Re: [PATCH v5 1/8] mm: Add a ptdesc flag to mark kernel page tables

[Baolu Lu <baolu.lu@linux.intel.com>]

On 10/9/25 03:56, Matthew Wilcox wrote:
> On Fri, Sep 19, 2025 at 01:39:59PM +0800, Lu Baolu wrote:
>> +static inline void ptdesc_set_kernel(struct ptdesc *ptdesc)
>> +{
>> +	struct folio *folio = ptdesc_folio(ptdesc);
>> +
>> +	folio_set_referenced(folio);
>> +}
> So this was the right way to do this at the time.  However, if you look
> at commit 522abd92279a this should now be ...
> 
>   enum pt_flags {
>   	PT_reserved = PG_reserved,
> +	PT_kernel = PG_referenced,
>   	/* High bits are used for zone/node/section */
>   };
> [...]
> 
> +static inline void ptdesc_set_kernel(struct ptdesc *ptdesc)
> +{
> +	set_bit(PT_kernel, &pt->pt_flags.f);
> +}

Thank you for the review comment. I updated the patch like the
following:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a3f97c551ad8..5abd427b6202 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(struct ptdesc *ptdesc)
+{
+       return test_bit(PT_kernel, &ptdesc->pt_flags.f);
+}
+
  /**
   * pagetable_alloc - Allocate pagetables
   * @gfp:    GFP flags

Thanks,
baolu