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Date: Thu, 6 Jun 2024 11:41:09 +0200
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Subject: Re: [PATCH v6 2/3] mm/rmap: integrate PMD-mapped folio splitting into
 pagewalk loop
To: Lance Yang <ioworker0@gmail.com>
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From: David Hildenbrand <david@redhat.com>
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On 06.06.24 11:38, Lance Yang wrote:
> On Thu, Jun 6, 2024 at 4:06 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 06.06.24 10:01, David Hildenbrand wrote:
>>> On 06.06.24 05:55, Lance Yang wrote:
>>>> On Wed, Jun 5, 2024 at 10:28 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>> On 05.06.24 16:20, Lance Yang wrote:
>>>>>> Hi David,
>>>>>>
>>>>>> On Wed, Jun 5, 2024 at 8:46 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>>>
>>>>>>> On 21.05.24 06:02, Lance Yang wrote:
>>>>>>>> In preparation for supporting try_to_unmap_one() to unmap PMD-mapped
>>>>>>>> folios, start the pagewalk first, then call split_huge_pmd_address() to
>>>>>>>> split the folio.
>>>>>>>>
>>>>>>>> Since TTU_SPLIT_HUGE_PMD will no longer perform immediately, we might
>>>>>>>> encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range during
>>>>>>>> the page walk. It’s probably necessary to mlock this THP to prevent it from
>>>>>>>> being picked up during page reclaim.
>>>>>>>>
>>>>>>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>>>>>>> Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>>>>>> Signed-off-by: Lance Yang <ioworker0@gmail.com>
>>>>>>>> ---
>>>>>>>
>>>>>>> [...] again, sorry for the late review.
>>>>>>
>>>>>> No worries at all, thanks for taking time to review!
>>>>>>
>>>>>>>
>>>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>>>>> index ddffa30c79fb..08a93347f283 100644
>>>>>>>> --- a/mm/rmap.c
>>>>>>>> +++ b/mm/rmap.c
>>>>>>>> @@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>>>>>>           if (flags & TTU_SYNC)
>>>>>>>>                   pvmw.flags = PVMW_SYNC;
>>>>>>>>
>>>>>>>> -     if (flags & TTU_SPLIT_HUGE_PMD)
>>>>>>>> -             split_huge_pmd_address(vma, address, false, folio);
>>>>>>>> -
>>>>>>>>           /*
>>>>>>>>            * For THP, we have to assume the worse case ie pmd for invalidation.
>>>>>>>>            * For hugetlb, it could be much worse if we need to do pud
>>>>>>>> @@ -1668,20 +1665,35 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>>>>>>           mmu_notifier_invalidate_range_start(&range);
>>>>>>>>
>>>>>>>>           while (page_vma_mapped_walk(&pvmw)) {
>>>>>>>> -             /* Unexpected PMD-mapped THP? */
>>>>>>>> -             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>>>>>>>> -
>>>>>>>>                   /*
>>>>>>>>                    * If the folio is in an mlock()d vma, we must not swap it out.
>>>>>>>>                    */
>>>>>>>>                   if (!(flags & TTU_IGNORE_MLOCK) &&
>>>>>>>>                       (vma->vm_flags & VM_LOCKED)) {
>>>>>>>>                           /* Restore the mlock which got missed */
>>>>>>>> -                     if (!folio_test_large(folio))
>>>>>>>> +                     if (!folio_test_large(folio) ||
>>>>>>>> +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
>>>>>>>>                                   mlock_vma_folio(folio, vma);
> 
> Should we still keep the '!pvmw.pte' here? Something like:
> 
> if (!folio_test_large(folio) || !pvmw.pte)
>      mlock_vma_folio(folio, vma);

I was wondering the same the whole time ...

> 
> We can mlock the THP to prevent it from being picked up during page reclaim.
> 
> David, I’d like to hear your thoughts on this ;)

but I think there is no need to for now, in the context of your patchset. :)

-- 
Cheers,

David / dhildenb