Re: [RFC 1/2] lib/vsprintf: Add support for pte_t

[Pedro Falcato <pfalcato@suse.de>]

On Wed, Jun 18, 2025 at 09:42:34AM +0530, Anshuman Khandual wrote:
> Add a new format for printing page table entries.
> 
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  Documentation/core-api/printk-formats.rst | 14 ++++++++++++++
>  lib/vsprintf.c                            | 20 ++++++++++++++++++++
>  mm/memory.c                               |  5 ++---
>  scripts/checkpatch.pl                     |  2 +-
>  4 files changed, 37 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> index 4b7f3646ec6ce..75a110b059ee1 100644
> --- a/Documentation/core-api/printk-formats.rst
> +++ b/Documentation/core-api/printk-formats.rst
> @@ -689,6 +689,20 @@ Rust
>  Only intended to be used from Rust code to format ``core::fmt::Arguments``.
>  Do *not* use it from C.
>  
> +Page Table Entry
> +----------------
> +
> +::
> +        %ppte
> +
> +Print standard page table entry pte_t.
> +
> +Passed by reference.
> +
> +Examples for a 64 bit page table entry, given &(u64)0xc0ffee::
> +
> +        %ppte   0x00c0ffee

Ok, so what's the point of this if you're just printing the number?

Could at least do something like:

%ppte 0xc0ff000|WRITE|DIRTY|PRESENT

no? Otherwise it's a not super useful wrapper around printing pte_val(*pte).

> +
>  Thanks
>  ======
>  
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3d85800757aa5..005490202ffb5 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2433,6 +2433,9 @@ 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
> + * - 'pte'	For a 64 bit page table entry, this prints its contents in
> + *              a hexa decimal 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
> @@ -2542,6 +2545,23 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  		default:
>  			return error_string(buf, end, "(einval)", spec);
>  		}
> +	case 'p':
> +		if (fmt[1] == 't' && fmt[2] == 'e') {
> +			pte_t *pte = (pte_t *)ptr;
> +
> +			spec.field_width = 10;
> +			spec.precision = 8;
> +			spec.base = 16;
> +			spec.flags = SPECIAL | SMALL | ZEROPAD;
> +			if (sizeof(pte_t) == sizeof(u64)) {
> +				u64 val = pte_val(*pte);
> +
> +				return number(buf, end, val, spec);
> +			}

As mentioned elsewhere in the thread, this obviously doesn't work for everything
32-bit, and 64-bit PAE, and all of the weird page table formats we have around.

-- 
Pedro