linux-kernel - Re: [PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS)

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Date:   Mon, 26 Jul 2021 14:04:23 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org,
        osalvador@...e.de, mhocko@...e.com, song.bao.hua@...ilicon.com,
        david@...hat.com, chenhuang5@...wei.com, bodeddub@...zon.com,
        corbet@....net
Cc:     duanxiongchun@...edance.com, fam.zheng@...edance.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, zhengqi.arch@...edance.com
Subject: Re: [PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL <<
 NR_PAGEFLAGS) - 1)

On 7/14/21 2:17 AM, Muchun Song wrote:
> Instead of hard-coding ((1UL << NR_PAGEFLAGS) - 1) everywhere, introducing
> PAGEFLAGS_MASK to make the code clear to get the page flags.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  include/linux/page-flags.h      | 4 +++-
>  include/trace/events/page_ref.h | 4 ++--
>  lib/test_printf.c               | 2 +-
>  lib/vsprintf.c                  | 2 +-
>  4 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5922031ffab6..358d3f6fa976 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -178,6 +178,8 @@ enum pageflags {
>  	PG_reported = PG_uptodate,
>  };
>  
> +#define PAGEFLAGS_MASK		(~((1UL << NR_PAGEFLAGS) - 1))

Can you explain why you chose this definition instead of

#define	PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)

and mostly use ~PAGEFLAGS_MASK below?
-- 
Mike Kravetz

> +
>  #ifndef __GENERATING_BOUNDS_H
>  
>  static inline unsigned long _compound_head(const struct page *page)
> @@ -859,7 +861,7 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)
>   * alloc-free cycle to prevent from reusing the page.
>   */
>  #define PAGE_FLAGS_CHECK_AT_PREP	\
> -	(((1UL << NR_PAGEFLAGS) - 1) & ~__PG_HWPOISON)
> +	(~(PAGEFLAGS_MASK | __PG_HWPOISON))
>  
>  #define PAGE_FLAGS_PRIVATE				\
>  	(1UL << PG_private | 1UL << PG_private_2)
> diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
> index 5d2ea93956ce..643b1b4e9f27 100644
> --- a/include/trace/events/page_ref.h
> +++ b/include/trace/events/page_ref.h
> @@ -38,7 +38,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
> +		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val)
> @@ -88,7 +88,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
> +		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val, __entry->ret)
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index 8ac71aee46af..eadcc6458b10 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -614,7 +614,7 @@ page_flags_test(int section, int node, int zone, int last_cpupid,
>  	bool append = false;
>  	int i;
>  
> -	flags &= BIT(NR_PAGEFLAGS) - 1;
> +	flags &= ~PAGEFLAGS_MASK;
>  	if (flags) {
>  		page_flags |= flags;
>  		snprintf(cmp_buf + size, BUF_SIZE - size, "%s", name);
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index dd006adfe853..ff7c180a2971 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2019,7 +2019,7 @@ static const struct page_flags_fields pff[] = {
>  static
>  char *format_page_flags(char *buf, char *end, unsigned long flags)
>  {
> -	unsigned long main_flags = flags & (BIT(NR_PAGEFLAGS) - 1);
> +	unsigned long main_flags = flags & ~PAGEFLAGS_MASK;
>  	bool append = false;
>  	int i;
>  
>