2023-11-21 12:03:39

by Alexander Gordeev

Subject: [PATCH] pgtable: do not expose _refcount field via ptdesc

Since commit d08d4e7cd6bf ("s390/mm: use full 4KB page for 2KB PTE")
_refcount field is not used for fragmented page tracking on s390 and
there is no other code left that accesses this field explicitly.

Suggested-by: Heiko Carstens <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>
---
include/linux/mm_types.h | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 957ce38768b2..0330e0ddca11 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -401,11 +401,10 @@ FOLIO_MATCH(compound_head, _head_2a);
* @pmd_huge_pte: Protected by ptdesc->ptl, used for THPs.
* @__page_mapping: Aliases with page->mapping. Unused for page tables.
* @pt_mm: Used for x86 pgds.
- * @pt_frag_refcount: For fragmented page table tracking. Powerpc and s390 only.
+ * @pt_frag_refcount: For fragmented page table tracking. Powerpc only.
* @_pt_pad_2: Padding to ensure proper alignment.
* @ptl: Lock for the page table.
* @__page_type: Same as page->page_type. Unused for page tables.
- * @_refcount: Same as page refcount. Used for s390 page tables.
* @pt_memcg_data: Memcg data. Tracked for page tables here.
*
* This struct overlays struct page for now. Do not modify without a good
@@ -438,7 +437,7 @@ struct ptdesc {
#endif
};
unsigned int __page_type;
- atomic_t _refcount;
+ unsigned int _pt_pad_3;
#ifdef CONFIG_MEMCG
unsigned long pt_memcg_data;
#endif
@@ -452,7 +451,6 @@ TABLE_MATCH(compound_head, _pt_pad_1);
TABLE_MATCH(mapping, __page_mapping);
TABLE_MATCH(rcu_head, pt_rcu_head);
TABLE_MATCH(page_type, __page_type);
-TABLE_MATCH(_refcount, _refcount);
#ifdef CONFIG_MEMCG
TABLE_MATCH(memcg_data, pt_memcg_data);
#endif
--
2.39.2

2023-11-21 14:50:54

by Heiko Carstens

Subject: Re: [PATCH] pgtable: do not expose _refcount field via ptdesc

On Tue, Nov 21, 2023 at 01:03:10PM +0100, Alexander Gordeev wrote:
> Since commit d08d4e7cd6bf ("s390/mm: use full 4KB page for 2KB PTE")
> _refcount field is not used for fragmented page tracking on s390 and
> there is no other code left that accesses this field explicitly.
>
> Suggested-by: Heiko Carstens <[email protected]>
> Signed-off-by: Alexander Gordeev <[email protected]>
> ---
> include/linux/mm_types.h | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 957ce38768b2..0330e0ddca11 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -401,11 +401,10 @@ FOLIO_MATCH(compound_head, _head_2a);
> * @pmd_huge_pte: Protected by ptdesc->ptl, used for THPs.
> * @__page_mapping: Aliases with page->mapping. Unused for page tables.
> * @pt_mm: Used for x86 pgds.
> - * @pt_frag_refcount: For fragmented page table tracking. Powerpc and s390 only.
> + * @pt_frag_refcount: For fragmented page table tracking. Powerpc only.
> * @_pt_pad_2: Padding to ensure proper alignment.
> * @ptl: Lock for the page table.
> * @__page_type: Same as page->page_type. Unused for page tables.
> - * @_refcount: Same as page refcount. Used for s390 page tables.

I would guess that you need to describe _pt_pad_3 instead here, just
like it is done for the other two pad members.

And most likely you need to add Andrew to "To:" so he doesn't miss
this :)

2023-11-21 15:49:04

by Alexander Gordeev

Subject: [PATCH v2] pgtable: do not expose _refcount field via ptdesc

Since commit d08d4e7cd6bf ("s390/mm: use full 4KB page for 2KB PTE")
_refcount field is not used for fragmented page tracking on s390 and
there is no other code left that accesses this field explicitly.

