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Message-ID: <fb5dff48-3181-4f5b-9be2-623785fc2da9@os.amperecomputing.com>
Date: Thu, 22 Jan 2026 13:41:28 -0800
From: Yang Shi <yang@...amperecomputing.com>
To: Anshuman Khandual <anshuman.khandual@....com>, catalin.marinas@....com,
will@...nel.org, ryan.roberts@....com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [v5 PATCH] arm64: mm: show direct mapping use in /proc/meminfo
On 1/21/26 9:09 PM, Anshuman Khandual wrote:
> Hello Yang,
>
> On 07/01/26 5:59 AM, Yang Shi wrote:
>> Since commit a166563e7ec3 ("arm64: mm: support large block mapping when
>> rodata=full"), the direct mapping may be split on some machines instead
>> keeping static since boot. It makes more sense to show the direct mapping
>> use in /proc/meminfo than before.
> I guess the direct mapping here refers to linear map ? IIUC it is called
> direct map on x86 and linear map on arm64 platforms. Then should not it
> be renamed as s/DirectMap/LinearMap instead ? This will align with names
> from ptdump as well.
Yes, linear map refers to direct map. They are interchangeable in this
patch. Using "DirectMap" keeps the compatibility with x86.
>
> Before the above mentioned commit, linear could get altered with memory
> hotplug and remove events as well.
>
>> This patch will make /proc/meminfo show the direct mapping use like the
>> below (4K base page size):
>> DirectMap4K: 94792 kB
>> DirectMap64K: 134208 kB
>> DirectMap2M: 1173504 kB
>> DirectMap32M: 5636096 kB
>> DirectMap1G: 529530880 kB
> If /proc/meminfo interface is getting updated via arch_report_meminfo()
> why not add stats for all kernel virtual address space ranges including
> vmemmap, vmalloc etc aka all address range headers in ptdump as many of
> those could change during system runtime. What makes linear mapping any
> special ?
Other than what Will suggested, /proc/meminfo does show vmalloc info:
VmallocTotal: 135288315904 kB
VmallocUsed: 114200 kB
VmallocChunk: 0 kB
AFAICT, large block mapping for vmalloc has not been widely used by
arm64 yet.
"cat /sys/kernel/debug/kernel_page_tables | grep -e BLK" doesn't show
any large block mapping on my AmpereOne machine with v6.19-rc4 kernel.
If we get more large block mappings for vmalloc, we can add the mapping
size info in the future.
Thanks,
Yang
>
>> Although just the machines which support BBML2_NOABORT can split the
>> direct mapping, show it on all machines regardless of BBML2_NOABORT so
>> that the users have consistent view in order to avoid confusion.
>>
>> Although ptdump also can tell the direct map use, but it needs to dump
>> the whole kernel page table. It is costly and overkilling. It is also
>> in debugfs which may not be enabled by all distros. So showing direct
>> map use in /proc/meminfo seems more convenient and has less overhead.
> Agreed a /proc/meminfo based broader kernel virtual address space stats
> display will complement ptdump which provides more granular information
> about their mapping (with additional cost and setup) but it should cover
> all the regions in kernel virtual space.
