Re: [PATCH V3] mm: Standardize printing for pgtable entries

[Andrew Morton]

On Thu,  9 Jul 2026 10:13:34 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:

> From: "David Hildenbrand (Arm)" <david@xxxxxxxxxx>
> 
> Bad page map reporting currently stores page table entry values in an
> unsigned long long and prints them with fixed 64-bit-oriented format
> strings. This is inconsistent across call sites and does not work well for
> architectures where page table entry values are not naturally represented
> as 64-bit values, such as 32-bit or 128-bit entries.

Well grumble.  It's a lot of fuss for something which nobody is hurting
from.  Or are they?  What's the actual utility here?

> Introduce a common helper to convert raw page table entry values into a
> fixed-width hexadecimal string based on the actual entry size. Use it for
> bad page map reporting and for dumping the page table walk in
> __print_bad_page_map_pgtable().
> 
> Pass page table entry values to the reporting path as raw bytes together
> with their size, instead of forcing them through an unsigned long long.
> It keeps the printed output consistent and avoids truncation or misleading
> formatting for non-64-bit page table entries.
> 
> --- a/mm/memory.c
> +++ b/mm/memory.c
>
> ...
>
> +#if defined(__SIZEOF_INT128__)
> +#define PTVAL_STR_MAX	(32 + 1) /* Max 128-bit value in hex + NUL */
> +#else
> +#define PTVAL_STR_MAX	(16 + 1) /* Max 64-bit value in hex + NUL */
> +#endif
> +
>  static void __print_bad_page_map_pgtable(struct mm_struct *mm, unsigned long addr)
>  {
> -	unsigned long long pgdv, p4dv, pudv, pmdv;
> +	char pgd_str[PTVAL_STR_MAX];
> +	char p4d_str[PTVAL_STR_MAX];
> +	char pud_str[PTVAL_STR_MAX];
> +	char pmd_str[PTVAL_STR_MAX];

That's another 128b of stack, in a potentially deep code path.

Hopefully gcc can reuse the same stack space for some of these but it's
not been good at this in the past.

>  	p4d_t p4d, *p4dp;
>  	pud_t pud, *pudp;
>  	pmd_t pmd, *pmdp;
> @@ -532,34 +575,34 @@ static void __print_bad_page_map_pgtable(struct mm_struct *mm, unsigned long add
>  	 * see locking requirements for print_bad_page_map().
>  	 */
>  	pgdp = pgd_offset(mm, addr);
> -	pgdv = pgd_val(*pgdp);
> +	ptval_to_str(pgd_str, pgd_val(*pgdp));
>  
>  	if (!pgd_present(*pgdp) || pgd_leaf(*pgdp)) {
> -		pr_alert("pgd:%08llx\n", pgdv);
> +		pr_alert("pgd:%s\n", pgd_str);

can this do
		pr_alert("pgd:%s\n", ptval_to_str(pgd_str, pgd_val(*pgdp)));

and eliminate a few locals?