Re: [RFC V2 1/3] lib/vsprintf: Add support for pgtable entries

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 10/06/26 4:43 PM, Usama Arif wrote:
> On Wed, 10 Jun 2026 05:35:43 +0100 Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
>> Add some print formats for pgtable entries at any pgtable level. These new
>> formats are %pp[g|4|u|m|t][d|e] i.e %ppgd, %pp4d, %ppud, %ppmd, and %ppte.
>> These currently support both 32 bit and 64 bit pgtable entries that can be
>> extended up to 128 bit when required.
>>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@kernel.org>
>> Cc: Lorenzo Stoakes <ljs@kernel.org>
>> Cc: Petr Mladek <pmladek@suse.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Jonathan Corbet <corbet@lwn.net>
>> Cc: Andy Whitcroft <apw@canonical.com>
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-doc@vger.kernel.org
>>
>>  Documentation/core-api/printk-formats.rst | 19 ++++++++
>>  lib/vsprintf.c                            | 58 +++++++++++++++++++++++
>>  scripts/checkpatch.pl                     |  2 +-
>>  3 files changed, 78 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
>> index c0b1b6089307..e69f91a9dd9d 100644
>> --- a/Documentation/core-api/printk-formats.rst
>> +++ b/Documentation/core-api/printk-formats.rst
>> @@ -696,6 +696,25 @@ Rust
>>  Only intended to be used from Rust code to format ``core::fmt::Arguments``.
>>  Do *not* use it from C.
>>  
>> +Page Table Entry
>> +----------------
>> +
>> +::
>> +
>> +        %p[pgd|p4dp|pud|pmd|pte]
> 
> s/p4dp/p4d to match others

Will fix.
> 
> 
>> +
>> +Print page table entry at any level.
>> +
>> +Passed by reference.
>> +
>> +Examples for a 64 bit page table entry, given &(u64)0xc0ffee::
>> +
>> +        %ppte   0x0000000000c0ffee
>> +        %ppmd   0x0000000000c0ffee
>> +        %ppud   0x0000000000c0ffee
>> +        %pp4d   0x0000000000c0ffee
>> +        %ppgd   0x0000000000c0ffee
>> +
>>  Thanks
>>  ======
>>  
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 9f359b31c8d1..d4ad3048a4db 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -856,6 +856,59 @@ static char *default_pointer(char *buf, char *end, const void *ptr,
>>  	return ptr_to_id(buf, end, ptr, spec);
>>  }
>>  
>> +static char *pxd_pointer(char *buf, char *end, const void *ptr,
>> +			 struct printf_spec spec, const char *fmt)
>> +{
>> +	if (check_pointer(&buf, end, ptr, spec))
>> +		return buf;
>> +
>> +	if (fmt[1] == 't' && fmt[2] == 'e') {
>> +		pte_t *pte = (pte_t *)ptr;
>> +
>> +		static_assert(sizeof(pte_t) == 4 ||
>> +			      sizeof(pte_t) == 8,
>> +			      "pte_t size must be 4 or 8 bytes");
>> +		return special_hex_number(buf, end, pte_val(ptep_get(pte)), sizeof(pte_t));
>> +	}
>> +
>> +	if (fmt[1] == 'm' && fmt[2] == 'd') {
>> +		pmd_t *pmd = (pmd_t *)ptr;
>> +
>> +		static_assert(sizeof(pmd_t) == 4 ||
>> +			      sizeof(pmd_t) == 8,
>> +			      "pmd_t size must be 4 or 8 bytes");
>> +		return special_hex_number(buf, end, pmd_val(pmdp_get(pmd)), sizeof(pmd_t));
>> +	}
>> +
>> +	if (fmt[1] == 'u' && fmt[2] == 'd') {
>> +		pud_t *pud = (pud_t *)ptr;
>> +
>> +		static_assert(sizeof(pud_t) == 4 ||
>> +			      sizeof(pud_t) == 8,
>> +			      "pud_t size must be 4 or 8 bytes");
>> +		return special_hex_number(buf, end, pud_val(pudp_get(pud)), sizeof(pud_t));
>> +	}
>> +
>> +	if (fmt[1] == '4' && fmt[2] == 'd') {
>> +		p4d_t *p4d = (p4d_t *)ptr;
>> +
>> +		static_assert(sizeof(p4d_t) == 4 ||
>> +			      sizeof(p4d_t) == 8,
>> +			      "p4d_t size must be 4 or 8 bytes");
>> +		return special_hex_number(buf, end, p4d_val(p4dp_get(p4d)), sizeof(p4d_t));
>> +	}
>> +
>> +	if (fmt[1] == 'g' && fmt[2] == 'd') {
>> +		pgd_t *pgd = (pgd_t *)ptr;
>> +
>> +		static_assert(sizeof(pgd_t) == 4 ||
>> +			      sizeof(pgd_t) == 8,
>> +			      "pgd_t size must be 4 or 8 bytes");
>> +		return special_hex_number(buf, end, pgd_val(pgdp_get(pgd)), sizeof(pgd_t));
> 
> You mentioned in the coverletter that pgdp_get() is the reason arm32 builds dont work.
> Just wanted to check what the issue is?
> 
> I had a look at arch/arm/include/asm/pgtable.h and I couldnt understand why
> it reads pgdp_get(pgpd) instead of pgdp_get(pgdp)?

