Re: [PATCH v4 1/8] mm: Add a ptdesc flag to mark kernel page tables

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Sep 05, 2025 at 01:50:56PM +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>
> ---
>  include/linux/page-flags.h | 51 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 51 insertions(+)

Seems like a good idea

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

> +/**
> + * 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(struct ptdesc *ptdesc)
> +{
> +	struct folio *folio = ptdesc_folio(ptdesc);
> +
> +	/*
> +	 * Note: the 'PG_referenced' bit does not strictly need to be
> +	 * tested before freeing the page. But this is nice for
> +	 * symmetry.
> +	 */
> +	return folio_test_referenced(folio);

Wrong comment, it was copied from the prior function

Jason