linux-kernel - Re: [PATCH v6 10/18] x86/mm: Physical address comparisons in fill

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Message-ID: <CAG_fn=Ut9JUpStLiO+GsoBpn3d_EyyttcuBby=EKzuxkKdcKcw@mail.gmail.com>
Date: Mon, 10 Nov 2025 17:24:38 +0100
From: Alexander Potapenko <glider@...gle.com>
To: Maciej Wieczor-Retman <m.wieczorretman@...me>
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Subject: Re: [PATCH v6 10/18] x86/mm: Physical address comparisons in fill_p*d/pte

On Wed, Oct 29, 2025 at 9:07 PM Maciej Wieczor-Retman
<m.wieczorretman@...me> wrote:
>
> From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
>
> Calculating page offset returns a pointer without a tag. When comparing
> the calculated offset to a tagged page pointer an error is raised
> because they are not equal.
>
> Change pointer comparisons to physical address comparisons as to avoid
> issues with tagged pointers that pointer arithmetic would create. Open
> code pte_offset_kernel(), pmd_offset(), pud_offset() and p4d_offset().
> Because one parameter is always zero and the rest of the function
> insides are enclosed inside __va(), removing that layer lowers the
> complexity of final assembly.
>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
> ---
> Changelog v2:
> - Open code *_offset() to avoid it's internal __va().
>
>  arch/x86/mm/init_64.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 0e4270e20fad..2d79fc0cf391 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -269,7 +269,10 @@ static p4d_t *fill_p4d(pgd_t *pgd, unsigned long vaddr)
>         if (pgd_none(*pgd)) {
>                 p4d_t *p4d = (p4d_t *)spp_getpage();
>                 pgd_populate(&init_mm, pgd, p4d);
> -               if (p4d != p4d_offset(pgd, 0))
> +
> +               if (__pa(p4d) != (pgtable_l5_enabled() ?
> +                                 __pa(pgd) :
> +                                 (unsigned long)pgd_val(*pgd) & PTE_PFN_MASK))

Did you test with both 4- and 5-level paging?
If I understand correctly, p4d and pgd are supposed to be the same
under !pgtable_l5_enabled().