Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

[Zi Yan <ziy@nvidia.com>]

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On 8 May 2024, at 10:35, Lance Yang wrote:

> On Wed, May 8, 2024 at 10:07 PM Zi Yan <ziy@nvidia.com> wrote:
>>
>> On 8 May 2024, at 1:43, Lance Yang wrote:
>>
>>> On Tue, May 7, 2024 at 11:26 PM Zi Yan <ziy@nvidia.com> wrote:
>>>>
>>>> On 1 May 2024, at 0:26, 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.
>>>>>
>>>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>>>> Signed-off-by: Lance Yang <ioworker0@gmail.com>
>>>>> ---
>>>>>  include/linux/huge_mm.h | 20 ++++++++++++++++++++
>>>>>  mm/huge_memory.c        | 42 +++++++++++++++++++++--------------------
>>>>>  mm/rmap.c               | 24 +++++++++++++++++------
>>>>>  3 files changed, 60 insertions(+), 26 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>> index c8d3ec116e29..38c4b5537715 100644
>>>>> --- a/include/linux/huge_mm.h
>>>>> +++ b/include/linux/huge_mm.h
>>>>> @@ -409,6 +409,20 @@ static inline bool thp_migration_supported(void)
>>>>>       return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
>>>>>  }
>>>>>
>>>>> +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
>>>>> +                        pmd_t *pmd, bool freeze, struct folio *folio);
>>>>> +
>>>>> +static inline void align_huge_pmd_range(struct vm_area_struct *vma,
>>>>> +                                     unsigned long *start,
>>>>> +                                     unsigned long *end)
>>>>> +{
>>>>> +     *start = ALIGN(*start, HPAGE_PMD_SIZE);
>>>>> +     *end = ALIGN_DOWN(*end, HPAGE_PMD_SIZE);
>>>>> +
>>>>> +     VM_WARN_ON_ONCE(vma->vm_start > *start);
>>>>> +     VM_WARN_ON_ONCE(vma->vm_end < *end);
>>>>> +}
>>>>> +
>>>>>  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
>>>>>
>>>>>  static inline bool folio_test_pmd_mappable(struct folio *folio)
>>>>> @@ -471,6 +485,12 @@ static inline void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>               unsigned long address, bool freeze, struct folio *folio) {}
>>>>>  static inline void split_huge_pmd_address(struct vm_area_struct *vma,
>>>>>               unsigned long address, bool freeze, struct folio *folio) {}
>>>>> +static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
>>>>> +                                      unsigned long address, pmd_t *pmd,
>>>>> +                                      bool freeze, struct folio *folio) {}
>>>>> +static inline void align_huge_pmd_range(struct vm_area_struct *vma,
>>>>> +                                     unsigned long *start,
>>>>> +                                     unsigned long *end) {}
>>>>>
>>>>>  #define split_huge_pud(__vma, __pmd, __address)      \
>>>>>       do { } while (0)
>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>> index 8261b5669397..145505a1dd05 100644
>>>>> --- a/mm/huge_memory.c
>>>>> +++ b/mm/huge_memory.c
>>>>> @@ -2584,6 +2584,27 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>       pmd_populate(mm, pmd, pgtable);
>>>>>  }
>>>>>
>>>>> +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
>>>>> +                        pmd_t *pmd, bool freeze, struct folio *folio)
>>>>> +{
>>>>> +     VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio));
>>>>> +     VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
>>>>> +     VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
>>>>> +     VM_BUG_ON(freeze && !folio);
>>>>> +
>>>>> +     /*
>>>>> +      * When the caller requests to set up a migration entry, we
>>>>> +      * require a folio to check the PMD against. Otherwise, there
>>>>> +      * is a risk of replacing the wrong folio.
>>>>> +      */
>>>>> +     if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
>>>>> +         is_pmd_migration_entry(*pmd)) {
>>>>> +             if (folio && folio != pmd_folio(*pmd))
>>>>> +                     return;
>>>>> +             __split_huge_pmd_locked(vma, pmd, address, freeze);
>>>>> +     }
>>>>> +}
>>>>> +
>>>>>  void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>               unsigned long address, bool freeze, struct folio *folio)
>>>>>  {
>>>>> @@ -2595,26 +2616,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>                               (address & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE);
>>>>>       mmu_notifier_invalidate_range_start(&range);
