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imf27.hostedemail.com; dkim=none; spf=pass (imf27.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1741165893; a=rsa-sha256; cv=none; b=n+ek3hPI3xw+19jwKWzKVXV42xqoctYgJ35477wqiIj5nLywnvYApHSJMJCqmb0u8bbgYK BGfRhooe1uqt0YgebOZoOy1FkVYQoO9hU7nLI0awlo4Zvy/g8t27xq5bzLPRWvXmvMDzxK 69sJ7yBGcLTpq5yoWE4Y7X7BNMUibMo= 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 886911007; Wed, 5 Mar 2025 01:11:45 -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 B68643F66E; Wed, 5 Mar 2025 01:11:18 -0800 (PST) Message-ID: <316c1ed4-bb22-4195-98ae-a4f50234131f@arm.com> Date: Wed, 5 Mar 2025 14:41:15 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 7/9] khugepaged: add mTHP support To: Nico Pache , Ryan Roberts Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 4599040007 X-Rspamd-Server: rspam09 X-Stat-Signature: digfrp6o5puqaw9b6u3ghp5ryiywqks6 X-HE-Tag: 1741165893-33743 X-HE-Meta: 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 K63sSp7b MTXvUpHt4XZXTl2JdEM/Neq0MXAgSOHdjKdiwLcMhP4M36bPgjrcmaSWvfjA9ER5AuSInxKfSqEEjEQMU2FiiFyOthK1gjY1V5iE9UtRLOjEVWvbwhQJixXqFtJ0Y1Ml0BwNU6Q7y8I6fUQ6L9NR/iL0fhOh0nzGnusJL3fDOqZGn9zLLInpbr2sUnwBvglgBcSBZXuFgozJgRM2gs3353Hd1xQGQnl1dFNQi3FR+lRimHWue/BROTjLC9vcgk9SugFIvohYlHxK2WIAffhgA2vKHA1s9toNPOhBX4mgrc2BkFGMtsc2Gh8kNf062SdlSlLeu X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 04/03/25 12:43 am, Nico Pache wrote: > On Wed, Feb 19, 2025 at 9:52 AM 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: > > Sorry for the delayed response, I needed to catch up on some other > work and wanted to make sure I looked into your questions before > answering. >> >> - 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? > > The previous locking behavior is kept; however, because we are not > installing a NEW pmd we need to repopulate the old PMD (like we do in > the fail case). The PMD entry was cleared to avoid GUP-fast races. >> >> - 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? > > Thanks for pointing that out, I corrected it by making sure they dont nest! > >> >> - 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. > > Correct, I believe we dont even need to take the PTL because of all > the write locks we took-- but for now i'm trying to keep the locking > changes to a minimum. We can focus on locking optimizations later. Even the current code is sprinkled with comments like "Probably unnecessary" when, for example taking the PTL around set_ptes(). IMHO let us follow the same logic for now, and we can think about dropping the spinlocks later, so I will prefer taking the PMD-PTL around pmd_populate(), and take the PTE-PTL around set_ptes(). > >> >> - I would have guessed the memory ordering you want from smp_wmb() would >> already be handled by the spin_unlock()? > > Yes I think that is correct, I noticed other callers doing this, but > on a second pass those are all lockless, so in this case we dont need > it. > >> >> >>> + } >>> >>> 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, >> >