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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/6/23 15:16, Dev Jain wrote:
> 
> 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.

Then 'folio_page(folio, i) != page' can catch this, which is why I 
suggest you move the 'nr_batch_ptes' calculation after the folio_page() 
check.

>> 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)
>>