>
>> Signed-off-by: Yang Shi <yang@...amperecomputing.com>
>> ---
>> v5: * Rebased to v6.19-rc4
>> * Fixed the build error for !CONFIG_PROC_FS
>> v4: * Used PAGE_END instead of _PAGE_END(VA_BITS_MIN) per Ryan
>> * Used shorter name for the helpers and variables per Ryan
>> * Fixed accounting for memory hotunplug
>> v3: * Fixed the over-accounting problems per Ryan
>> * Introduced helpers for add/sub direct map use and #ifdef them with
>> CONFIG_PROC_FS per Ryan
>> * v3 is a fix patch on top of v2
>> v2: * Counted in size instead of the number of entries per Ryan
>> * Removed shift array per Ryan
>> * Use lower case "k" per Ryan
>> * Fixed a couple of build warnings reported by kernel test robot
>> * Fixed a couple of poential miscounts
>>
>> arch/arm64/mm/mmu.c | 202 +++++++++++++++++++++++++++++++++++++++-----
>> 1 file changed, 181 insertions(+), 21 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 8e1d80a7033e..422441c9a992 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -29,6 +29,7 @@
>> #include <linux/mm_inline.h>
>> #include <linux/pagewalk.h>
>> #include <linux/stop_machine.h>
>> +#include <linux/proc_fs.h>
>>
>> #include <asm/barrier.h>
>> #include <asm/cputype.h>
>> @@ -171,6 +172,85 @@ static void init_clear_pgtable(void *table)
>> dsb(ishst);
>> }
>>
>> +enum dm_type {
>> + PTE,
>> + CONT_PTE,
>> + PMD,
>> + CONT_PMD,
>> + PUD,
>> + NR_DM_TYPE,
>> +};
>> +
>> +#ifdef CONFIG_PROC_FS
>> +static unsigned long dm_meminfo[NR_DM_TYPE];
>> +
>> +void arch_report_meminfo(struct seq_file *m)
>> +{
>> + char *size[NR_DM_TYPE];
>> +
>> +#if defined(CONFIG_ARM64_4K_PAGES)
>> + size[PTE] = "4k";
>> + size[CONT_PTE] = "64k";
>> + size[PMD] = "2M";
>> + size[CONT_PMD] = "32M";
>> + size[PUD] = "1G";
>> +#elif defined(CONFIG_ARM64_16K_PAGES)
>> + size[PTE] = "16k";
>> + size[CONT_PTE] = "2M";
>> + size[PMD] = "32M";
>> + size[CONT_PMD] = "1G";
>> +#elif defined(CONFIG_ARM64_64K_PAGES)
>> + size[PTE] = "64k";
>> + size[CONT_PTE] = "2M";
>> + size[PMD] = "512M";
>> + size[CONT_PMD] = "16G";
>> +#endif
>> +
>> + seq_printf(m, "DirectMap%s: %8lu kB\n",
>> + size[PTE], dm_meminfo[PTE] >> 10);
>> + seq_printf(m, "DirectMap%s: %8lu kB\n",
>> + size[CONT_PTE],
>> + dm_meminfo[CONT_PTE] >> 10);
>> + seq_printf(m, "DirectMap%s: %8lu kB\n",
>> + size[PMD], dm_meminfo[PMD] >> 10);
>> + seq_printf(m, "DirectMap%s: %8lu kB\n",
>> + size[CONT_PMD],
>> + dm_meminfo[CONT_PMD] >> 10);
>> + if (pud_sect_supported())
>> + seq_printf(m, "DirectMap%s: %8lu kB\n",
>> + size[PUD], dm_meminfo[PUD] >> 10);
>> +}
>> +
>> +static inline bool is_dm_addr(unsigned long addr)
>> +{
>> + return (addr >= PAGE_OFFSET) && (addr < PAGE_END);
>> +}
>> +
>> +static inline void dm_meminfo_add(unsigned long addr, unsigned long size,
>> + enum dm_type type)
>> +{
>> + if (is_dm_addr(addr))
>> + dm_meminfo[type] += size;
>> +}
>> +
>> +static inline void dm_meminfo_sub(unsigned long addr, unsigned long size,
>> + enum dm_type type)
>> +{
>> + if (is_dm_addr(addr))
>> + dm_meminfo[type] -= size;
>> +}
>> +#else
>> +static inline void dm_meminfo_add(unsigned long addr, unsigned long size,
>> + enum dm_type type)
>> +{
>> +}
>> +
>> +static inline void dm_meminfo_sub(unsigned long addr, unsigned long size,
>> + enum dm_type type)
>> +{
>> +}
>> +#endif
>> +
>> static void init_pte(pte_t *ptep, unsigned long addr, unsigned long end,
>> phys_addr_t phys, pgprot_t prot)
>> {
>> @@ -236,6 +316,11 @@ static int alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>>
>> init_pte(ptep, addr, next, phys, __prot);
>>
>> + if (pgprot_val(__prot) & PTE_CONT)
>> + dm_meminfo_add(addr, (next - addr), CONT_PTE);
>> + else
>> + dm_meminfo_add(addr, (next - addr), PTE);
>> +
>> ptep += pte_index(next) - pte_index(addr);
>> phys += next - addr;
>> } while (addr = next, addr != end);
>> @@ -266,6 +351,17 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
>> (flags & NO_BLOCK_MAPPINGS) == 0) {
>> pmd_set_huge(pmdp, phys, prot);
>>
>> + /*
>> + * It is possible to have mappings allow cont mapping
>> + * but disallow block mapping. For example,
>> + * map_entry_trampoline().
