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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Jun 2025 07:17:29.3972 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0c80a2d4-7a71-4261-0317-08ddb2260393 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AMS1EPF00000041.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS4PR08MB8219 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 >> --- >> >> 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) >