Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in page->flags

From	Wei Yang <richard.weiyang@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in page->flags
Date	2017-03-19 16:10 +0100
Message-ID	<tmKCZ-7uc-9@gated-at.bofh.it> (permalink)
References	<tmaJb-6zz-1@gated-at.bofh.it> <tmKjD-75i-1@gated-at.bofh.it>
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On Sun, Mar 19, 2017 at 10:30:13AM -0400, Michal Hocko wrote:
>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
>> long".
>> 
>> 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
>> SECTIONS_PGOFF
>> 
>>     #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.
>

Hi, Michal

Glad to hear from you.

I think the purpose of definition BITS_PER_LONG is more easily to let audience
know it is the number of bits of type long. If it has no improvement, we don't
need to define a specific macro .

And as you could see, several related macros use BITS_PER_LONG in their
definition. After this change, all of them will have a consistent definition.

After this change, code looks more neat :-)

So it looks more reasonable to use this.

-- 
Wei Yang
Help you, Help me

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[PATCH] mm: use BITS_PER_LONG to unify the definition in page->flags Wei Yang <richard.weiyang@gmail.com> - 2017-03-18 01:50 +0100
  Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in  page->flags Michal Hocko <mhocko@kernel.org> - 2017-03-19 15:50 +0100
    Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in  page->flags Michal Hocko <mhocko@kernel.org> - 2017-03-19 16:10 +0100
      Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in  page->flags Wei Yang <richard.weiyang@gmail.com> - 2017-03-19 17:10 +0100
    Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in  page->flags Wei Yang <richard.weiyang@gmail.com> - 2017-03-19 16:10 +0100

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