Re: [PATCH v7 2/8] mm: Add a ptdesc flag to mark kernel page tables

[Mike Rapoport <rppt@kernel.org>]

On Wed, Oct 22, 2025 at 04:26:28PM +0800, Lu Baolu wrote:
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> The page tables used to map the kernel and userspace often have very
> different handling rules. There are frequently *_kernel() variants of
> functions just for kernel page tables. That's not great and has lead
> to code duplication.
> 
> Instead of having completely separate call paths, allow a 'ptdesc' to
> be marked as being for kernel mappings. Introduce helpers to set and
> clear this status.
> 
> Note: this uses the PG_referenced bit. Page flags are a great fit for
> this since it is truly a single bit of information.  Use PG_referenced
> itself because it's a fairly benign flag (as opposed to things like
> PG_lock). It's also (according to Willy) unlikely to go away any time
> soon.
> 
> PG_referenced is not in PAGE_FLAGS_CHECK_AT_FREE. It does not need to
> be cleared before freeing the page, and pages coming out of the
> allocator should have it cleared. Regardless, introduce an API to
> clear it anyway. Having symmetry in the API makes it easier to change
> the underlying implementation later, like if there was a need to move
> to a PAGE_FLAGS_CHECK_AT_FREE bit.
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Acked-by: David Hildenbrand <david@redhat.com>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  include/linux/mm.h | 41 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d16b33bacc32..354d7925bf77 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2940,6 +2940,7 @@ static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long a
>  #endif /* CONFIG_MMU */
>  
>  enum pt_flags {
> +	PT_kernel = PG_referenced,
>  	PT_reserved = PG_reserved,
>  	/* High bits are used for zone/node/section */
>  };
> @@ -2965,6 +2966,46 @@ static inline bool pagetable_is_reserved(struct ptdesc *pt)
>  	return test_bit(PT_reserved, &pt->pt_flags.f);
>  }
>  
> +/**
> + * ptdesc_set_kernel - Mark a ptdesc used to map the kernel
> + * @ptdesc: The ptdesc to be marked
> + *
> + * Kernel page tables often need special handling. Set a flag so that
> + * the handling code knows this ptdesc will not be used for userspace.
> + */
> +static inline void ptdesc_set_kernel(struct ptdesc *ptdesc)
> +{
> +	set_bit(PT_kernel, &ptdesc->pt_flags.f);
> +}
> +
> +/**
> + * ptdesc_clear_kernel - Mark a ptdesc as no longer used to map the kernel
> + * @ptdesc: The ptdesc to be unmarked
> + *
> + * Use when the ptdesc is no longer used to map the kernel and no longer
> + * needs special handling.
> + */
> +static inline void ptdesc_clear_kernel(struct ptdesc *ptdesc)
> +{
> +	/*
> +	 * Note: the 'PG_referenced' bit does not strictly need to be
> +	 * cleared before freeing the page. But this is nice for
> +	 * symmetry.
> +	 */
> +	clear_bit(PT_kernel, &ptdesc->pt_flags.f);
> +}
> +
> +/**
> + * ptdesc_test_kernel - Check if a ptdesc is used to map the kernel
> + * @ptdesc: The ptdesc being tested
> + *
> + * Call to tell if the ptdesc used to map the kernel.
> + */
> +static inline bool ptdesc_test_kernel(const struct ptdesc *ptdesc)
> +{
> +	return test_bit(PT_kernel, &ptdesc->pt_flags.f);
> +}
> +
>  /**
>   * pagetable_alloc - Allocate pagetables
>   * @gfp:    GFP flags
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.