Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in page->flags
- Date: Sun, 19 Mar 2017 10:30:13 -0400
- From: Michal Hocko <mhocko@xxxxxxxxxx>
- Subject: Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in page->flags
On Sat 18-03-17 08:39:14, Wei Yang wrote:
> The field page->flags is defined as unsigned long and is divided into
> several parts to store different information of the page, like section,
> node, zone. Which means all parts must sit in the one "unsigned
> BITS_PER_LONG is used in several places to ensure this applies.
> #if SECTIONS_WIDTH+NODES_WIDTH+ZONES_WIDTH > BITS_PER_LONG - NR_PAGEFLAGS
> #if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
> #if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
> While we use "sizeof(unsigned long) * 8" in the definition of
> #define SECTIONS_PGOFF ((sizeof(unsigned long)*8) - SECTIONS_WIDTH)
> This may not be that obvious for audience to catch the point.
> This patch replaces the "sizeof(unsigned long) * 8" with BITS_PER_LONG to
> make all this consistent.
I am not really sure this is an improvement. page::flags is unsigned
long nad the current code reflects that type.
> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> include/linux/mm.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b84615b0f64c..a5d80de089ff 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -684,7 +684,7 @@ int finish_mkwrite_fault(struct vm_fault *vmf);
> /* Page flags: | [SECTION] | [NODE] | ZONE | [LAST_CPUPID] | ... | FLAGS | */
> -#define SECTIONS_PGOFF ((sizeof(unsigned long)*8) - SECTIONS_WIDTH)
> +#define SECTIONS_PGOFF (BITS_PER_LONG - SECTIONS_WIDTH)
> #define NODES_PGOFF (SECTIONS_PGOFF - NODES_WIDTH)
> #define ZONES_PGOFF (NODES_PGOFF - ZONES_WIDTH)
> #define LAST_CPUPID_PGOFF (ZONES_PGOFF - LAST_CPUPID_WIDTH)