From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8B59B201270; Wed, 5 Mar 2025 09:08:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741165697; cv=none; b=An0FQ7nQng8SDR7CWWhHCzCYDH8wDCst1S+3wZ6UIZ9jZDLTiEOCeRcLPwU+7Qwlx595056o7NGZlgnBini7abv1E/EILmOf7hCyGwCEAYiwd9jJXHeSH2PCuDtxsJMJaN8pnbY8aVRZqWqx5tMzIXwZNgE0+gFECG0/A5nAcwo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741165697; c=relaxed/simple; bh=g7nZ2uqeH8LwJ98yMTZXa6Mh806OPQyUp5YRPfaTrPk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QcXLcS4EkUYte5sfaxbif76Uozwx5T/cjO6hddrmao1TvTaJCr0HqQPBHMaBArnyyfmimD/LqmGrpJKwxFJaEGgFSRB3Bk/ZEkhQiiqqRda8vHnhk5/QQXiYF5LSV4GCKzGnHfnXDie3Z5HS9YLZbMPC0GMglgXNJmZBZJYKGyk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 035C5FEC; Wed, 5 Mar 2025 01:08:28 -0800 (PST) Received: from [10.162.42.28] (K4MQJ0H1H2.blr.arm.com [10.162.42.28]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id F32E03F66E; Wed, 5 Mar 2025 01:08:00 -0800 (PST) Message-ID: Date: Wed, 5 Mar 2025 14:37:57 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 7/9] khugepaged: add mTHP support To: Ryan Roberts , Nico Pache , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org Cc: anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, jack@suse.cz, srivatsa@csail.mit.edu, haowenchao22@gmail.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, ioworker0@gmail.com, wangkefeng.wang@huawei.com, ziy@nvidia.com, jglisse@google.com, surenb@google.com, vishal.moola@gmail.com, zokeefe@google.com, zhengqi.arch@bytedance.com, jhubbard@nvidia.com, 21cnbao@gmail.com, willy@infradead.org, kirill.shutemov@linux.intel.com, david@redhat.com, aarcange@redhat.com, raquini@redhat.com, sunnanyong@huawei.com, usamaarif642@gmail.com, audra@redhat.com, akpm@linux-foundation.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, tiwai@suse.de References: <20250211003028.213461-1-npache@redhat.com> <20250211003028.213461-8-npache@redhat.com> <0319c841-cde9-42f6-a230-39b050659f1a@arm.com> Content-Language: en-US From: Dev Jain In-Reply-To: <0319c841-cde9-42f6-a230-39b050659f1a@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 19/02/25 10:22 pm, Ryan Roberts wrote: > On 11/02/2025 00:30, Nico Pache wrote: >> Introduce the ability for khugepaged to collapse to different mTHP sizes. >> While scanning a PMD range for potential collapse candidates, keep track >> of pages in MIN_MTHP_ORDER chunks via a bitmap. Each bit represents a >> utilized region of order MIN_MTHP_ORDER ptes. We remove the restriction >> of max_ptes_none during the scan phase so we dont bailout early and miss >> potential mTHP candidates. >> >> After the scan is complete we will perform binary recursion on the >> bitmap to determine which mTHP size would be most efficient to collapse >> to. max_ptes_none will be scaled by the attempted collapse order to >> determine how full a THP must be to be eligible. >> >> If a mTHP collapse is attempted, but contains swapped out, or shared >> pages, we dont perform the collapse. >> >> Signed-off-by: Nico Pache >> --- >> mm/khugepaged.c | 122 ++++++++++++++++++++++++++++++++---------------- >> 1 file changed, 83 insertions(+), 39 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index c8048d9ec7fb..cd310989725b 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1127,13 +1127,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> { >> LIST_HEAD(compound_pagelist); >> pmd_t *pmd, _pmd; >> - pte_t *pte; >> + pte_t *pte, mthp_pte; >> pgtable_t pgtable; >> struct folio *folio; >> spinlock_t *pmd_ptl, *pte_ptl; >> int result = SCAN_FAIL; >> struct vm_area_struct *vma; >> struct mmu_notifier_range range; >> + unsigned long _address = address + offset * PAGE_SIZE; >> VM_BUG_ON(address & ~HPAGE_PMD_MASK); >> >> /* >> @@ -1148,12 +1149,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> *mmap_locked = false; >> } >> >> - result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER); >> + result = alloc_charge_folio(&folio, mm, cc, order); >> if (result != SCAN_SUCCEED) >> goto out_nolock; >> >> mmap_read_lock(mm); >> - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER); >> + *mmap_locked = true; >> + result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order); >> if (result != SCAN_SUCCEED) { >> mmap_read_unlock(mm); >> goto out_nolock; >> @@ -1171,13 +1173,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> * released when it fails. So we jump out_nolock directly in >> * that case. Continuing to collapse causes