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

[David Hildenbrand <david@redhat.com>]

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@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?

I assume it could fail if we race with concurrent split? Or isn't that 
possible?

-- 
Cheers,

David / dhildenb