>> + * So we have to increase CONT_PMD and PMD size here
>> + * to avoid double counting.
>> + */
>> + if (pgprot_val(prot) & PTE_CONT)
>> + dm_meminfo_add(addr, (next - addr), CONT_PMD);
>> + else
>> + dm_meminfo_add(addr, (next - addr), PMD);
>> /*
>> * After the PMD entry has been populated once, we
>> * only allow updates to the permission attributes.
>> @@ -389,6 +485,7 @@ static int alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
>> (flags & NO_BLOCK_MAPPINGS) == 0) {
>> pud_set_huge(pudp, phys, prot);
>>
>> + dm_meminfo_add(addr, (next - addr), PUD);
>> /*
>> * After the PUD entry has been populated once, we
>> * only allow updates to the permission attributes.
>> @@ -575,16 +672,21 @@ pgd_pgtable_alloc_special_mm(enum pgtable_type pgtable_type)
>> return __pgd_pgtable_alloc(NULL, GFP_PGTABLE_KERNEL, pgtable_type);
>> }
>>
>> -static void split_contpte(pte_t *ptep)
>> +static void split_contpte(unsigned long addr, pte_t *ptep)
>> {
>> int i;
>>
>> + dm_meminfo_sub(addr, CONT_PTE_SIZE, CONT_PTE);
>> +
>> 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)));
>> +
>> + dm_meminfo_add(addr, CONT_PTE_SIZE, PTE);
>> }
>>
>> -static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>> +static int split_pmd(unsigned long addr, pmd_t *pmdp, pmd_t pmd, gfp_t gfp,
>> + bool to_cont)
>> {
>> pmdval_t tableprot = PMD_TYPE_TABLE | PMD_TABLE_UXN | PMD_TABLE_AF;
>> unsigned long pfn = pmd_pfn(pmd);
>> @@ -606,8 +708,13 @@ static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>> if (to_cont)
>> prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>>
>> + dm_meminfo_sub(addr, PMD_SIZE, PMD);
>> for (i = 0; i < PTRS_PER_PTE; i++, ptep++, pfn++)
>> __set_pte(ptep, pfn_pte(pfn, prot));
>> + if (to_cont)
>> + dm_meminfo_add(addr, PMD_SIZE, CONT_PTE);
>> + else
>> + dm_meminfo_add(addr, PMD_SIZE, PTE);
>>
>> /*
>> * Ensure the pte entries are visible to the table walker by the time
>> @@ -619,16 +726,21 @@ static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>> return 0;
>> }
>>
>> -static void split_contpmd(pmd_t *pmdp)
>> +static void split_contpmd(unsigned long addr, pmd_t *pmdp)
>> {
>> int i;
>>
>> + dm_meminfo_sub(addr, CONT_PMD_SIZE, CONT_PMD);
>> +
>> 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)));
>> +
>> + dm_meminfo_add(addr, CONT_PMD_SIZE, PMD);
>> }
>>
>> -static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
>> +static int split_pud(unsigned long addr, pud_t *pudp, pud_t pud, gfp_t gfp,
>> + bool to_cont)
>> {
>> pudval_t tableprot = PUD_TYPE_TABLE | PUD_TABLE_UXN | PUD_TABLE_AF;
>> unsigned int step = PMD_SIZE >> PAGE_SHIFT;
>> @@ -651,8 +763,13 @@ static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
>> if (to_cont)
>> prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>>
>> + dm_meminfo_sub(addr, PUD_SIZE, PUD);
>> for (i = 0; i < PTRS_PER_PMD; i++, pmdp++, pfn += step)