inconsistency. >> */ >> - result = __collapse_huge_page_swapin(mm, vma, address, pmd, >> - referenced, HPAGE_PMD_ORDER); >> + result = __collapse_huge_page_swapin(mm, vma, _address, pmd, >> + referenced, order); >> if (result != SCAN_SUCCEED) >> goto out_nolock; >> } >> >> mmap_read_unlock(mm); >> + *mmap_locked = false; >> /* >> * Prevent all access to pagetables with the exception of >> * gup_fast later handled by the ptep_clear_flush and the VM >> @@ -1187,7 +1190,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> * mmap_lock. >> */ >> mmap_write_lock(mm); >> - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER); >> + result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order); >> if (result != SCAN_SUCCEED) >> goto out_up_write; >> /* check if the pmd is still valid */ >> @@ -1198,11 +1201,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> vma_start_write(vma); >> anon_vma_lock_write(vma->anon_vma); >> >> - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address, >> - address + HPAGE_PMD_SIZE); >> + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, _address, >> + _address + (PAGE_SIZE << order)); >> mmu_notifier_invalidate_range_start(&range); >> >> pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */ >> + >> /* >> * This removes any huge TLB entry from the CPU so we won't allow >> * huge and small TLB entries for the same virtual address to >> @@ -1216,10 +1220,10 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> mmu_notifier_invalidate_range_end(&range); >> tlb_remove_table_sync_one(); >> >> - pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); >> + pte = pte_offset_map_lock(mm, &_pmd, _address, &pte_ptl); >> if (pte) { >> - result = __collapse_huge_page_isolate(vma, address, pte, cc, >> - &compound_pagelist, HPAGE_PMD_ORDER); >> + result = __collapse_huge_page_isolate(vma, _address, pte, cc, >> + &compound_pagelist, order); >> spin_unlock(pte_ptl); >> } else { >> result = SCAN_PMD_NULL; >> @@ -1248,8 +1252,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> anon_vma_unlock_write(vma->anon_vma); >> >> result = __collapse_huge_page_copy(pte, folio, pmd, _pmd, >> - vma, address, pte_ptl, >> - &compound_pagelist, HPAGE_PMD_ORDER); >> + vma, _address, pte_ptl, >> + &compound_pagelist, order); >> pte_unmap(pte); >> if (unlikely(result != SCAN_SUCCEED)) >> goto out_up_write; >> @@ -1260,20 +1264,37 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, >> * write. >> */ >> __folio_mark_uptodate(folio); >> - pgtable = pmd_pgtable(_pmd); >> - >> - _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot); >> - _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma); >> - >> - spin_lock(pmd_ptl); >> - BUG_ON(!pmd_none(*pmd)); >> - folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE); >> - folio_add_lru_vma(folio, vma); >> - pgtable_trans_huge_deposit(mm, pmd, pgtable); >> - set_pmd_at(mm, address, pmd, _pmd); >> - update_mmu_cache_pmd(vma, address, pmd); >> - deferred_split_folio(folio, false); >> - spin_unlock(pmd_ptl); >> + if (order == HPAGE_PMD_ORDER) { >> + pgtable = pmd_pgtable(_pmd); >> + _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot); >> + _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma); >> + >> + spin_lock(pmd_ptl); >> + BUG_ON(!pmd_none(*pmd)); >> + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE); >> + folio_add_lru_vma(folio, vma); >> + pgtable_trans_huge_deposit(mm, pmd, pgtable); >> + set_pmd_at(mm, address, pmd, _pmd); >> + update_mmu_cache_pmd(vma, address, pmd); >> + deferred_split_folio(folio, false); >> + spin_unlock(pmd_ptl); >> + } else { //mTHP >> + mthp_pte = mk_pte(&folio->page, vma->vm_page_prot); >> + mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma); >> + >> + spin_lock(pmd_ptl); >> + folio_ref_add(folio, (1 << order) - 1); >> + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE); >> + folio_add_lru_vma(folio, vma); >> + spin_lock(pte_ptl); >> + set_ptes(vma->vm_mm, _address, pte, mthp_pte, (1 << order)); >> + update_mmu_cache_range(NULL, vma, _address, pte, (1 << order)); >> + spin_unlock(pte_ptl); >> + smp_wmb(); /* make pte visible before pmd */ >> + pmd_populate(mm, pmd, pmd_pgtable(_pmd)); >> + deferred_split_folio(folio, false); >> + spin_unlock(pmd_ptl); > > I've only stared at this briefly, but it feels like there might be some bugs: > > - Why are you taking the pmd ptl? and calling pmd_populate? Surely the pte > table already exists and is attached to the pmd? So we are only need to update > the pte entries here? Or perhaps the whole pmd was previously isolated? > > - I think some arches use a single PTL for all levels of the pgtable? So in > this case it's probably not a good idea to nest the pmd and pte spin lock? > > - Given the pte PTL is dropped then reacquired, is there any way that the ptes > could have changed under us? Is any revalidation required? Perhaps not if pte > table was removed from the PMD. > > - I would have guessed the memory ordering you want from smp_wmb() would > already be handled by the spin_unlock()? Not sure if I understand this; in pmd_install(), we take the PMD lock and before dropping the lock, do smp_wmb(), how is this case different? > > >> + } >> >> folio = NULL; >> >> @@ -1353,21 +1374,27 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, >> { >> pmd_t *pmd; >> pte_t *pte, *_pte; >> + int i; >> int result = SCAN_FAIL, referenced = 0; >> int none_or_zero = 0, shared = 0; >> struct page *page = NULL; >> struct folio *folio = NULL; >> unsigned long _address; >> + unsigned long enabled_orders; >> spinlock_t *ptl; >> int node = NUMA_NO_NODE, unmapped = 0; >> bool writable = false; >> - >> + int chunk_none_count = 0; >> + int scaled_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - MIN_MTHP_ORDER); >> + unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0; >> VM_BUG_ON(address & ~HPAGE_PMD_MASK); >> >> result = find_pmd_or_thp_or_none(mm, address, &pmd); >> if (result != SCAN_SUCCEED) >> goto out; >> >> + bitmap_zero(cc->mthp_bitmap, MAX_MTHP_BITMAP_SIZE); >> + bitmap_zero(cc->mthp_bitmap_temp, MAX_MTHP_BITMAP_SIZE); >> memset(cc->node_load, 0, sizeof(cc->node_load)); >> nodes_clear(cc->alloc_nmask); >> pte = pte_offset_map_lock(mm, pmd, address, &ptl); >> @@ -1376,8 +1403,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, >> goto out; >> } >> >> - for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR; >> - _pte++, _address += PAGE_SIZE) { >> + for (i = 0; i < HPAGE_PMD_NR; i++) { >> + if (i % MIN_MTHP_NR == 0) >> + chunk_none_count = 0; >> + >> + _pte = pte + i; >> + _address = address + i * PAGE_SIZE; >> pte_t pteval = ptep_get(_pte); >> if (is_swap_pte(pteval)) { >> ++unmapped; >> @@ -1400,16 +1431,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, >> } >> } >> if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >> + ++chunk_none_count; >> ++none_or_zero; >> - if (!userfaultfd_armed(vma) && >> - (!cc->is_khugepaged || >> - none_or_zero <= khugepaged_max_ptes_none)) { >> - continue; >> - } else { >> + if (userfaultfd_armed(vma)) { >> result = SCAN_EXCEED_NONE_PTE; >> count_vm_event(THP_SCAN_EXCEED_NONE_PTE); >> goto out_unmap; >> } >> + continue; >> } >> if (pte_uffd_wp(pteval)) { >> /* >> @@ -1500,7 +1529,16 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, >> folio_test_referenced(folio) || mmu_notifier_test_young(vma->vm_mm, >> address))) >> referenced++; >> + >> + /* >> + * we are reading in MIN_MTHP_NR page chunks. if there are no empty >> + * pages keep track of it in the bitmap for mTHP collapsing. >> + */ >> + if (chunk_none_count < scaled_none && >> + (i + 1) % MIN_MTHP_NR == 0) >> + bitmap_set(cc->mthp_bitmap, i / MIN_MTHP_NR, 1); >> } >> + >> if (!writable) { >> result = SCAN_PAGE_RO; >> } else if (cc->is_khugepaged && >> @@ -1513,10 +1551,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, >> out_unmap: >> pte_unmap_unlock(pte, ptl); >> if (result == SCAN_SUCCEED) { >> - result = collapse_huge_page(mm, address, referenced, >> - unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0); >> - /* collapse_huge_page will return with the mmap_lock released */ >> - *mmap_locked = false; >> + enabled_orders = thp_vma_allowable_orders(vma, vma->vm_flags, >> + tva_flags, THP_ORDERS_ALL_ANON); >> + result = khugepaged_scan_bitmap(mm, address, referenced, unmapped, cc, >> + mmap_locked, enabled_orders); >> + if (result > 0) >> + result = SCAN_SUCCEED; >> + else >> + result = SCAN_FAIL; >> } >> out: >> trace_mm_khugepaged_scan_pmd(mm, &folio->page, writable, referenced, >> @@ -2476,11 +2518,13 @@ static int khugepaged_collapse_single_pmd(unsigned long addr, struct mm_struct * >> fput(file); >> if (result == SCAN_PTE_MAPPED_HUGEPAGE) { >> mmap_read_lock(mm); >> + *mmap_locked = true; >> if (khugepaged_test_exit_or_disable(mm)) >> goto end; >> result = collapse_pte_mapped_thp(mm, addr, >> !cc->is_khugepaged); >> mmap_read_unlock(mm); >> + *mmap_locked = false; >> } >> } else { >> result = khugepaged_scan_pmd(mm, vma, addr, >