp()
Not limited to !is_khugepaged. collapse_single_pmd() only checked the
result, then passed !cc->is_khugepaged as install_pmd.
Now the same return goes through mthp_collapse() first:
collapse_single_pmd()
-> collapse_scan_file()
-> mthp_collapse()
-> collapse_file()
-> SCAN_PTE_MAPPED_HUGEPAGE
-> goto next_offset
So collapse_single_pmd() never sees SCAN_PTE_MAPPED_HUGEPAGE for this
case anymore. Hopefully Im not missing something here :)
Cheers, Lance
