Re: [v6 PATCH] arm64: mm: show direct mapping use in /proc/meminfo

[Will Deacon <will@kernel.org>]

On Mon, Mar 16, 2026 at 12:29:33PM -0700, 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>
> ---
> v6: * Rebased to v7.0-rc3
>     * Rebased on top of Anshuman's v5 "arm64/mm: Enable batched TLB flush
>       in unmap_hotplug_range()"
>     * Used const for direct map type array per Will
>     * Defined PUD size for 16K/64K even though it is not used per Will
>     * Removed the misleading comment in init_pmd() per Will
> 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 | 197 +++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 176 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 5fb9a66f0754..7e95dbc69d57 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 +172,87 @@ 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)
> +{
> +	const 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";
> +	size[PUD] = "64G";
> +#elif defined(CONFIG_ARM64_64K_PAGES)
> +	size[PTE] = "64k";
> +	size[CONT_PTE] = "2M";
> +	size[PMD] = "512M";
> +	size[CONT_PMD] = "16G";
> +	size[PUD] = "4T";
> +#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);
> +}

Just use __is_lm_address()?

For better or worse, the arm64 arch code tends to talk about the "linear
map" rather than the "direct map", so a little bit of renaming would be
good (i.e. s/dm/lm/). I'm fine if you want to keep the user-visible
strings as "DirectMap".

Will