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

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Message-ID: <d64116c5-4851-4658-9b68-fdba3886b4cd@linux.alibaba.com>
Date: Tue, 29 Apr 2025 14:53:35 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Nico Pache <npache@...hat.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
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Subject: Re: [PATCH v4 08/12] khugepaged: skip collapsing mTHP to smaller
 orders



On 2025/4/28 23:44, Nico Pache wrote:
> On Thu, Apr 24, 2025 at 1:49 AM Baolin Wang
> <baolin.wang@...ux.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@....com>
>>> Signed-off-by: Dev Jain <dev.jain@....com>
>>> Signed-off-by: Nico Pache <npache@...hat.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.