Re: [PATCH v3 2/2] mm: Optimize mremap() by PTE batching

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

On 27/05/25 4:15 pm, Lorenzo Stoakes wrote:
> On Tue, May 27, 2025 at 01:20:49PM +0530, Dev Jain wrote:
>> Use folio_pte_batch() to optimize move_ptes(). On arm64, if the ptes
>> are painted with the contig bit, then ptep_get() will iterate through all 16
>> entries to collect a/d bits. Hence this optimization will result in a 16x
>> reduction in the number of ptep_get() calls. Next, ptep_get_and_clear()
>> will eventually call contpte_try_unfold() on every contig block, thus
>> flushing the TLB for the complete large folio range. Instead, use
>> get_and_clear_full_ptes() so as to elide TLBIs on each contig block, and only
>> do them on the starting and ending contig block.
> But you're also making this applicable to non-contpte cases?
>
> See below, but the commit message shoud clearly point out this is general
> for page table split large folios (unless I've missed something of course!
> :)
>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   mm/mremap.c | 40 +++++++++++++++++++++++++++++++++-------
>>   1 file changed, 33 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/mremap.c b/mm/mremap.c
>> index 0163e02e5aa8..580b41f8d169 100644
>> --- a/mm/mremap.c
>> +++ b/mm/mremap.c
>> @@ -170,6 +170,24 @@ static pte_t move_soft_dirty_pte(pte_t pte)
>>   	return pte;
>>   }
>>
>> +/* mremap a batch of PTEs mapping the same large folio */
> I think this comment is fairly useless, it basically spells out the function
> name.
>
> I'd prefer something like 'determine if a PTE contains physically contiguous
> entries which map the same large folio'.

I'd rather prefer dropping the comment altogether, the function is fairly obvious : )


>> +static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr,
>> +		pte_t *ptep, pte_t pte, int max_nr)
>> +{
>> +	const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> +	struct folio *folio;
>> +
>> +	if (max_nr == 1)
>> +		return 1;
>> +
>> +	folio = vm_normal_folio(vma, addr, pte);
>> +	if (!folio || !folio_test_large(folio))
>> +		return 1;
>> +
>> +	return folio_pte_batch(folio, addr, ptep, pte, max_nr, flags, NULL,
>> +			       NULL, NULL);
>> +}
> The code is much better however! :)
>
>> +
>>   static int move_ptes(struct pagetable_move_control *pmc,
>>   		unsigned long extent, pmd_t *old_pmd, pmd_t *new_pmd)
>>   {
>> @@ -177,7 +195,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
>>   	bool need_clear_uffd_wp = vma_has_uffd_without_event_remap(vma);
>>   	struct mm_struct *mm = vma->vm_mm;
>>   	pte_t *old_ptep, *new_ptep;
>> -	pte_t pte;
>> +	pte_t old_pte, pte;
>>   	pmd_t dummy_pmdval;
>>   	spinlock_t *old_ptl, *new_ptl;
>>   	bool force_flush = false;
>> @@ -185,6 +203,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
>>   	unsigned long new_addr = pmc->new_addr;
>>   	unsigned long old_end = old_addr + extent;
>>   	unsigned long len = old_end - old_addr;
>> +	int max_nr_ptes;
>> +	int nr_ptes;
>>   	int err = 0;
>>
>>   	/*
>> @@ -236,12 +256,14 @@ static int move_ptes(struct pagetable_move_control *pmc,
>>   	flush_tlb_batched_pending(vma->vm_mm);
>>   	arch_enter_lazy_mmu_mode();
>>
>> -	for (; old_addr < old_end; old_ptep++, old_addr += PAGE_SIZE,
>> -				   new_ptep++, new_addr += PAGE_SIZE) {
>> -		if (pte_none(ptep_get(old_ptep)))
>> +	for (; old_addr < old_end; old_ptep += nr_ptes, old_addr += nr_ptes * PAGE_SIZE,
>> +		new_ptep += nr_ptes, new_addr += nr_ptes * PAGE_SIZE) {
>> +		nr_ptes = 1;
>> +		max_nr_ptes = (old_end - old_addr) >> PAGE_SHIFT;
>> +		old_pte = ptep_get(old_ptep);
>> +		if (pte_none(old_pte))
>>   			continue;
>>
>> -		pte = ptep_get_and_clear(mm, old_addr, old_ptep);
>>   		/*
>>   		 * If we are remapping a valid PTE, make sure
>>   		 * to flush TLB before we drop the PTL for the
>> @@ -253,8 +275,12 @@ static int move_ptes(struct pagetable_move_control *pmc,
>>   		 * the TLB entry for the old mapping has been
>>   		 * flushed.
>>   		 */
>> -		if (pte_present(pte))
>> +		if (pte_present(old_pte)) {
>> +			nr_ptes = mremap_folio_pte_batch(vma, old_addr, old_ptep,
>> +							 old_pte, max_nr_ptes);
>>   			force_flush = true;
>> +		}
>> +		pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr_ptes, 0);
> Just to clarify, in the previous revision you said:
>
> "Split THPs won't be batched; you can use pte_batch() (from David's refactoring)
> and figure the split THP batch out, but then get_and_clear_full_ptes() will be
> gathering a/d bits and smearing them across the batch, which will be incorrect."
>
> But... this will be triggered for page table split large folio no?
>
> So is there something wrong here or not?

Since I am using folio_pte_batch (and not the hypothetical pte_batch() I was
saying in the other email), the batch must belong to the same folio. Since split
THP means a small folio, nr_ptes will be 1.



>
>>   		pte = move_pte(pte, old_addr, new_addr);
>>   		pte = move_soft_dirty_pte(pte);
>>
>> @@ -267,7 +293,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
>>   				else if (is_swap_pte(pte))
>>   					pte = pte_swp_clear_uffd_wp(pte);
>>   			}
>> -			set_pte_at(mm, new_addr, new_ptep, pte);
>> +			set_ptes(mm, new_addr, new_ptep, pte, nr_ptes);
> The code looks much better here after refactoring, however!
>
>>   		}
>>   	}
>>
>> --
>> 2.30.2
>>