diff mbox series

[v1,4/5] mm: Add new ptep_deref() helper to fully encapsulate pte_t

Message ID 20230511125848.78621-5-ryan.roberts@arm.com
State New
Headers
Series [v1,1/5] mm: vmalloc must set pte via arch code |

Commit Message

Ryan Roberts May 11, 2023, 12:58 p.m. UTC 
  There are many call sites that directly dereference a pte_t pointer.
This makes it very difficult to properly encapsulate a page table in the
arch code without having to allocate shadow page tables. ptep_deref()
aims to solve this by replacing all direct dereferences with a call to
this function.

The default implementation continues to just dereference the pointer
(*ptep), so generated code should be exactly the same. However, it is
possible for the architecture to override the default with their own
implementation, that can (e.g.) hide certain bits from the core code, or
determine young/dirty status by mixing in state from another source.

While ptep_get() and ptep_get_lockless() already exist, these are
implemented as atomic accesses (e.g. READ_ONCE() in the default case).
So rather than using ptep_get() and risking performance regressions,
introduce an new variant.

Call sites will be converted to use the accessor in future commits.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 include/linux/pgtable.h | 7 +++++++
 1 file changed, 7 insertions(+)

Comments

Ryan Roberts May 11, 2023, 1:14 p.m. UTC | #1 
My appologies for the noise: A blank line between Cc and Subject has broken the
subject and grouping in lore.

Please Ignore this, I will resend.


On 11/05/2023 13:58, Ryan Roberts wrote:
> There are many call sites that directly dereference a pte_t pointer.
> This makes it very difficult to properly encapsulate a page table in the
> arch code without having to allocate shadow page tables. ptep_deref()
> aims to solve this by replacing all direct dereferences with a call to
> this function.
> 
> The default implementation continues to just dereference the pointer
> (*ptep), so generated code should be exactly the same. However, it is
> possible for the architecture to override the default with their own
> implementation, that can (e.g.) hide certain bits from the core code, or
> determine young/dirty status by mixing in state from another source.
> 
> While ptep_get() and ptep_get_lockless() already exist, these are
> implemented as atomic accesses (e.g. READ_ONCE() in the default case).
> So rather than using ptep_get() and risking performance regressions,
> introduce an new variant.
> 
> Call sites will be converted to use the accessor in future commits.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  include/linux/pgtable.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index c5a51481bbb9..1161beab2492 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -204,6 +204,13 @@ static inline int pudp_set_access_flags(struct vm_area_struct *vma,
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  #endif
>  
> +#ifndef ptep_deref
> +static inline pte_t ptep_deref(pte_t *ptep)
> +{
> +	return *(pte_t *)ptep;
> +}
> +#endif
> +
>  #ifndef __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
>  static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>  					    unsigned long address,
diff mbox series

Patch

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index c5a51481bbb9..1161beab2492 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -204,6 +204,13 @@  static inline int pudp_set_access_flags(struct vm_area_struct *vma,
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 #endif
 
+#ifndef ptep_deref
+static inline pte_t ptep_deref(pte_t *ptep)
+{
+	return *(pte_t *)ptep;
+}
+#endif
+
 #ifndef __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
 static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
 					    unsigned long address,