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Date: Thu, 13 Jun 2024 10:34:45 +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 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?
> + continue;
> + }
> +
> + /* Unexpected PMD-mapped THP? */
> + VM_BUG_ON_FOLIO(!pvmw.pte, folio);
> +
> pfn = pte_pfn(ptep_get(pvmw.pte));
> subpage = folio_page(folio, pfn - folio_pfn(folio));
> address = pvmw.address;
--
Cheers,
David / dhildenb
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