>>>>>       ptl = pmd_lock(vma->vm_mm, pmd);
>>>>> -
>>>>> -     /*
>>>>> -      * If caller asks to setup a migration entry, we need a folio to check
>>>>> -      * pmd against. Otherwise we can end up replacing wrong folio.
>>>>> -      */
>>>>> -     VM_BUG_ON(freeze && !folio);
>>>>> -     VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
>>>>> -
>>>>> -     if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
>>>>> -         is_pmd_migration_entry(*pmd)) {
>>>>> -             /*
>>>>> -              * It's safe to call pmd_page when folio is set because it's
>>>>> -              * guaranteed that pmd is present.
>>>>> -              */
>>>>> -             if (folio && folio != pmd_folio(*pmd))
>>>>> -                     goto out;
>>>>> -             __split_huge_pmd_locked(vma, pmd, range.start, freeze);
>>>>> -     }
>>>>> -
>>>>> -out:
>>>>> +     split_huge_pmd_locked(vma, range.start, pmd, freeze, folio);
>>>>>       spin_unlock(ptl);
>>>>>       mmu_notifier_invalidate_range_end(&range);
>>>>>  }
>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>> index 7e2575d669a9..432601154583 100644
>>>>> --- a/mm/rmap.c
>>>>> +++ b/mm/rmap.c
>>>>> @@ -1636,9 +1636,6 @@ 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
>>>>> @@ -1650,6 +1647,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>>>       range.end = vma_address_end(&pvmw);
>>>>>       mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
>>>>>                               address, range.end);
>>>>> +     if (flags & TTU_SPLIT_HUGE_PMD)
>>>>> +             align_huge_pmd_range(vma, &range.start, &range.end);
>>>>>       if (folio_test_hugetlb(folio)) {
>>>>>               /*
>>>>>                * If sharing is possible, start and end will be adjusted
>>>>> @@ -1664,9 +1663,6 @@ 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.
>>>>>                */
>>>>> @@ -1678,6 +1674,22 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>>>                       goto walk_done_err;
>>>>>               }
>>>>>
>>>>> +             if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
>>>>> +                     /*
>>>>> +                      * We temporarily have to drop the PTL and start once
>>>>> +                      * again from that now-PTE-mapped page table.
>>>>> +                      */
>>>>> +                     split_huge_pmd_locked(vma, range.start, pvmw.pmd, false,
>>>>> +                                           folio);
>>>>
>>>> Just in case you might miss here, since you will no longer align
>>>> range.start as Baolin mentioned in another email and you have a VM_WARN_ONCE
>>>> in split_huge_pmd_locked(), you will need to align the input address now.
>>>
>>> Thanks for bringing that up!
>>>
>>> I do miss the alignment here when I decide to no longer align range.start
>>> in another email - thanks!
>>>
>> No problem.
>>
>>> Zi, could I move the alignment here?
>>> IIUC, we will not encounter a partially mapped THP here, and range.start
>>> and range.end should also not beyond the VMA limits.
>>>
>>>                      align_huge_pmd_range(vma, &range.start, &range.end);
>>>                      split_huge_pmd_locked(vma, range.start, pvmw.pmd, false,
>>>                                            folio);
>>
>> I think you can just do
>>
>> split_huge_pmd_locked(vma, ALIGN(range.start, HPAGE_PMD_SIZE), pvmw.pmd, false, folio);
>>
>> since range will later be used by mmu_notifier_invalidate_range_end() and changing
>> it might cause secondary TLB invalidation issues.
>
> Ok, makes sense to me - thanks!
>
> But we probably cannot use the HPAGE_PMD_SIZE here; it will cause
> broken compilation as seen in v3[1].
>
> Perhaps we still need to add a new alignment function for the huge PMD?
>
> [1] https://lore.kernel.org/linux-mm/20240429202040.187453-1-sj@kernel.org/
>
> Thanks again for the review!
>
Or you can adjust the alignment inside split_huge_pmd_locked(), since it can
be called other than __split_huge_pmd().

Hmm, I notice that split_huge_pmd_address() has mmu_notifier ops but your
split_huge_pmd_locked() does not include them, I wonder if that could cause
issues with mmu_notifier issues. Adding mmu_notifier people to confirm.


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Best Regards,
Yan, Zi

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