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Message-ID: <f787f046-6b35-425f-91ab-80aa6e8fd35b@arm.com>
Date: Thu, 4 Sep 2025 12:15:52 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Yang Shi <yang@...amperecomputing.com>, Ard Biesheuvel <ardb@...nel.org>,
Dev Jain <dev.jain@....com>, scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCH v7 3/6] arm64: mm: support large block mapping when
rodata=full
On 29/08/2025 12:52, Ryan Roberts wrote:
> From: Yang Shi <yang@...amperecomputing.com>
>
> When rodata=full is specified, kernel linear mapping has to be mapped at
> PTE level since large page table can't be split due to break-before-make
> rule on ARM64.
>
> This resulted in a couple of problems:
> - performance degradation
> - more TLB pressure
> - memory waste for kernel page table
>
> With FEAT_BBM level 2 support, splitting large block page table to
> smaller ones doesn't need to make the page table entry invalid anymore.
> This allows kernel split large block mapping on the fly.
>
> Add kernel page table split support and use large block mapping by
> default when FEAT_BBM level 2 is supported for rodata=full. When
> changing permissions for kernel linear mapping, the page table will be
> split to smaller size.
>
> The machine without FEAT_BBM level 2 will fallback to have kernel linear
> mapping PTE-mapped when rodata=full.
>
> With this we saw significant performance boost with some benchmarks and
> much less memory consumption on my AmpereOne machine (192 cores, 1P)
> with 256GB memory.
>
> * Memory use after boot
> Before:
> MemTotal: 258988984 kB
> MemFree: 254821700 kB
>
> After:
> MemTotal: 259505132 kB
> MemFree: 255410264 kB
>
> Around 500MB more memory are free to use. The larger the machine, the
> more memory saved.
>
> * Memcached
> We saw performance degradation when running Memcached benchmark with
> rodata=full vs rodata=on. Our profiling pointed to kernel TLB pressure.
> With this patchset we saw ops/sec is increased by around 3.5%, P99
> latency is reduced by around 9.6%.
> The gain mainly came from reduced kernel TLB misses. The kernel TLB
> MPKI is reduced by 28.5%.
>
> The benchmark data is now on par with rodata=on too.
>
> * Disk encryption (dm-crypt) benchmark
> Ran fio benchmark with the below command on a 128G ramdisk (ext4) with
> disk encryption (by dm-crypt).
> fio --directory=/data --random_generator=lfsr --norandommap \
> --randrepeat 1 --status-interval=999 --rw=write --bs=4k --loops=1 \
> --ioengine=sync --iodepth=1 --numjobs=1 --fsync_on_close=1 \
> --group_reporting --thread --name=iops-test-job --eta-newline=1 \
> --size 100G
>
> The IOPS is increased by 90% - 150% (the variance is high, but the worst
> number of good case is around 90% more than the best number of bad
> case). The bandwidth is increased and the avg clat is reduced
> proportionally.
>
> * Sequential file read
> Read 100G file sequentially on XFS (xfs_io read with page cache
> populated). The bandwidth is increased by 150%.
>
> Co-developed-by: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: Yang Shi <yang@...amperecomputing.com>
> ---
> arch/arm64/include/asm/cpufeature.h | 2 +
> arch/arm64/include/asm/mmu.h | 1 +
> arch/arm64/include/asm/pgtable.h | 5 +
> arch/arm64/kernel/cpufeature.c | 7 +-
> arch/arm64/mm/mmu.c | 248 +++++++++++++++++++++++++++-
> arch/arm64/mm/pageattr.c | 4 +
> 6 files changed, 261 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index bf13d676aae2..e223cbf350e4 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -871,6 +871,8 @@ static inline bool system_supports_pmuv3(void)
> return cpus_have_final_cap(ARM64_HAS_PMUV3);
> }
>
> +bool cpu_supports_bbml2_noabort(void);
> +
> static inline bool system_supports_bbml2_noabort(void)
> {
> return alternative_has_cap_unlikely(ARM64_HAS_BBML2_NOABORT);
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index 6e8aa8e72601..56fca81f60ad 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/arch/arm64/include/asm/mmu.h
> @@ -71,6 +71,7 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
> pgprot_t prot, bool page_mappings_only);
> extern void *fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot);
