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

[Usama Arif <usama.arif@linux.dev>]

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


> +
> +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)?
   

> +	}
> +	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
> 
>