Re: [v2 PATCH] arm64: mm: show direct mapping use in /proc/meminfo

Yang Shi
2025-11-10T23:06:16Z

Hi Ryan,

Gently ping. Thanks for reviewing the v1 patch. Any comment for this
version?

Yang

On 10/23/25 2:52 PM, 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.
> 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
>
> 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.
>
> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
> ---
> arch/arm64/mm/mmu.c | 86 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 86 insertions(+)
>
> 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
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index b8d37eb037fc..7207b55d0046 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>
> @@ -51,6 +52,17 @@
>
> DEFINE_STATIC_KEY_FALSE(arm64_ptdump_lock_key);
>
> +enum direct_map_type {
> + PTE,
> + CONT_PTE,
> + PMD,
> + CONT_PMD,
> + PUD,
> + NR_DIRECT_MAP_TYPE,
> +};
> +
> +static unsigned long direct_map_size[NR_DIRECT_MAP_TYPE];
> +
> u64 kimage_voffset __ro_after_init;
> EXPORT_SYMBOL(kimage_voffset);
>
> @@ -171,6 +183,45 @@ static void init_clear_pgtable(void *table)
> dsb(ishst);
> }
>
> +#ifdef CONFIG_PROC_FS
> +void arch_report_meminfo(struct seq_file *m)
> +{
> + char *size[NR_DIRECT_MAP_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], direct_map_size[PTE] >> 10);
> + seq_printf(m, "DirectMap%s: %8lu kB\n",
> + size[CONT_PTE],
> + direct_map_size[CONT_PTE] >> 10);
> + seq_printf(m, "DirectMap%s: %8lu kB\n",
> + size[PMD], direct_map_size[PMD] >> 10);
> + seq_printf(m, "DirectMap%s: %8lu kB\n",
> + size[CONT_PMD],
> + direct_map_size[CONT_PMD] >> 10);
> + if (pud_sect_supported())
> + seq_printf(m, "DirectMap%s: %8lu kB\n",
> + size[PUD], direct_map_size[PUD] >> 10);
> +}
> +#endif
> +
> static void init_pte(pte_t *ptep, unsigned long addr, unsigned long end,
> phys_addr_t phys, pgprot_t prot)
> {
> @@ -234,6 +285,11 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>
> init_pte(ptep, addr, next, phys, __prot);
>
> + if (pgprot_val(__prot) & PTE_CONT)
> + direct_map_size[CONT_PTE] += next - addr;
> + else
> + direct_map_size[PTE] += next - addr;
> +
> ptep += pte_index(next) - pte_index(addr);
> phys += next - addr;
> } while (addr = next, addr != end);
> @@ -262,6 +318,17 @@ static void 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)
> + direct_map_size[CONT_PMD] += next - addr;
> + else
> + direct_map_size[PMD] += next - addr;
> /*
> * After the PMD entry has been populated once, we
> * only allow updates to the permission attributes.
> @@ -368,6 +435,7 @@ static void alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
> (flags & NO_BLOCK_MAPPINGS) == 0) {
> pud_set_huge(pudp, phys, prot);
>
> + direct_map_size[PUD] += next - addr;
> /*
> * After the PUD entry has been populated once, we
> * only allow updates to the permission attributes.
> @@ -532,9 +600,13 @@ static void split_contpte(pte_t *ptep)
> {
> int i;
>
> + direct_map_size[CONT_PTE] -= CONT_PTE_SIZE;
> +
> 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)));
> +
> + direct_map_size[PTE] += CONT_PTE_SIZE;
> }
>
> static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
> @@ -559,8 +631,13 @@ 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);
>
> + direct_map_size[PMD] -= PMD_SIZE;
> for (i = 0; i < PTRS_PER_PTE; i++, ptep++, pfn++)
> __set_pte(ptep, pfn_pte(pfn, prot));
> + if (to_cont)
> + direct_map_size[CONT_PTE] += PMD_SIZE;
> + else
> + direct_map_size[PTE] += PMD_SIZE;
>
> /*
> * Ensure the pte entries are visible to the table walker by the time
> @@ -576,9 +653,13 @@ static void split_contpmd(pmd_t *pmdp)
> {
> int i;
>
> + direct_map_size[CONT_PMD] -= CONT_PMD_SIZE;
> +
> 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)));
> +
> + direct_map_size[PMD] += CONT_PMD_SIZE;
> }
>
> static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
> @@ -604,8 +685,13 @@ 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);
>
> + direct_map_size[PUD] -= PUD_SIZE;
> for (i = 0; i < PTRS_PER_PMD; i++, pmdp++, pfn += step)
> set_pmd(pmdp, pfn_pmd(pfn, prot));
> + if (to_cont)
> + direct_map_size[CONT_PMD] += PUD_SIZE;
> + else
> + direct_map_size[PMD] += PUD_SIZE;
>
> /*
> * Ensure the pmd entries are visible to the table walker by the time