> extern void mark_linear_text_alias_ro(void);
> +extern int split_kernel_leaf_mapping(unsigned long start, unsigned long end);
>
> /*
> * This check is triggered during the early boot before the cpufeature
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index abd2dee416b3..aa89c2e67ebc 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -371,6 +371,11 @@ static inline pmd_t pmd_mkcont(pmd_t pmd)
> return __pmd(pmd_val(pmd) | PMD_SECT_CONT);
> }
>
> +static inline pmd_t pmd_mknoncont(pmd_t pmd)
> +{
> + return __pmd(pmd_val(pmd) & ~PMD_SECT_CONT);
> +}
> +
> #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> static inline int pte_uffd_wp(pte_t pte)
> {
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index b93f4ee57176..a8936c1023ea 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2217,7 +2217,7 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
> return arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_HVHE);
> }
>
> -static bool has_bbml2_noabort(const struct arm64_cpu_capabilities *caps, int scope)
> +bool cpu_supports_bbml2_noabort(void)
> {
> /*
> * We want to allow usage of BBML2 in as wide a range of kernel contexts
> @@ -2251,6 +2251,11 @@ static bool has_bbml2_noabort(const struct arm64_cpu_capabilities *caps, int sco
> return true;
> }
>
> +static bool has_bbml2_noabort(const struct arm64_cpu_capabilities *caps, int scope)
> +{
> + return cpu_supports_bbml2_noabort();
> +}
> +
> #ifdef CONFIG_ARM64_PAN
> static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
> {
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 34e5d78af076..114b88216b0c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -481,6 +481,8 @@ void create_kpti_ng_temp_pgd(pgd_t *pgdir, phys_addr_t phys, unsigned long virt,
> int flags);
> #endif
>
> +#define INVALID_PHYS_ADDR -1
> +
> static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm,
> enum pgtable_type pgtable_type)
> {
> @@ -488,7 +490,9 @@ static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm,
> struct ptdesc *ptdesc = pagetable_alloc(GFP_PGTABLE_KERNEL & ~__GFP_ZERO, 0);
> phys_addr_t pa;
>
> - BUG_ON(!ptdesc);
> + if (!ptdesc)
> + return INVALID_PHYS_ADDR;
> +
> pa = page_to_phys(ptdesc_page(ptdesc));
>
> switch (pgtable_type) {
> @@ -509,16 +513,240 @@ static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm,
> return pa;
> }
>
> +static phys_addr_t
> +try_pgd_pgtable_alloc_init_mm(enum pgtable_type pgtable_type)
> +{
> + return __pgd_pgtable_alloc(&init_mm, pgtable_type);
> +}
> +
> static phys_addr_t __maybe_unused
> pgd_pgtable_alloc_init_mm(enum pgtable_type pgtable_type)
> {
> - return __pgd_pgtable_alloc(&init_mm, pgtable_type);
> + phys_addr_t pa;
> +
> + pa = __pgd_pgtable_alloc(&init_mm, pgtable_type);
> + BUG_ON(pa == INVALID_PHYS_ADDR);
> + return pa;
> }
>
> static phys_addr_t
> pgd_pgtable_alloc_special_mm(enum pgtable_type pgtable_type)
> {
> - return __pgd_pgtable_alloc(NULL, pgtable_type);
> + phys_addr_t pa;
> +
> + pa = __pgd_pgtable_alloc(NULL, pgtable_type);
> + BUG_ON(pa == INVALID_PHYS_ADDR);
> + return pa;
> +}
> +
> +static void split_contpte(pte_t *ptep)
> +{
> + int i;
> +
> + ptep = PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
> + for (i = 0; i < CONT_PTES; i++, ptep++)
> + __set_pte(ptep, pte_mknoncont(__ptep_get(ptep)));
> +}
> +
> +static int split_pmd(pmd_t *pmdp, pmd_t pmd)
> +{
> + pmdval_t tableprot = PMD_TYPE_TABLE | PMD_TABLE_UXN | PMD_TABLE_AF;
> + unsigned long pfn = pmd_pfn(pmd);
> + pgprot_t prot = pmd_pgprot(pmd);
> + phys_addr_t pte_phys;
> + pte_t *ptep;
> + int i;
> +
> + pte_phys = try_pgd_pgtable_alloc_init_mm(TABLE_PTE);
> + if (pte_phys == INVALID_PHYS_ADDR)
> + return -ENOMEM;
> + ptep = (pte_t *)phys_to_virt(pte_phys);
> +
> + if (pgprot_val(prot) & PMD_SECT_PXN)
> + tableprot |= PMD_TABLE_PXN;
> +
> + prot = __pgprot((pgprot_val(prot) & ~PTE_TYPE_MASK) | PTE_TYPE_PAGE);
> + prot = __pgprot(pgprot_val(prot) | PTE_CONT);
> +
> + for (i = 0; i < PTRS_PER_PTE; i++, ptep++, pfn++)
> + __set_pte(ptep, pfn_pte(pfn, prot));
> +
> + /*
> + * Ensure the pte entries are visible to the table walker by the time
> + * the pmd entry that points to the ptes is visible.
