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Date: Thu, 13 Jun 2024 10:46:57 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: 21cnbao@...il.com, akpm@...ux-foundation.org,
baolin.wang@...ux.alibaba.com, fengwei.yin@...el.com,
libang.li@...group.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
maskray@...gle.com, mhocko@...e.com, minchan@...nel.org, peterx@...hat.com,
ryan.roberts@....com, shy828301@...il.com, sj@...nel.org,
songmuchun@...edance.com, wangkefeng.wang@...wei.com, willy@...radead.org,
xiehuan09@...il.com, ziy@...dia.com, zokeefe@...gle.com
Subject: Re: [PATCH v7 3/4] mm/rmap: integrate PMD-mapped folio splitting into
pagewalk loop
On 13.06.24 10:45, David Hildenbrand wrote:
> On 13.06.24 10:34, David Hildenbrand wrote:
>> On 10.06.24 14:06, 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>
>>> Suggested-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>> Signed-off-by: Lance Yang <ioworker0@...il.com>
>>> ---
>>> include/linux/huge_mm.h | 6 ++++++
>>> mm/huge_memory.c | 42 +++++++++++++++++++++--------------------
>>> mm/rmap.c | 21 +++++++++++++++------
>>> 3 files changed, 43 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index 088d66a54643..4670c6ee118b 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -415,6 +415,9 @@ 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);
>>> +
>>> #else /* CONFIG_TRANSPARENT_HUGEPAGE */
>>>
>>> static inline bool folio_test_pmd_mappable(struct folio *folio)
>>> @@ -477,6 +480,9 @@ 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) {}
>>>
>>> #define split_huge_pud(__vma, __pmd, __address) \
>>> do { } while (0)
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index e6d26c2eb670..d2697cc8f9d4 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2581,6 +2581,27 @@ 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);
>>
>> Curious: could we actually end up here without a folio right now? That
>> would mean, that try_to_unmap_one() would be called with folio==NULL.
>>
>>> +
>>> + /*
>>> + * 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)
>>> {
>>> @@ -2592,26 +2613,7 @@ 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 ddffa30c79fb..b77f88695588 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
>>> @@ -1668,9 +1665,6 @@ 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.
>>> */
>>> @@ -1682,6 +1676,21 @@ 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, pvmw.address, pvmw.pmd,
>>> + false, folio);
>>> + flags &= ~TTU_SPLIT_HUGE_PMD;
>>> + page_vma_mapped_walk_restart(&pvmw);
>>
>> If, for some reason, split_huge_pmd_locked() would fail, we would keep
>> looping and never hit the VM_BUG_ON_FOLIO() below. Maybe we'd want to
>> let split_huge_pmd_locked() return whether splitting succeeded, and
>> handle that case differently?
>
> I assume it could fail if we race with concurrent split? Or isn't that
> possible?
>
(I assume we hold the PTL, so such races should not be possible)
--
Cheers,
David / dhildenb
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