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Message-ID: <30b7eb92-c1cc-4cab-b056-1200938f3633@arm.com>
Date: Thu, 26 Jun 2025 10:24:16 +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 26/06/25 10:17 am, Lorenzo Stoakes wrote:
> On Thu, Jun 26, 2025 at 09:18:47AM +0530, Dev Jain wrote:
>> 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.
> Right but I'm saying mapped_folio == folio right? You're literally asserting it
> here? So there's no need to assign mapped_folio at all, just reference folio no?
Ah I get you now, we can do the PTE batching directly on the folio, thanks.
>
>>>> + 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.
> I thought you eliminated nr_ptes as a variable here? Where else is it used?
>
> Oh how this code needs refactoring...
>
>>>> 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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