> + */
> + dsb(ishst);
> + __pmd_populate(pmdp, pte_phys, tableprot);
> +
> + return 0;
> +}
> +
> +static void split_contpmd(pmd_t *pmdp)
> +{
> + int i;
> +
> + pmdp = PTR_ALIGN_DOWN(pmdp, sizeof(*pmdp) * CONT_PMDS);
> + for (i = 0; i < CONT_PMDS; i++, pmdp++)
> + set_pmd(pmdp, pmd_mknoncont(pmdp_get(pmdp)));
> +}
> +
> +static int split_pud(pud_t *pudp, pud_t pud)
> +{
> + pudval_t tableprot = PUD_TYPE_TABLE | PUD_TABLE_UXN | PUD_TABLE_AF;
> + unsigned int step = PMD_SIZE >> PAGE_SHIFT;
> + unsigned long pfn = pud_pfn(pud);
> + pgprot_t prot = pud_pgprot(pud);
> + phys_addr_t pmd_phys;
> + pmd_t *pmdp;
> + int i;
> +
> + pmd_phys = try_pgd_pgtable_alloc_init_mm(TABLE_PMD);
> + if (pmd_phys == INVALID_PHYS_ADDR)
> + return -ENOMEM;
> + pmdp = (pmd_t *)phys_to_virt(pmd_phys);
> +
> + if (pgprot_val(prot) & PMD_SECT_PXN)
> + tableprot |= PUD_TABLE_PXN;
> +
> + prot = __pgprot((pgprot_val(prot) & ~PMD_TYPE_MASK) | PMD_TYPE_SECT);
> + prot = __pgprot(pgprot_val(prot) | PTE_CONT);
> +
> + for (i = 0; i < PTRS_PER_PMD; i++, pmdp++, pfn += step)
> + set_pmd(pmdp, pfn_pmd(pfn, prot));
> +
> + /*
> + * Ensure the pmd entries are visible to the table walker by the time
> + * the pud entry that points to the pmds is visible.
> + */
> + dsb(ishst);
> + __pud_populate(pudp, pmd_phys, tableprot);
> +
> + return 0;
> +}
> +
> +static int split_kernel_leaf_mapping_locked(unsigned long addr)
> +{
> + pgd_t *pgdp, pgd;
> + p4d_t *p4dp, p4d;
> + pud_t *pudp, pud;
> + pmd_t *pmdp, pmd;
> + pte_t *ptep, pte;
> + int ret = 0;
> +
> + /*
> + * PGD: If addr is PGD aligned then addr already describes a leaf
> + * boundary. If not present then there is nothing to split.
> + */
> + if (ALIGN_DOWN(addr, PGDIR_SIZE) == addr)
> + goto out;
> + pgdp = pgd_offset_k(addr);
> + pgd = pgdp_get(pgdp);
> + if (!pgd_present(pgd))
> + goto out;
> +
> + /*
> + * P4D: If addr is P4D aligned then addr already describes a leaf
> + * boundary. If not present then there is nothing to split.
> + */
> + if (ALIGN_DOWN(addr, P4D_SIZE) == addr)
> + goto out;
> + p4dp = p4d_offset(pgdp, addr);
> + p4d = p4dp_get(p4dp);
> + if (!p4d_present(p4d))
> + goto out;
> +
> + /*
> + * PUD: If addr is PUD aligned then addr already describes a leaf
> + * boundary. If not present then there is nothing to split. Otherwise,
> + * if we have a pud leaf, split to contpmd.
> + */
> + if (ALIGN_DOWN(addr, PUD_SIZE) == addr)
> + goto out;
> + pudp = pud_offset(p4dp, addr);
> + pud = pudp_get(pudp);
> + if (!pud_present(pud))
> + goto out;
> + if (pud_leaf(pud)) {
> + ret = split_pud(pudp, pud);
> + if (ret)
> + goto out;
> + }
> +
> + /*
> + * CONTPMD: If addr is CONTPMD aligned then addr already describes a
> + * leaf boundary. If not present then there is nothing to split.
