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Subject: Re: [PATCH 4/6] mm: Make compound_head const-preserving
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
References: <20210416231531.2521383-1-willy@infradead.org>
 <20210416231531.2521383-5-willy@infradead.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <228b6174-5fa7-23d8-5a17-f85f6725068b@suse.cz>
Date:   Fri, 23 Apr 2021 16:58:14 +0200
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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;
>