Re: [PATCH v7 3/4] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

[David Hildenbrand <david@redhat.com>]

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@redhat.com>
> Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: Lance Yang <ioworker0@gmail.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