> + * Otherwise, if we have a contpmd leaf, split to pmd.
> + */
> + if (ALIGN_DOWN(addr, CONT_PMD_SIZE) == addr)
> + goto out;
> + pmdp = pmd_offset(pudp, addr);
> + pmd = pmdp_get(pmdp);
> + if (!pmd_present(pmd))
> + goto out;
> + if (pmd_leaf(pmd)) {
> + if (pmd_cont(pmd))
> + split_contpmd(pmdp);
> + /*
> + * PMD: If addr is PMD aligned then addr already describes a
> + * leaf boundary. Otherwise, split to contpte.
> + */
> + if (ALIGN_DOWN(addr, PMD_SIZE) == addr)
> + goto out;
> + ret = split_pmd(pmdp, pmd);
> + if (ret)
> + goto out;
> + }
> +
> + /*
> + * CONTPTE: If addr is CONTPTE aligned then addr already describes a
> + * leaf boundary. If not present then there is nothing to split.
> + * Otherwise, if we have a contpte leaf, split to pte.
> + */
> + if (ALIGN_DOWN(addr, CONT_PTE_SIZE) == addr)
> + goto out;
> + ptep = pte_offset_kernel(pmdp, addr);
> + pte = __ptep_get(ptep);
> + if (!pte_present(pte))
> + goto out;
> + if (pte_cont(pte))
> + split_contpte(ptep);
> +
> +out:
> + return ret;
> +}
> +
> +static DEFINE_MUTEX(pgtable_split_lock);
> +
> +int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
> +{
> + int ret;
> +
> + /*
> + * !BBML2_NOABORT systems should not be trying to change permissions on
> + * anything that is not pte-mapped in the first place. Just return early
> + * and let the permission change code raise a warning if not already
> + * pte-mapped.
> + */
> + if (!system_supports_bbml2_noabort())
> + return 0;
> +
> + /*
> + * Ensure start and end are at least page-aligned since this is the
> + * finest granularity we can split to.
> + */
> + if (start != PAGE_ALIGN(start) || end != PAGE_ALIGN(end))
> + return -EINVAL;
> +
> + mutex_lock(&pgtable_split_lock);
> + arch_enter_lazy_mmu_mode();
There is a spec issue here: We are inside a lazy mmu section, for which the
documentation says is an atomic context so we can't sleep. But
split_kernel_leaf_mapping_locked() will allocate pgtable memory if needed in a
manner that might sleep.
This isn't a problem in practice for arm64 since it's lazy mmu implementation
allows sleeping. I propose just adding a comment here to explain this and leave
the logic as is. Are people happy with this approach?
> +
> + ret = split_kernel_leaf_mapping_locked(start);
> + if (!ret)
> + ret = split_kernel_leaf_mapping_locked(end);
> +
> + arch_leave_lazy_mmu_mode();
> + mutex_unlock(&pgtable_split_lock);
> + return ret;
> }
>
> /*
> @@ -640,6 +868,16 @@ static inline void arm64_kfence_map_pool(phys_addr_t kfence_pool, pgd_t *pgdp) {
>
> #endif /* CONFIG_KFENCE */
>
> +static inline bool force_pte_mapping(void)
> +{
> + bool bbml2 = system_capabilities_finalized() ?
> + system_supports_bbml2_noabort() : cpu_supports_bbml2_noabort();
> +
> + return (!bbml2 && (rodata_full || arm64_kfence_can_set_direct_map() ||
> + is_realm_world())) ||
> + debug_pagealloc_enabled();
> +}
> +
> static void __init map_mem(pgd_t *pgdp)
> {
> static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN);
> @@ -665,7 +903,7 @@ static void __init map_mem(pgd_t *pgdp)
>
> early_kfence_pool = arm64_kfence_alloc_pool();
>
> - if (can_set_direct_map())
> + if (force_pte_mapping())
> flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>
> /*
> @@ -1367,7 +1605,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
>
> VM_BUG_ON(!mhp_range_allowed(start, size, true));
>
> - if (can_set_direct_map())
> + if (force_pte_mapping())
> flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>
> __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 6da8cbc32f46..0aba80a38cef 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -140,6 +140,10 @@ static int update_range_prot(unsigned long start, unsigned long size,
> data.set_mask = set_mask;
> data.clear_mask = clear_mask;
>
> + ret = split_kernel_leaf_mapping(start, start + size);
> + if (WARN_ON_ONCE(ret))
> + return ret;
> +
> arch_enter_lazy_mmu_mode();
>
> /*
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