Suggested-by: Heiko Carstens <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>
---
include/linux/mm_types.h | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 957ce38768b2..ce0cfc6e4d94 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -401,11 +401,11 @@ FOLIO_MATCH(compound_head, _head_2a);
* @pmd_huge_pte: Protected by ptdesc->ptl, used for THPs.
* @__page_mapping: Aliases with page->mapping. Unused for page tables.
* @pt_mm: Used for x86 pgds.
- * @pt_frag_refcount: For fragmented page table tracking. Powerpc and s390 only.
+ * @pt_frag_refcount: For fragmented page table tracking. Powerpc only.
* @_pt_pad_2: Padding to ensure proper alignment.
* @ptl: Lock for the page table.
* @__page_type: Same as page->page_type. Unused for page tables.
- * @_refcount: Same as page refcount. Used for s390 page tables.
+ * @_pt_pad_3: Padding that aliases with page's refcount.
* @pt_memcg_data: Memcg data. Tracked for page tables here.
*
* This struct overlays struct page for now. Do not modify without a good
@@ -438,7 +438,7 @@ struct ptdesc {
#endif
};
unsigned int __page_type;
- atomic_t _refcount;
+ unsigned int _pt_pad_3;
#ifdef CONFIG_MEMCG
unsigned long pt_memcg_data;
#endif
@@ -452,7 +452,6 @@ TABLE_MATCH(compound_head, _pt_pad_1);
TABLE_MATCH(mapping, __page_mapping);
TABLE_MATCH(rcu_head, pt_rcu_head);
TABLE_MATCH(page_type, __page_type);
-TABLE_MATCH(_refcount, _refcount);
#ifdef CONFIG_MEMCG
TABLE_MATCH(memcg_data, pt_memcg_data);
#endif
--
2.39.2

2023-11-21 16:47:23

by Vishal Moola

Subject: Re: [PATCH v2] pgtable: do not expose _refcount field via ptdesc

On Tue, Nov 21, 2023 at 04:45:21PM +0100, Alexander Gordeev wrote:
> Since commit d08d4e7cd6bf ("s390/mm: use full 4KB page for 2KB PTE")
> _refcount field is not used for fragmented page tracking on s390 and
> there is no other code left that accesses this field explicitly.
>
> Suggested-by: Heiko Carstens <[email protected]>
> Signed-off-by: Alexander Gordeev <[email protected]>
> ---
> include/linux/mm_types.h | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 957ce38768b2..ce0cfc6e4d94 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -401,11 +401,11 @@ FOLIO_MATCH(compound_head, _head_2a);
> * @pmd_huge_pte: Protected by ptdesc->ptl, used for THPs.
> * @__page_mapping: Aliases with page->mapping. Unused for page tables.
> * @pt_mm: Used for x86 pgds.
> - * @pt_frag_refcount: For fragmented page table tracking. Powerpc and s390 only.
> + * @pt_frag_refcount: For fragmented page table tracking. Powerpc only.
> * @_pt_pad_2: Padding to ensure proper alignment.
> * @ptl: Lock for the page table.
> * @__page_type: Same as page->page_type. Unused for page tables.
> - * @_refcount: Same as page refcount. Used for s390 page tables.
> + * @_pt_pad_3: Padding that aliases with page's refcount.

I like updating the documentation, but I'd rather see _refcount renamed
to __page_refcount similar to the other unused page fields. _pt_pad_*
is used for variables that aren't present in struct page (and are
required for padding).

> * @pt_memcg_data: Memcg data. Tracked for page tables here.
> *
> * This struct overlays struct page for now. Do not modify without a good
> @@ -438,7 +438,7 @@ struct ptdesc {
> #endif
> };
> unsigned int __page_type;
> - atomic_t _refcount;
> + unsigned int _pt_pad_3;
> #ifdef CONFIG_MEMCG
> unsigned long pt_memcg_data;
> #endif
> @@ -452,7 +452,6 @@ TABLE_MATCH(compound_head, _pt_pad_1);
> TABLE_MATCH(mapping, __page_mapping);
> TABLE_MATCH(rcu_head, pt_rcu_head);
> TABLE_MATCH(page_type, __page_type);
> -TABLE_MATCH(_refcount, _refcount);

Its still a good idea to keep this check. In the case of !CONFIG_MEMCG,
_refcount is the last variable of ptdesc and if some change were to happen
that caused it to be misaligned we will want to know.

> #ifdef CONFIG_MEMCG
> TABLE_MATCH(memcg_data, pt_memcg_data);
> #endif
> --
> 2.39.2
>
>