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Tue, 07 Jul 2026 20:11:33 -0700 (PDT) Date: Wed, 8 Jul 2026 03:11:10 +0000 Mime-Version: 1.0 X-Mailer: git-send-email 2.55.0.795.g602f6c329a-goog Message-ID: <20260708031129.3503195-1-jthoughton@google.com> Subject: [PATCH 00/18] Another attempt at HVO support on arm64 From: James Houghton To: Will Deacon , Catalin Marinas , Muchun Song , Oscar Salvador Cc: Nikos Nikoleris , Linu Cherian , Mark Rutland , David Hildenbrand , Andrew Morton , Ryan Roberts , Nanyong Sun , Yu Zhao , Frank van der Linden , David Rientjes , James Houghton , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260707_201136_983542_7A943201 X-CRM114-Status: GOOD ( 25.79 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi everyone, This patch series uses a trick with the Access Flag on CPUs that support hardware update of the AF to update vmemmap page table entries without introducing a time window where CPUs accessing the vmemmap might fault. By avoiding faults, the HugeTLB vmemmap optimization (HVO) can be implemented correctly on arm64 in a much more straightforward way than previously attempted, most recently here[1] (please see [1] for a breakdown of the other approaches attempted before). For large-memory systems that allocate most of their available memory to HugeTLB, HVO saves a huge amount of memory (1.5% of system memory). This series has four parts: 1. Some preparatory changes (patches 1-3) 1. Bare minimum HVO support (patches 4-10) 2. Drop BBML2_NOABORT requirement for HVO (patches 11-13) 3. Drop the user-configurable Kconfig for HVO (patches 14-18) Parts 3 and 4 are technically optional. More details below. The main functional caveat with this series is that bootmem HugeTLB pages are not "pre-HVOed". They will be HVOed, but because at pre-HVO time SMP CPUs have not been enabled, we cannot query for full system support. This series is based on 7.2-rc2 (0e35b9b6ec0f). This series almost 100% cleanly applies to mm-new, which has some of Muchun's HVO patches, with one trivial conflict. I imagine this series will conflict pretty heavily with some of Muchun's other patches[2]. -- The AF trick -- The trick is that translations with the AF unset cannot be cached in the TLB (see Rule R_DWZCQ in the Arm ARM), so they can be atomically updated without needing a full break-before-make sequence. So the PTE update sequence becomes: 1. Atomically clear the AF on the existing PTE. 2. Invalidate the TLB. 3. cmpxchg the AF=0 PTE with the new PTE. If this fails, goto 1. If there is a CPU on the system that does not support hardware access flag updates, clearing the AF is problematic, as those CPUs might fault on the vmemmap usage. Therefore, HVO compatbility checks all CPUs for HW AF updates. -- Application to HVO -- HVO relies on the following page table transitions: - When enabling HVO for a page, PMD block entries in the vmemmap are shattered into PMD table entries. The first PTE remains mapped normally (RW mapping to a real page of struct pages), but the remaining PTEs in the vmemmap are mapped read-only to a shared page of struct pages (that is, there is an OA change and a permissions change). - When disabling HVO for a page, the RO PTEs are remapped back to RW PTEs that point to newly reallocated pages of struct pages. The PMD block -> table transition is not undone. In patches 4-10 of this series, I use the Access Flag trick to do the PTE OA and permissions updates. We rely on BBML2_NOABORT for the PMD block -> table transition. In patches 11-13, I re-use the Access Flag trick to do the PMD block -> table transition without needing BBML2_NOABORT. For systems that support BBML2_NOABORT, the logic is unchanged. I am aware of Linu's BBML3 patches; I've opted not to rebase onto them for now, but I am happy to do so later. -- Late-onlining of CPUs that do not support HW AF -- One of the complications with this series is how to handle late-onlining