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

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Message-ID: <20170319150345.GA34657@WeideMacBook-Pro.local>
Date:   Sun, 19 Mar 2017 23:03:45 +0800
From:   Wei Yang <richard.weiyang@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Wei Yang <richard.weiyang@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in
 page->flags

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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