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Message-ID: <0c20196b-f5bd-4238-bbb9-316f6ac3078e@arm.com>
Date: Mon, 23 Jun 2025 12:46:50 +0530
From: Dev Jain <dev.jain@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
david@...hat.com
Cc: ziy@...dia.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] khugepaged: Optimize collapse_pte_mapped_thp() for large
folios by PTE batching
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@....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)
>
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