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

[Yang Shi <yang@os.amperecomputing.com>]

On 5/12/26 8:22 AM, Will Deacon wrote:
> 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()?

Yeah, good idea.

>
> 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".

Sure, I will rename the internal APIs to "lm" to keep the consistency. I 
think we'd better to keep the user-visible strings as "DirectMap".

Thanks,
Yang

>
> Will