>> set_pmd(pmdp, pfn_pmd(pfn, prot));
>> + if (to_cont)
>> + dm_meminfo_add(addr, PUD_SIZE, CONT_PMD);
>> + else
>> + dm_meminfo_add(addr, PUD_SIZE, PMD);
>>
>> /*
>> * Ensure the pmd entries are visible to the table walker by the time
>> @@ -707,7 +824,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>> if (!pud_present(pud))
>> goto out;
>> if (pud_leaf(pud)) {
>> - ret = split_pud(pudp, pud, GFP_PGTABLE_KERNEL, true);
>> + ret = split_pud(addr, pudp, pud, GFP_PGTABLE_KERNEL, true);
>> if (ret)
>> goto out;
>> }
>> @@ -725,14 +842,14 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>> goto out;
>> if (pmd_leaf(pmd)) {
>> if (pmd_cont(pmd))
>> - split_contpmd(pmdp);
>> + split_contpmd(addr, 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, GFP_PGTABLE_KERNEL, true);
>> + ret = split_pmd(addr, pmdp, pmd, GFP_PGTABLE_KERNEL, true);
>> if (ret)
>> goto out;
>> }
>> @@ -749,7 +866,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>> if (!pte_present(pte))
>> goto out;
>> if (pte_cont(pte))
>> - split_contpte(ptep);
>> + split_contpte(addr, ptep);
>>
>> out:
>> return ret;
>> @@ -835,7 +952,7 @@ static int split_to_ptes_pud_entry(pud_t *pudp, unsigned long addr,
>> int ret = 0;
>>
>> if (pud_leaf(pud))
>> - ret = split_pud(pudp, pud, gfp, false);
>> + ret = split_pud(addr, pudp, pud, gfp, false);
>>
>> return ret;
>> }
>> @@ -849,8 +966,8 @@ static int split_to_ptes_pmd_entry(pmd_t *pmdp, unsigned long addr,
>>
>> if (pmd_leaf(pmd)) {
>> if (pmd_cont(pmd))
>> - split_contpmd(pmdp);
>> - ret = split_pmd(pmdp, pmd, gfp, false);
>> + split_contpmd(addr, pmdp);
>> + ret = split_pmd(addr, pmdp, pmd, gfp, false);
>>
>> /*
>> * We have split the pmd directly to ptes so there is no need to
>> @@ -868,7 +985,7 @@ static int split_to_ptes_pte_entry(pte_t *ptep, unsigned long addr,
>> pte_t pte = __ptep_get(ptep);
>>
>> if (pte_cont(pte))
>> - split_contpte(ptep);
>> + split_contpte(addr, ptep);
>>
>> return 0;
>> }
>> @@ -1444,37 +1561,57 @@ static bool pgtable_range_aligned(unsigned long start, unsigned long end,
>> return true;
>> }
>>
>> -static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
>> +static void unmap_hotplug_pte_range(pte_t *ptep, unsigned long addr,
>> unsigned long end, bool free_mapped,
>> struct vmem_altmap *altmap)
>> {
>> - pte_t *ptep, pte;
>> + pte_t pte;
>>
>> do {
>> - ptep = pte_offset_kernel(pmdp, addr);
>> pte = __ptep_get(ptep);
>> if (pte_none(pte))
>> continue;
>>
>> WARN_ON(!pte_present(pte));
>> __pte_clear(&init_mm, addr, ptep);
>> + dm_meminfo_sub(addr, PAGE_SIZE, PTE);
>> flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
>> if (free_mapped)
>> free_hotplug_page_range(pte_page(pte),
>> PAGE_SIZE, altmap);
>> - } while (addr += PAGE_SIZE, addr < end);
>> + } while (ptep++, addr += PAGE_SIZE, addr < end);
>> +}
>> +
