Re: [PATCH] khugepaged: Optimize collapse_pte_mapped_thp() for large folios by PTE batching

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

On 23/06/25 12:10 pm, Baolin Wang wrote:
>
>
> On 2025/6/18 23:56, Dev Jain wrote:
>> Use PTE batching to optimize collapse_pte_mapped_thp().
>>
>> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for 
>> collapse.
>> Then, calling ptep_clear() for every pte will cause a TLB flush for 
>> every
>> contpte block. Instead, clear_full_ptes() does a
>> contpte_try_unfold_partial() which will flush the TLB only for the 
>> (if any)
>> starting and ending contpte block, if they partially overlap with the 
>> range
>> khugepaged is looking at.
>>
>> For all arches, there should be a benefit due to batching atomic 
>> operations
>> on mapcounts due to folio_remove_rmap_ptes().
>>
>> Note that we do not need to make a change to the check
>> "if (folio_page(folio, i) != page)"; if i'th page of the folio is equal
>> to the first page of our batch, then i + 1, .... i + nr_batch_ptes - 1
>> pages of the folio will be equal to the corresponding pages of our
>> batch mapping consecutive pages.
>>
>> No issues were observed with mm-selftests.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>
>> This is rebased on:
>> https://lore.kernel.org/all/20250618102607.10551-1-dev.jain@arm.com/
>> If there will be a v2 of either version I'll send them together.
>>
>>   mm/khugepaged.c | 38 +++++++++++++++++++++++++-------------
>>   1 file changed, 25 insertions(+), 13 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 649ccb2670f8..7d37058eda5b 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1499,15 +1499,16 @@ static int set_huge_pmd(struct vm_area_struct 
>> *vma, unsigned long addr,
>>   int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>>                   bool install_pmd)
>>   {
>> +    int nr_mapped_ptes = 0, nr_batch_ptes, result = SCAN_FAIL;
>>       struct mmu_notifier_range range;
>>       bool notified = false;
>>       unsigned long haddr = addr & HPAGE_PMD_MASK;
>> +    unsigned long end = haddr + HPAGE_PMD_SIZE;
>>       struct vm_area_struct *vma = vma_lookup(mm, haddr);
>>       struct folio *folio;
>>       pte_t *start_pte, *pte;
>>       pmd_t *pmd, pgt_pmd;
>>       spinlock_t *pml = NULL, *ptl;
>> -    int nr_ptes = 0, result = SCAN_FAIL;
>>       int i;
>>         mmap_assert_locked(mm);
>> @@ -1620,12 +1621,17 @@ int collapse_pte_mapped_thp(struct mm_struct 
>> *mm, unsigned long addr,
>>       if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd))))
>>           goto abort;
>>   +    i = 0, addr = haddr, pte = start_pte;
>>       /* step 2: clear page table and adjust rmap */
>> -    for (i = 0, addr = haddr, pte = start_pte;
>> -         i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
>> +    do {
>> +        const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> +        int max_nr_batch_ptes = (end - addr) >> PAGE_SHIFT;
>> +        struct folio *this_folio;
>>           struct page *page;
>>           pte_t ptent = ptep_get(pte);
>>   +        nr_batch_ptes = 1;
>> +
>>           if (pte_none(ptent))
>>               continue;
>>           /*
>> @@ -1639,6 +1645,11 @@ int collapse_pte_mapped_thp(struct mm_struct 
>> *mm, unsigned long addr,
>>               goto abort;
>>           }
>>           page = vm_normal_page(vma, addr, ptent);
>> +        this_folio = page_folio(page);
>> +        if (folio_test_large(this_folio) && max_nr_batch_ptes != 1)
>> +            nr_batch_ptes = folio_pte_batch(this_folio, addr, pte, 
>> ptent,
>> +                    max_nr_batch_ptes, flags, NULL, NULL, NULL);
>> +
>>           if (folio_page(folio, i) != page)
>>               goto abort;
>
> IMO, 'this_folio' is always equal 'folio', right? Can't we just use 
> 'folio'?

I don't think so. What if we have mremapped some bytes of this PMD range

to point to another folio.

>
> In addition, I think the folio_test_large() and max_nr_batch_ptes 
> checks are redundant, since the 'folio' must be PMD-sized large folio 
> after 'folio_page(folio, i) != page' check.

As an improvement we can at least do likely(folio_test_large()) since 
this is very likely.


>
> So I think we can move the 'nr_batch_ptes' calculation after the 
> folio_page() check, then shoule be:
>
> nr_batch_ptes = folio_pte_batch(folio, addr, pte, ptent,
>             max_nr_batch_ptes, flags, NULL, NULL, NULL);
>
>> @@ -1647,18 +1658,19 @@ int collapse_pte_mapped_thp(struct mm_struct 
>> *mm, unsigned long addr,
>>            * TLB flush can be left until pmdp_collapse_flush() does it.
>>            * PTE dirty? Shmem page is already dirty; file is read-only.
>>            */
>> -        ptep_clear(mm, addr, pte);
>> -        folio_remove_rmap_pte(folio, page, vma);
>> -        nr_ptes++;
>> -    }
>> +        clear_full_ptes(mm, addr, pte, nr_batch_ptes, false);
>> +        folio_remove_rmap_ptes(folio, page, nr_batch_ptes, vma);
>> +        nr_mapped_ptes += nr_batch_ptes;
>> +    } while (i += nr_batch_ptes, addr += nr_batch_ptes * PAGE_SIZE,
>> +         pte += nr_batch_ptes, i < HPAGE_PMD_NR);
>>         if (!pml)
>>           spin_unlock(ptl);
>>         /* step 3: set proper refcount and mm_counters. */
>> -    if (nr_ptes) {
>> -        folio_ref_sub(folio, nr_ptes);
>> -        add_mm_counter(mm, mm_counter_file(folio), -nr_ptes);
>> +    if (nr_mapped_ptes) {
>> +        folio_ref_sub(folio, nr_mapped_ptes);
>> +        add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes);
>>       }
>>         /* step 4: remove empty page table */
>> @@ -1691,10 +1703,10 @@ int collapse_pte_mapped_thp(struct mm_struct 
>> *mm, unsigned long addr,
>>               : SCAN_SUCCEED;
>>       goto drop_folio;
>>   abort:
>> -    if (nr_ptes) {
>> +    if (nr_mapped_ptes) {
>>           flush_tlb_mm(mm);
>> -        folio_ref_sub(folio, nr_ptes);
>> -        add_mm_counter(mm, mm_counter_file(folio), -nr_ptes);
>> +        folio_ref_sub(folio, nr_mapped_ptes);
>> +        add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes);
>>       }
>>   unlock:
>>       if (start_pte)
>