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Message-ID: <086fae7a-87ae-4da2-a10b-f01af184c418@arm.com>
Date: Mon, 23 Jun 2025 12:55:56 +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 12:51 pm, Baolin Wang wrote:
>
>
> 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@....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.
Ah. page will be part of the folio, so the folio derived from page is
equal to folio, so it is large. Thanks.
>
>>> 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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