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

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Message-ID: <20170319160501.GB1187@WeideMBP.lan>
Date:   Mon, 20 Mar 2017 00:05:01 +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 11:08:22AM -0400, Michal Hocko wrote:
>On Sun 19-03-17 23:03:45, Wei Yang wrote:
>> 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.
>
>I do not think that this is sufficient to justify the change.
>

Fine~ Thanks for comments~

>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me

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