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[4/6] mm: Make compound_head const-preserving

Message ID 20210416231531.2521383-5-willy@infradead.org (mailing list archive)
State New
Headers show
Series Constify struct page arguments | expand

Commit Message

Matthew Wilcox April 16, 2021, 11:15 p.m. UTC
If you pass a const pointer to compound_head(), you get a const pointer
back; if you pass a mutable pointer, you get a mutable pointer back.
Also remove an unnecessary forward definition of struct page; we're about
to dereference page->compound_head, so it must already have been defined.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/page-flags.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

Comments

Vlastimil Babka April 23, 2021, 2:58 p.m. UTC | #1
On 4/17/21 1:15 AM, Matthew Wilcox (Oracle) wrote:
> If you pass a const pointer to compound_head(), you get a const pointer
> back; if you pass a mutable pointer, you get a mutable pointer back.

Hmm, nice trick.

> Also remove an unnecessary forward definition of struct page; we're about
> to dereference page->compound_head, so it must already have been defined.

Right.

> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  include/linux/page-flags.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 04a34c08e0a6..d8e26243db25 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -177,17 +177,17 @@ enum pageflags {
>  
>  #ifndef __GENERATING_BOUNDS_H
>  
> -struct page;	/* forward declaration */
> -
> -static inline struct page *compound_head(struct page *page)
> +static inline unsigned long _compound_head(const struct page *page)
>  {
>  	unsigned long head = READ_ONCE(page->compound_head);
>  
>  	if (unlikely(head & 1))
> -		return (struct page *) (head - 1);
> -	return page;
> +		return head - 1;
> +	return (unsigned long)page;
>  }
>  
> +#define compound_head(page)	((typeof(page))_compound_head(page))
> +
>  static __always_inline int PageTail(struct page *page)
>  {
>  	return READ_ONCE(page->compound_head) & 1;
>
Anshuman Khandual April 27, 2021, 3:31 a.m. UTC | #2
On 4/17/21 4:45 AM, Matthew Wilcox (Oracle) wrote:
> If you pass a const pointer to compound_head(), you get a const pointer
> back; if you pass a mutable pointer, you get a mutable pointer back.
> Also remove an unnecessary forward definition of struct page; we're about
> to dereference page->compound_head, so it must already have been defined.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/page-flags.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 04a34c08e0a6..d8e26243db25 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -177,17 +177,17 @@ enum pageflags {
>  
>  #ifndef __GENERATING_BOUNDS_H
>  
> -struct page;	/* forward declaration */
> -
> -static inline struct page *compound_head(struct page *page)
> +static inline unsigned long _compound_head(const struct page *page)
>  {
>  	unsigned long head = READ_ONCE(page->compound_head);
>  
>  	if (unlikely(head & 1))
> -		return (struct page *) (head - 1);
> -	return page;
> +		return head - 1;
> +	return (unsigned long)page;
>  }
>  
> +#define compound_head(page)	((typeof(page))_compound_head(page))
> +
>  static __always_inline int PageTail(struct page *page)
>  {
>  	return READ_ONCE(page->compound_head) & 1;
> 

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
diff mbox series

Patch

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 04a34c08e0a6..d8e26243db25 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -177,17 +177,17 @@  enum pageflags {
 
 #ifndef __GENERATING_BOUNDS_H
 
-struct page;	/* forward declaration */
-
-static inline struct page *compound_head(struct page *page)
+static inline unsigned long _compound_head(const struct page *page)
 {
 	unsigned long head = READ_ONCE(page->compound_head);
 
 	if (unlikely(head & 1))
-		return (struct page *) (head - 1);
-	return page;
+		return head - 1;
+	return (unsigned long)page;
 }
 
+#define compound_head(page)	((typeof(page))_compound_head(page))
+
 static __always_inline int PageTail(struct page *page)
 {
 	return READ_ONCE(page->compound_head) & 1;