Re: [RFC PATCH 08/12] khugepaged: Abstract PMD-THP collapse

[Dev Jain <dev.jain@arm.com>]

On 18/12/24 12:54 am, Ryan Roberts wrote:
> On 16/12/2024 16:51, Dev Jain wrote:
>> Abstract away taking the mmap_lock exclusively, copying page contents, and
>> setting the PMD, into vma_collapse_anon_folio_pmd().
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   mm/khugepaged.c | 119 +++++++++++++++++++++++++++---------------------
>>   1 file changed, 66 insertions(+), 53 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 078794aa3335..88beebef773e 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1111,58 +1111,17 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
>>   	return SCAN_SUCCEED;
>>   }
>>   
>> -static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>> -			      int referenced, int unmapped, int order,
>> -			      struct collapse_control *cc)
>> +static int vma_collapse_anon_folio_pmd(struct mm_struct *mm, unsigned long address,
>> +		struct vm_area_struct *vma, struct collapse_control *cc, pmd_t *pmd,
>> +		struct folio *folio)
>>   {
>> +	struct mmu_notifier_range range;
>> +	spinlock_t *pmd_ptl, *pte_ptl;
>>   	LIST_HEAD(compound_pagelist);
>> -	pmd_t *pmd, _pmd;
>> -	pte_t *pte;
>>   	pgtable_t pgtable;
>> -	struct folio *folio;
>> -	spinlock_t *pmd_ptl, *pte_ptl;
>> -	int result = SCAN_FAIL;
>> -	struct vm_area_struct *vma;
>> -	struct mmu_notifier_range range;
>> -
>> -	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>> -
>> -	/*
>> -	 * Before allocating the hugepage, release the mmap_lock read lock.
>> -	 * The allocation can take potentially a long time if it involves
>> -	 * sync compaction, and we do not need to hold the mmap_lock during
>> -	 * that. We will recheck the vma after taking it again in write mode.
>> -	 */
>> -	mmap_read_unlock(mm);
>> -
>> -	result = alloc_charge_folio(&folio, mm, order, cc);
>> -	if (result != SCAN_SUCCEED)
>> -		goto out_nolock;
>> -
>> -	mmap_read_lock(mm);
>> -	result = hugepage_vma_revalidate(mm, address, true, &vma, order, cc);
>> -	if (result != SCAN_SUCCEED) {
>> -		mmap_read_unlock(mm);
>> -		goto out_nolock;
>> -	}
>> -
>> -	result = find_pmd_or_thp_or_none(mm, address, &pmd);
>> -	if (result != SCAN_SUCCEED) {
>> -		mmap_read_unlock(mm);
>> -		goto out_nolock;
>> -	}
>> -
>> -	if (unmapped) {
>> -		/*
>> -		 * __collapse_huge_page_swapin will return with mmap_lock
>> -		 * released when it fails. So we jump out_nolock directly in
>> -		 * that case.  Continuing to collapse causes inconsistency.
>> -		 */
>> -		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
>> -						     referenced, order);
>> -		if (result != SCAN_SUCCEED)
>> -			goto out_nolock;
>> -	}
>> +	int result;
>> +	pmd_t _pmd;
>> +	pte_t *pte;
>>   
>>   	mmap_read_unlock(mm);
>>   	/*
>> @@ -1174,7 +1133,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>>   	 * mmap_lock.
>>   	 */
>>   	mmap_write_lock(mm);
>> -	result = hugepage_vma_revalidate(mm, address, true, &vma, order, cc);
>> +
>> +	result = hugepage_vma_revalidate(mm, address, true, &vma, HPAGE_PMD_ORDER, cc);
>>   	if (result != SCAN_SUCCEED)
>>   		goto out_up_write;
>>   	/* check if the pmd is still valid */
>> @@ -1206,7 +1166,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>>   	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
>>   	if (pte) {
>>   		result = __collapse_huge_page_isolate(vma, address, pte, cc,
>> -						      &compound_pagelist, order);
>> +						      &compound_pagelist, HPAGE_PMD_ORDER);
>>   		spin_unlock(pte_ptl);
>>   	} else {
>>   		result = SCAN_PMD_NULL;
>> @@ -1262,11 +1222,64 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>>   	deferred_split_folio(folio, false);
>>   	spin_unlock(pmd_ptl);
>>   
>> -	folio = NULL;
>> -
>>   	result = SCAN_SUCCEED;
>>   out_up_write:
>>   	mmap_write_unlock(mm);
>> +	return result;
>> +}
>> +
>> +static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>> +			      int referenced, int unmapped, int order,
>> +			      struct collapse_control *cc)
>> +{
>> +	struct vm_area_struct *vma;
>> +	int result = SCAN_FAIL;
>> +	struct folio *folio;
>> +	pmd_t *pmd;
>> +
>> +	/*
>> +	 * Before allocating the hugepage, release the mmap_lock read lock.
>> +	 * The allocation can take potentially a long time if it involves
>> +	 * sync compaction, and we do not need to hold the mmap_lock during
>> +	 * that. We will recheck the vma after taking it again in write mode.
>> +	 */
>> +	mmap_read_unlock(mm);
>> +
>> +	result = alloc_charge_folio(&folio, mm, order, cc);
>> +	if (result != SCAN_SUCCEED)
>> +		goto out_nolock;
>> +
>> +	mmap_read_lock(mm);
>> +	result = hugepage_vma_revalidate(mm, address, true, &vma, order, cc);
>> +	if (result != SCAN_SUCCEED) {
>> +		mmap_read_unlock(mm);
>> +		goto out_nolock;
>> +	}
>> +
>> +	result = find_pmd_or_thp_or_none(mm, address, &pmd);
>> +	if (result != SCAN_SUCCEED) {
>> +		mmap_read_unlock(mm);
>> +		goto out_nolock;
>> +	}
>> +
>> +	if (unmapped) {
>> +		/*
>> +		 * __collapse_huge_page_swapin will return with mmap_lock
>> +		 * released when it fails. So we jump out_nolock directly in
>> +		 * that case.  Continuing to collapse causes inconsistency.
>> +		 */
>> +		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
>> +						     referenced, order);
>> +		if (result != SCAN_SUCCEED)
>> +			goto out_nolock;
>> +	}
>> +
>> +	if (order == HPAGE_PMD_ORDER)
>> +		result = vma_collapse_anon_folio_pmd(mm, address, vma, cc, pmd, folio);
> I think the locking is broken here? collapse_huge_page() used to enter with the
> mmamp read lock and exit without the lock held at all. After the change, this is
> only true for order == HPAGE_PMD_ORDER. For other orders, you exit with the mmap
> read lock still held. Perhaps:
>
> 	else
> 		mmap_read_unlock(mm);

I have completely messed up sequential building of patches :) I will take care next time.

>
>> +
>> +	if (result == SCAN_SUCCEED)
>> +		folio = NULL;
>> +
>>   out_nolock:
>>   	if (folio)
>>   		folio_put(folio);