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Message-ID: <8109236a-9288-4935-8321-dbff361dc529@arm.com>
Date: Thu, 26 Jun 2025 09:18:47 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
baolin.wang@...ux.alibaba.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 v2 2/3] khugepaged: Optimize collapse_pte_mapped_thp() for
large folios by PTE batching
On 25/06/25 6:41 pm, Lorenzo Stoakes wrote:
> On Wed, Jun 25, 2025 at 11:28:05AM +0530, 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>
>> ---
>> mm/khugepaged.c | 38 ++++++++++++++++++++++++++------------
>> 1 file changed, 26 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 3944b112d452..4c8d33abfbd8 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);
>> @@ -1621,11 +1622,17 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>> goto abort;
>>
>> /* 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++) {
>> + for (i = 0, addr = haddr, pte = start_pte; i < HPAGE_PMD_NR;
>> + i += nr_batch_ptes, addr += nr_batch_ptes * PAGE_SIZE,
>> + pte += nr_batch_ptes) {
>> + const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> + int max_nr_batch_ptes = (end - addr) >> PAGE_SHIFT;
>> + struct folio *mapped_folio;
>> struct page *page;
>> pte_t ptent = ptep_get(pte);
>>
>> + nr_batch_ptes = 1;
>> +
>> if (pte_none(ptent))
>> continue;
>> /*
>> @@ -1639,26 +1646,33 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>> goto abort;
>> }
>> page = vm_normal_page(vma, addr, ptent);
>> + mapped_folio = page_folio(page);
>> +
>> if (folio_page(folio, i) != page)
>> goto abort;
> Isn't this asserting that folio == mapped_folio here? We're saying page is the
> ith page of folio, so why do we need to look up mapped_folio?
We need to check for all PTEs whether they map the right page or not. This may
get disturbed due to mremap and stuff.
>
>> + mapped_folio = page_folio(page);
> You're assigning this twice.
Forgot to remove, thanks.
>
>> + nr_batch_ptes = folio_pte_batch(mapped_folio, addr, pte, ptent,
>> + max_nr_batch_ptes, flags,
>> + NULL, NULL, NULL);
>> +
>> /*
>> * Must clear entry, or a racing truncate may re-remove it.
>> * 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, /* full = */ false);
>> + folio_remove_rmap_ptes(folio, page, nr_batch_ptes, vma);
>> + nr_mapped_ptes += nr_batch_ptes;
>> }
>>
>> 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 +1705,10 @@ 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) {
> I know it's ironic coming from me :P but I'm not sure why we need to churn this
> up by renaming?
Because nr_ptes is an existing variable and I need a new variable to make
the jump at the end of the PTE batch.
>
>> 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)
>> --
>> 2.30.2
>>
> V
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