>> +static void unmap_hotplug_cont_pte_range(pmd_t *pmdp, unsigned long addr,
>> + unsigned long end, bool free_mapped,
>> + struct vmem_altmap *altmap)
>> +{
>> + unsigned long next;
>> + pte_t *ptep, pte;
>> +
>> + do {
>> + next = pte_cont_addr_end(addr, end);
>> + ptep = pte_offset_kernel(pmdp, addr);
>> + pte = __ptep_get(ptep);
>> +
>> + if (pte_present(pte) && pte_cont(pte)) {
>> + dm_meminfo_sub(addr, CONT_PTE_SIZE, CONT_PTE);
>> + dm_meminfo_add(addr, CONT_PTE_SIZE, PTE);
>> + }
>> +
>> + unmap_hotplug_pte_range(ptep, addr, next, free_mapped, altmap);
>> + } while (addr = next, addr < end);
>> }
>>
>> -static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
>> +static void unmap_hotplug_pmd_range(pmd_t *pmdp, unsigned long addr,
>> unsigned long end, bool free_mapped,
>> struct vmem_altmap *altmap)
>> {
>> unsigned long next;
>> - pmd_t *pmdp, pmd;
>> + pmd_t pmd;
>>
>> do {
>> next = pmd_addr_end(addr, end);
>> - pmdp = pmd_offset(pudp, addr);
>> pmd = READ_ONCE(*pmdp);
>> if (pmd_none(pmd))
>> continue;
>> @@ -1482,6 +1619,7 @@ static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
>> WARN_ON(!pmd_present(pmd));
>> if (pmd_sect(pmd)) {
>> pmd_clear(pmdp);
>> + dm_meminfo_sub(addr, PMD_SIZE, PMD);
>>
>> /*
>> * One TLBI should be sufficient here as the PMD_SIZE
>> @@ -1494,7 +1632,28 @@ static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
>> continue;
>> }
>> WARN_ON(!pmd_table(pmd));
>> - unmap_hotplug_pte_range(pmdp, addr, next, free_mapped, altmap);
>> + unmap_hotplug_cont_pte_range(pmdp, addr, next, free_mapped, altmap);
>> + } while (pmdp++, addr = next, addr < end);
>> +}
>> +
>> +static void unmap_hotplug_cont_pmd_range(pud_t *pudp, unsigned long addr,
>> + unsigned long end, bool free_mapped,
>> + struct vmem_altmap *altmap)
>> +{
>> + unsigned long next;
>> + pmd_t *pmdp, pmd;
>> +
>> + do {
>> + next = pmd_cont_addr_end(addr, end);
>> + pmdp = pmd_offset(pudp, addr);
>> + pmd = READ_ONCE(*pmdp);
>> +
>> + if (pmd_leaf(pmd) && pmd_cont(pmd)) {
>> + dm_meminfo_sub(addr, CONT_PMD_SIZE, CONT_PMD);
>> + dm_meminfo_add(addr, CONT_PMD_SIZE, PMD);
>> + }
>> +
>> + unmap_hotplug_pmd_range(pmdp, addr, next, free_mapped, altmap);
>> } while (addr = next, addr < end);
>> }
>>
>> @@ -1515,6 +1674,7 @@ static void unmap_hotplug_pud_range(p4d_t *p4dp, unsigned long addr,
>> WARN_ON(!pud_present(pud));
>> if (pud_sect(pud)) {
>> pud_clear(pudp);
>> + dm_meminfo_sub(addr, PUD_SIZE, PUD);
>>
>> /*
>> * One TLBI should be sufficient here as the PUD_SIZE
>> @@ -1527,7 +1687,7 @@ static void unmap_hotplug_pud_range(p4d_t *p4dp, unsigned long addr,
>> continue;
>> }
>> WARN_ON(!pud_table(pud));
>> - unmap_hotplug_pmd_range(pudp, addr, next, free_mapped, altmap);
>> + unmap_hotplug_cont_pmd_range(pudp, addr, next, free_mapped, altmap);
>> } while (addr = next, addr < end);
>> }
>>
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