of CPUs that do not have HW AF when HVO is in use. Naively, if HVO (HW AF) is supported on all boot CPUs and the kernel is compiled with HVO support, late CPUs that do not support HVO will not be onlined. This is a regression. This series provides two ways of dealing with this. First, add a default-off Kconfig for users to enable HVO support, avoiding the regression. This is not ideal. Patches 14-18 get rid of the new Kconfig by allowing onlining of HVO-incompatible late CPUs as long as HVO is not actively in use. -- Litmus test -- The following Herd litmus test demonstrates the PTE update routine: AArch64 TTDFaultlessUpdate Variant=vmsa TTHM=HA { uint64_t x=1; uint64_t y=2; [PTE(x)]=(oa:PA(x), af:1); 0:X0=PTE(x); 1:X0=PTE(x); 0:X1=x; 1:X1=x; pteval_t 0:X2=(oa:PA(x), af:0); pteval_t 0:X3=(oa:PA(y), af:1); } P0 | P1 ; LDR X4,[X0] | L0: ; MOV X5,X4 | LDR X2,[X1] ; CAS X4,X2,[X0] | ; DSB ISHST | ; LSR X9,X1,#12 | ; TLBI VAALE1IS,X9 | ; DSB ISH | ; ISB | ; CAS X2,X3,[X0] | ; exists 0:X5=0:X4 /\ (* First CAS must succeed *) (fault(P1:L0) \/ ~(1:X2=2 \/ 1:X2=1)) (* This test should not violate BBM requirements. *) This test must be run with herdtools with Nikos's changes[3] to more accurately model BBM requirements. When tried, the output will notably *not* contain the "Warning-BBM-expected" flag. -- Testing -- I haven't yet done extensive testing of this series. HVO is correctly freeing pages on my system, and the hugetlb-vmemmap test passes. Freeing HVOed HugeTLB pages also seems to function normally. [1] https://lore.kernel.org/linux-arm-kernel/20241107202033.2721681-1-yuzhao@google.com/ [2] https://lore.kernel.org/linux-mm/20260513130542.35604-1-songmuchun@bytedance.com/ [3] https://github.com/herd/herdtools7/pull/1864 James Houghton (18): hugetlb_vmemmap: Always flush TLB if needed upon PTE remapping hugetlb_vmemmap: Move vmemmap_get_tail up hugetlb_vmemmap: Leave pages partially HVOed upon restore failure hugetlb_vmemmap: Use try_update_vmemmap_pte to update in-use PTEs hugetlb_vmemmap: Allow architectures not to allow HVO at runtime arm64: Rename cpu_has_hw_af to system_has_hw_af arm64: Add system_supports_hvo arm64: Implement try_update_vmemmap_pte using the AF trick arm64: Prevent HVO if the HVO system feature is not enabled arm64: Support hugetlb vmemmap optimization hugetlb_vmemmap: Use try_populate_vmemmap_pmd for replacing in-use PMDs arm64: Implement try_populate_vmemmap_pmd using AF trick arm64: Drop BBML2_NOABORT requirement for HVO hugetlb_vmemmap: Rename mm/hugetlb_vmemmap.h to mm/hugetlb_vmemmap_internal.h hugetlb_vmemmap: Add a way to permanently disable HVO when needed arm64: Allow "optional" CPU features to be required sometimes arm64: Permit onlining of HVO-incompatible late CPUs if HVO is not in use arm64: Remove user-selectable HVO Kconfig MAINTAINERS | 3 +- arch/arm64/Kconfig | 1 + arch/arm64/include/asm/cpucaps.h | 2 + arch/arm64/include/asm/cpufeature.h | 39 ++- arch/arm64/include/asm/hugetlb.h | 7 + arch/arm64/include/asm/pgalloc.h | 54 ++++ arch/arm64/include/asm/pgtable.h | 57 ++++- arch/arm64/kernel/cpufeature.c | 43 ++++ arch/arm64/tools/cpucaps | 1 + arch/loongarch/include/asm/pgalloc.h | 8 + arch/loongarch/include/asm/pgtable.h | 8 + arch/riscv/include/asm/pgalloc.h | 8 + arch/riscv/include/asm/pgtable.h | 8 + arch/x86/include/asm/pgalloc.h | 8 + arch/x86/include/asm/pgtable.h | 8 + include/asm-generic/hugetlb.h | 7 + include/linux/hugetlb_vmemmap.h | 20 ++ include/linux/pgalloc.h | 20 ++ include/linux/pgtable.h | 21 ++ mm/hugetlb.c | 2 +- mm/hugetlb_sysfs.c | 2 +- mm/hugetlb_vmemmap.c | 237 +++++++++++++----- ...b_vmemmap.h => hugetlb_vmemmap_internal.h} | 6 +- mm/sparse-vmemmap.c | 2 +- 24 files changed, 489 insertions(+), 83 deletions(-) create mode 100644 include/linux/hugetlb_vmemmap.h rename mm/{hugetlb_vmemmap.h => hugetlb_vmemmap_internal.h} (95%) base-commit: 0e35b9b6ec0ffcc5e23cbdec09f5c622ad532b53 -- 2.55.0.795.g602f6c329a-goog