Right - that's a typo.

Following arm32 pgtable header change enables the build.

diff --git a/arch/arm/include/asm/pgtable-2level-types.h b/arch/arm/include/asm/pgtable-2level-types.h
index 650e793f4142..3f1d52402129 100644
--- a/arch/arm/include/asm/pgtable-2level-types.h
+++ b/arch/arm/include/asm/pgtable-2level-types.h
@@ -31,6 +31,7 @@ typedef struct { pteval_t pgprot; } pgprot_t;
 #define __pte(x)        ((pte_t) { (x) } )
 #define __pmd(x)        ((pmd_t) { (x) } )
 #define __pgprot(x)     ((pgprot_t) { (x) } )
+#define __pgd(x)        ((pgd_t) { { (x), 0 } })

 #else
 /*
@@ -49,6 +50,7 @@ typedef pteval_t pgprot_t;
 #define __pte(x)        (x)
 #define __pmd(x)        (x)
 #define __pgprot(x)     (x)
+#define __pgd(x)        { (x), 0 }

 #endif /* STRICT_MM_TYPECHECKS */

diff --git a/arch/arm/include/asm/pgtable.h b/arch/arm/include/asm/pgtable.h
index 982795cf4563..349e1f819385 100644
--- a/arch/arm/include/asm/pgtable.h
+++ b/arch/arm/include/asm/pgtable.h
@@ -141,7 +141,7 @@ extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,

 extern pgd_t swapper_pg_dir[PTRS_PER_PGD];

-#define pgdp_get(pgpd)         READ_ONCE(*pgdp)
+#define pgdp_get(pgdp)         READ_ONCE(*pgdp)

 #define pud_page(pud)          pmd_page(__pmd(pud_val(pud)))
 #define pud_write(pud)         pmd_write(__pmd(pud_val(pud)))
>    
> 
>> +	}
>> +	return default_pointer(buf, end, ptr, spec);
>> +}
>> +
>>  int kptr_restrict __read_mostly;
>>  
>>  static noinline_for_stack
>> @@ -2506,6 +2559,9 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
>>   *		Without an option prints the full name of the node
>>   *		f full name
>>   *		P node name, including a possible unit address
>> + * - 'p[g|4|u|m|t|][d|e]' For a page table entry, this prints its
>> + *			  contents in a hexadecimal format
>> + *
>>   * - 'x' For printing the address unmodified. Equivalent to "%lx".
>>   *       Please read the documentation (path below) before using!
>>   * - '[ku]s' For a BPF/tracing related format specifier, e.g. used out of
>> @@ -2615,6 +2671,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>>  		default:
>>  			return error_string(buf, end, "(einval)", spec);
>>  		}
>> +	case 'p':
>> +		return pxd_pointer(buf, end, ptr, spec, fmt);
>>  	default:
>>  		return default_pointer(buf, end, ptr, spec);
>>  	}
>> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
>> index 0492d6afc9a1..f68955858e29 100755
>> --- a/scripts/checkpatch.pl
>> +++ b/scripts/checkpatch.pl
>> @@ -6975,7 +6975,7 @@ sub process {
>>  				my $fmt = get_quoted_string($lines[$count - 1], raw_line($count, 0));
>>  				$fmt =~ s/%%//g;
>>  
>> -				while ($fmt =~ /(\%[\*\d\.]*p(\w)(\w*))/g) {
>> +				while ($fmt =~ /(\%[\*\d\.]*p(\w)(\w*)(pte|pmd|pud|p4d|pgd))/g) {
>>  					$specifier = $1;
>>  					$extension = $2;
>>  					$qualifier = $3;
>> -- 
>> 2.30.2
>>
>>