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Date: Wed, 08 May 2024 10:48:02 -0400
From: Zi Yan <ziy@...dia.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org,
maskray@...gle.com, ryan.roberts@....com, david@...hat.com,
21cnbao@...il.com, mhocko@...e.com, fengwei.yin@...el.com,
zokeefe@...gle.com, shy828301@...il.com, xiehuan09@...il.com,
libang.li@...group.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com,
peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Alistair Popple <apopple@...dia.com>, Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into
pagewalk loop
On 8 May 2024, at 10:35, Lance Yang wrote:
> On Wed, May 8, 2024 at 10:07 PM Zi Yan <ziy@...dia.com> wrote:
>>
>> On 8 May 2024, at 1:43, Lance Yang wrote:
>>
>>> On Tue, May 7, 2024 at 11:26 PM Zi Yan <ziy@...dia.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@...hat.com>
>>>>> Signed-off-by: Lance Yang <ioworker0@...il.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.
--
Best Regards,
Yan, Zi
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