Re: [PATCH v4 08/12] khugepaged: skip collapsing mTHP to smaller orders

Baolin Wang
2025-04-29T06:53:46Z

On 2025/4/28 23:44, Nico Pache wrote:
> On Thu, Apr 24, 2025 at 1:49 AM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2025/4/17 08:02, Nico Pache wrote:
>>> khugepaged may try to collapse a mTHP to a smaller mTHP, resulting in
>>> some pages being unmapped. Skip these cases until we have a way to check
>>> if its ok to collapse to a smaller mTHP size (like in the case of a
>>> partially mapped folio).
>>>
>>> This patch is inspired by Dev Jain's work on khugepaged mTHP support [1].
>>>
>>> [1] https://lore.kernel.org/lkml/20241216165105.56185-11-dev.jain@arm.com/
>>>
>>> Co-developed-by: Dev Jain <dev.jain@arm.com>
>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>> Signed-off-by: Nico Pache <npache@redhat.com>
>>> ---
>>> mm/khugepaged.c | 7 ++++++-
>>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index ece39fd71fe6..383aff12cd43 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -625,7 +625,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>>> folio = page_folio(page);
>>> VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
>>>
>>> - /* See hpage_collapse_scan_pmd(). */
>>> + if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) {
>>> + result = SCAN_PTE_MAPPED_HUGEPAGE;
>>> + goto out;
>>> + }
>>
>> Should we also add this check in hpage_collapse_scan_pmd() to abort the
>> scan early?
> No I dont think so, we can't abort there because we dont know the
> attempted collapse order, and we dont want to miss potential mTHP
> collapses (by bailing out early and not populating the bitmap).

OK. That makes sense.